AMMRC CTR 73-30

THERMOMECHANICAL PROCESSING B TITANIUM

Technical Report by

D. H. Avery and N . W . Polan &own University Providence, Rhode Island

September 1973

Final Report Contract Number DAAG46-72-C-0165 D/A Project : lW564603D385 AMCMS Code: 554C.12.62000 T i t l e of Project : P ro jec t i l e , XM517 Agency Accession Number: DAOD4810

Approved for public release; d i s t r ibu t ion unlimited.

Prepared for

ARMY MATERIALS AND MECHANICS RESEARCH CENTER Watertown, Massachusetts 02172

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FOREWORD

This repor t covers t h e work done in t h e period May 11, 1972 t o August 1 0 ,

1973 under the general t i t l e ''Themnomechanical Processing B Titanium".

work was sponsored by t he Army Materials and Mechanics Research Center, on

Contract No. DAAG46-72-C-0165, D/A Project lW564603D385, AMCMS Code

554C.12.62000. Mr. A . Zarkades acted as Contracting Officer's Representative.

The research described was conducted a t Brown University by D . H. Avery and

N. w. Polan.

The

This pro jec t has been accomplished as p a r t of t h e U. S. Army Manufacturing

Methods and Technology Program, which has as its objec t ive t h e timely estab-

lishment of manufacturing processes, techniques or equipment t o insure t h e ef-

f icient production of current or fu tu re defense programs.

CONTENTS

Page

FOREWORD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

SCALE-UPPROGRAM . . e 2

OPTIMIZATION OF MECHANICAL PROPERTIES . . . . . . . . . . . . . . . . . . 5

Design criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Application of t h e criteria t o $-Ti a l loys . . . . . . . . . . . . . 8

Effects of cold work on polygonized and a-$ s t ruc tures

Effects of cold-reduction temperature . . . . . . . . . . . . . . . . 13

. . . . . . . 13

Multi-step aging . . . . . . . . . . . . . . . . . . . . . . . . . . 14

STRESS CORROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 5

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6

I BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

- v -

ABSTRACT

The r e s u l t s of t h i s invest igat ion include a set of cri teria f o r t h e design

of very strong yet duc t i l e materials. By appl icat ion of these cri teria, thermo-

mechanical processing has resu l ted in t he attainment i n a @-titanium a l loy of

highly reproducible tensile s t rengths in excess of 320,000 p s i with a cup-cone

duc t i l e shear failure of 15% RA, exhibi t ing d u c t i l e behavior a t elastic strains

grea te r than 2%.

j,js i ‘ /?%e response of t h e 13 a l loy TS6 ( T i : 1 0 C r , 7 V , 3.5 Mo, 3 All, t o a wide

var ie ty of thermomechanical treatments has been invest igated, including t h e

effects of cold work on polygonized s t ruc tures , t h e effects of cold-reduction

temperature, and multi-stage aging.

be manipulated within a wide range by varying t h e amount of cold work and the

Combinations of s t rength and d u c t i l i t y can J aging treatment. Mechanical propert ies are s t rongly dependent upon the mode of

deformation during cold working and s l i g h t var ia t ions i n a l l o y chemistry.

S t ress corrosion tests i n H 0-3%NaCl and CH O H - l % H C l ind ica te superior perform- 2 3

fj? ’ ? ance of TS6 compared with s imi la r ly t r ea t ed B-120VCA.

A scale-up of ingot melting and processing techniques has been accomplished.

Ingots of TS6 weighing twenty-eight pounds have been prepared using a master-

alicly technique.

has been successfully cold press-swaged 20% and cold r o l l e d 90%.

Hot extrusion a t 18OOOF has yielded two-inch diameter rod which

Compared w i t h

previous one-pound heats o f TS6, t h i s material overages earlier, a t somewhat lower

hardness l eve l s , and with more rapid kinet ics .

INTRODUCTION

The fabr ica t ion of non-equilibrium microstructures is a f resh approach t o

materials design which has been r e l a t i v e l y neglected but which can produce sub-

s t a n t i a l l y improved physical propert ies .

maximization of one property or the optimization of a combination of propert ies

is based on the control led placement of t h e const i tuents of the microstructure,

involving new processing techniques t o overcome t h e r e s t r a i n t s imposed by the

phase diagram and t r a d i t i o n a l cas t ing and heat t r e a t i n g methods.

design of non-equilibrium microstructures is t o optimize and o r i en t the desired

propert ies i n the f i n a l fabr icated item.

including s t rength , elastic modulus, fracture toughness, electrical conductivity,

and magnetic permeability are generally needed only in spec i f i c d i rec t ions i n an

object .

d i r ec t iona l i t y , as i n the maximizing of the permeability of s i l icon-iron t rans-

former sheets ; s imi la r ly , the elastic modulus in the plane of a panel subject t o

buckling could be maximized.

microstructures include the ausfonning and patenting processes in steels.

The development of materials for t h e

One aim in the

Critical values of spec i f ic propert ies

This approach has been used i n u t i l i z i n g crystal lographic texture and

Other familiar explo i ta t ions of non-equilibrium

The research described below is an example of t he r e s u l t s of such an ap-

proach--the design of a very s t rong, highly d u c t i l e t i tanium alloy through the

development of a very fine cellular dis locat ion s t ruc tu re which is amenable t o

low temperature aging treatments. The invest igat ion of thermomechanical proc-

essing of t h e 6-titanium a l l o y TS6 ( T i :

under t h i s contract has resu l ted i n fu r the r charac te r iza t ion , manipulation,

and improvement of the proper t ies of the material.

s t rengths i n excess of 320,000 p s i with cup-cone duc t i l e shea r failures of 15%

RA have been a t ta ined .

has been s tudied, concurrent with the development of a set of cri teria which

1 0 C r , 7 V , 3.5 Mo, 3 A l ) continued

Highly reproducible t e n s i l e

The response t o a var ie ty of thermomechanical treatments

r

- 2 -

apply t o t h i s a l l o y and character ize i n general t h e optimum design of very

s t rong, highly d u c t i l e metallic a l loys through the development of u l t r a - f ine

grain size.

which w i l l result in l a rger -sca le sect ion s i z e s cons is ten t w i t h s t r u c t u r a l ap-

p l ica t ions .

i n a l l o y chemistry has been completed, and stress corrosion proper t ies of the

a l loy have been characterized.

A scale-up of melting and processing techniques was i n i t i a t e d

A preliminary invest igat ion of t he effects of s l i g h t var ia t ions

SCALE-UP PROGRAM

The @ - T i a l l oy t h a t w e are developing has exhibited such superior mechani-

cal proper t ies t h a t an e f f o r t has been made t o scale up current melting and

processing techniques t o y ie ld la rger -sca le sec t ion s i zes consis tent with s t ruc-

tural appl icat ions.

men sizes--up t o .160 inch diameter--since the as-received ingots have been i n

the form of one-pound hot-forged rods less that 1 / 2 inch i n diameter and the

subsequent mechanical deformation has been la rge . The scale-up program being

pursued promises t o y ie ld 3/8-inch diameter rod after a 95% reduction of area

through cold swaging; t h i s material w i l l have su f f i c i en t sec t ion s i z e t o inves-

t i g a t e the effects of thermomechanical processing on notch toughness and t o

check the correspondence of scaled-up t e n s i l e propert ies .

In t h e past we have been l imited t o small t e n s i l e speci-

Two twenty-eight pound ingots of TS6 have been prepared a t AMMRC by consum-

able-electrode melting.

found by X-ray radio-aphy t o contain t h e molybdenum const i tuent of the alloy

dis t r ibu ted along the a x i s of the cast ing i n the form of cy l ind r i ca l p e l l e t s

which remained unmelted during ingot preparation. The second ingot was pre-

pared by first combining the 10% chromium and 3.5% molybdenum cons t i tuents of

TS6 t o form a master a l l o y with a melting point of 186OOC ra the r than the 2625Oc

The first was received during August, 1972, and was

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and 1900OC of Mo and Cr separately.

t o chips o r powder which is then r ead i ly dissolved i n t h e remaining const i tuents

of TS6.

was received i n December, 1972.

of t h e ingot and sectioned i n t o one-inch cubes.

on these using a hand-operated hydraulic press a t temperatures between 1950O

This a l loy is b r i t t l e and can be reduced

This method proved successful, and the second twenty-eight pound ingot

A one-inch th ick d i s c was cu t f r o m t h e bottom

A forging study was conducted

and 13OOOF.

but rose above 75 k s i as the t e s t i n g temperature was lowered below 1500OF.

specimens were reduced about 50% and showed no s igns of crack formation at the

low rates of loading employed; however, the l a rge increase i n y ie ld s t rength

suggests t h a t a prec ip i ta t ion must be avoided by forging a t 1 6 O O O F or above.

A t 180OOF the y ie ld s t rength was observed t o be less than 1 5 k s i

A U

The second ingot was then machined to 4.750 inch diameter, vacuum sealed

i n a steel can and successful ly extruded t o two-inch diameter a t 180OOF by

Nuclear Metals, Inc. of Concord, Mass. A two-inch length was cut h m the

t r a i l i n g end of the extrusion.

and two specimens were machined and ro l l ed without d i f f i c u l t y t o 90% reduction

of thickness.

425OC with hardness measurements taken a t in t e rva l s as shown i n Figure 1.

pared with heats #1 and #2 (one-pound hea ts ) , heat #5 ( t h i s twenty-eight pound

heat) appears t o overage earlier, at somewhat lower hardness l eve l s , and with

s l i g h t l y more rapid k ine t ics .

qu i te s a t i s f a c t o r i l y , and high workability has been retained.

The can was removed with a solut ion of HN3-H20,

One was first solut ion t r ea t ed a t 850OC. Then both were aged a t

Com-

Otherwise, t he scale-up has been accomplished

The e n t i r e rod was then solut ion t rea ted a t 85OOC f o r one hour, water

quenched, pickled t o remove the steel can, and l i g h t l y surface ground t o two-

inch diameter.

being pursued but has not ye t been accomplished.

The f i n a l s tage of cold working t o 3 /8-hch diameter rod is now

Two major problems encountered

- 4 -

have been t h e d i f f i c u l t i e s i n scal ing up ingot preparation and t h e unavailabil-

i t y of large-scale cold working f a c i l i t i e s or t he re luctance of p r iva t e indus-

t r ies t o attempt heavy cold reduction of t i t an ium a l l o y s using t h e i r equipment.

The first problem has been solved, but it delayed fur ther development u n t i l

mid-December 1972 when the first sa t i s f ac to ry l a rge ingot was produced.

The second problem was ant ic ipated before the proposal of t h i s p ro jec t ,

and many indus t r ies with t h e capabi l i ty of la rge scale cold working--either rod

r o l l i n g , swaging or drawing--were contacted.

i n undertaking the work, believing it t o be within t h e i r capacity. Now that the

material has been prepared, however, and formal commitment t o assume t h e pro jec t

is sought, several new d i f f i c u l t i e s have ar i sen . Previous experience with t i ta-

nium alloys i n general has prejudiced many possible sources against attempting

the work despi te our own extraordinary results w i t h cold reduction of both t h e

one and twenty-eight pound ingots of TS6. Most of those who do feel that they

may have the capacity and are wi l l ing t o make the attempt requi re equipment

breakage guarantees and patent r i g h t s from t h i s contractor , both of which are

Several of t hese expressed i n t e r e s t

unacceptable.

O f the major firms contacted, including U. S . Stee l , Alcoa, Wyman-Gordon,

Bethlehem S tee l , Washburn Wire, and Carpenter Technology, only two agreed t o

sa t i s f ac to ry terms within the f inanc ia l l imi t a t ions of t h i s contract . Wyman-

Gordon attempted t o cold forge a sect ion of t he two-inch diameter rod t o one-

inch diameter which would then be cold extruded t o 3/8 inch using ava i lab le

equipment. This attempt was unsuccessful, however, s ince only flat forging

d i e s were avai lab le , producing a completely unacceptable state of stress in the

mater ia l r e su l t i ng i n fracture after less then 15% RA.

The second alternative--use of a 1000 ton press and l a rge capacity swaging

equipment a t the Bethlehem S tee l Co.--has a l s o been pursued and has demonstrated

- 5 -

t h a t cold reduction of the scaled-up material is, indeed, feas ib le . The two-

inch diameter rod was reduced 20% RA by cold press-swaging,deformation of the

rod being homogeneous with no cracking occuring.

attempted due t o damage t o the r e l a t i v e l y soft R 43 press dies which deformed

p l a s t i c a l l y during loading. The ava i lab le ro t a ry swaging equipment also proved

inadequate since t h e included angle ( 8 O ) of the dies was excessive. Neverthe-

less, given appropriately designed equipment, cold reduction of TS6 on a l a rge

scale seems commercially p rac t i ca l .

Further reduction was not

C

While scaled-up thermomechanical processing has been accomplished, it is

our opinion t h a t t he most appropriate appl icat ion of t h i s material does not l i e

i n l a rge scale s t ruc tu res but in fabr ica t ion on a smaller scale where the re-

quired mechanical operations are qui te r ead i ly and e f f i c i e n t l y obtained--high

s t rength wire cable and fas teners are prime examples.

t i o n s are t o be pursued, i n view of t he experiences indicated above, separate

i n d u s t r i a l contract ing t o develop the capaci ty for heavy cold deformation of

l a rge sect ion s i z e s is required.

If l a rge scale applica-

OPTIMIZATION OF MECHANICAL PROPERTIES

The invest igat ion of t h e mechanical and stress corrosion propert ies of t h e

a l loy was conducted on the one-pound ingots produced a t AMMRC (heats #2 and #3) .

While the results presented below are extraordinary, representing d u c t i l e behav-

i o r i n metals a t elastic s t r a i n s i n excess of 2.3%, they are not , we believe,

unique t o t h i s specific a l loy .

u la r u l t r a f i n e f ibrous microstructure previously exploited only i n heavily drawn

p e a r l i t e . Concurrent wi th t he empirical investigation,we have proposed a set

of cri teria for the character izat ion of an optimal material for high s t rength

and high d u c t i l i t y through t h e development of ultra-fine grain s i ze .

Rather they represent the behavior of a par t ic -

We now

ou t l ine the r a t iona le f o r these cri teria and discuss the results of t h e i r suc-

cessful appl icat ion t o t h e pa r t i cu la r case a t hand--the thermomechanical proc-

ess ing o f B titanium.

Design criteria

1.

2.

3 .

4.

5.

The s t a r t i n g material cons is t s o f a metastable fcc o r bcc s ing le phase.

The material has a low strain-hardening exponent and good workability.

In order t o form s t a b l e subgrain cells t h e material has a low stacking-

fault energy and a high recovery temperature.

A hardening mechanism ac t ive below t h e recovery temperature is ava i l -

ab le , i .e. metastable decomposition or loca l ordering, not so l id -

solut ion diffusion-controlled prec ip i ta t ion .

The flow stress is r e l a t i v e l y insens i t ive t o temperature.

Rat ionale

The c e n t r a l requirement of t h i s ana lys i s is t h e development of a very f i n e

subgrain cel l s t ruc tu re which i s k ine t i ca l ly homogeneous and responds t o a rela-

t i v e l y low-temperature hardening mechanism. Work hardening must be discounted

a t t he ou t se t s ince the c r i t e r i o n f o r t he maintenance of mechanical s t ab i l i t y - -

du/de > a--becomes impossible a t very high stresses. If t h i s c r i t e r i o n were t o

be m e t , after very small s t r a i n s it would impose stress l e v e l s i n excess of any

conceivable fracture stress. However, i f a very fine-grained cellular substruc-

ture can be produced, d i s loca t ion motion is l imited and t h e y ie ld s t rength

raised according t o t h e Hall-Petch r e l a t i o n ,

Simultaneously, t he fracture s t rength is increased by

K, A. u f = u + - fo J;i

- 7 -

i f microstructurally dimensioned microcracks or voids are the control l ing frac-

ture mechanism.

exploited i n heavily drawn two-phase p e a r l i t e C1,21.

t i ons , however, p l a s t i c deformation of single-phase mater ia l results in the for-

mation of a s t ab le cell structure which can be reduced in dimension upon f u r t h e r

deformation.

Such a microstructural strengthening was first observed and

Under t h e proper condi-

The cri t ical dimension is then given by

d = d m , 0

where

ing) the material and do

schematically i n Figure 2. Then

CW is the reduction of area due t o cold working (e.g. swaging or d r a w -

is t h e hypothetical i n i t i a l subgrain s ize as shown

-1/4 U = Q t K'(1 - CW) ,

Y Y, Y U

and

-1/4 Uf = Uf t Ki(1 - CW) .

0

The development of s t a b l e e f f ec t ive substructures generally requires cold

deformations of a t least 30% t o 40%; and, since the desired f i n e submicron cells

require fu r the r reduction, good workability is essential. For t h i s reason, t h i s

type of processing is considered t o be l imited t o fcc or bcc so l id solut ion

a l loys with a r e l a t i v e l y low strain-hardening exponent.

In order t o s t a b i l i z e and maintain t h e cell s t ruc ture , a low stacking-fault

energy and high recovery temperature are necessary.

duced transformations have been avoided, the highly deformed material is essen-

t i a l l y k ine t i ca l ly homogeneous and amenable t o fu r the r strengthening through

transformation or ordering of t he metastable s t a r t i n g material.

thermal hardening mechanism must occur without a l t e r i n g the cell geometry.

Therefore, only metastable decompositions, i n t e r s t i t i a l d i f fus ion , and 1-

Assuming deformation in-

However, any

- 8 -

ordering react ions seem feas ib l e , and so l id solut ion diffusion-controlled pre-

c i p i t a t i o n mechanisms are excluded. A material which satisfies these cri teria

can be heavily cold worked a t a moderately low yie ld s t rength t o develop the

substructure; subsequent aging p rec ip i t a t e s a second phase t o produce maximum

s t rength with the re ten t ion of some d u c t i l i t y since no p re fe ren t i a l segregation

occurs a t grain boundaries.

The d e s i r a b i l i t y of a low s e n s i t i v i t y of flow stress t o temperature is d ic-

t a t e d by the attempt t o avoid low-energy-absorption ad iaba t ic shear fractures.

For a material showing exponential hardening--a = KE --in a first order approx-

imation completely ad iaba t ic flow would l o c a l l y reduce t h e y ie ld s t rength t o

n

E

where c is the heat capacity. Thus w e favor a low value of d ~ ' da

l a rge c and n are des i rab le a l so . However, most high s t rength metals are

characterized by a low s t r a i n hardening exponent; and, s ince w e previously re-

quired good workabili ty, some tendency toward local ized non-cleavage type defor-

mation must be ant ic ipa ted .

Optimally, P

P

Application of the criteria t o B-titanium a l loys

Promising candidates for t h i s type of thermomechanical strengthening treat-

ment include the metastable B a l loys of t i tanium.

temperature bcc phase is re ta ined a t room temperature by rapid quenching.

sequent aging can permit prec ip i ta t ion hardening by decomposition of t h e meta-

stable 8 phase.

grain boundaries, w i t h the at tendant degradation of s t rength , d u c t i l i t y , and

stress corrosion propert ies .

k ine t i ca l ly homogeneous material and is the basis for ordinary thermomechanical

In these a l loys , t he high

Sub-

A common problem, though, is p re fe ren t i a l p rec ip i ta t ion a t

Deformation p r i o r t o aging tends t o produce a

- 9 -

treatments.

posit ion T i :

chanical treatment.

tate the f ine ly dispersed hcp a phase produced a maximum ultimate t e n s i l e s t rength

of 234,000 p s i compared with the solution t rea ted and quenched s t rength of

119,000 ps i .

low, approximately 1 5 erg/cm2 C41.

of thermomechanical processing of t h e 6-Ti a l loys , and, i n view of the results,

TS6 was chosen for study i n t h i s investigation.

ent i re class of B a l loys is suscept ible in some degree t o the treatment de-

Lerinman, e t a l . C31 reported work i n which the a l loy TS6, of com-

10.8 Cr, 7.8 V , 3.4 Mo, 3.2 Al, responded favorably t o thermome-

Cold reductions up t o 40% and subsequent aging t o precipi-

The stacking-fault energy for the a l loy was determined t o be very

These were among the earliest investigations

We believe, however, t h a t the

scribed here. For example, TS6 is similar t o Crucible B-120VCA.

Several one-pound ingots of composition similar t o TS6--Ti: 1 0 C r y 7 V ,

3.5 Mo, 3 Al- -were produced a t AMMRC by conventional titanium melting tech-

niques--nonconsumable-electrode arc melting followed by forging and remelting

i n a consumable-electrode arc furnace. These ingots were then hot forged and

I swaged t o 0.4-inch diameter rods, machined t o 0.350 inch diameter, solut ion

t rea ted i n vacuo a t 85OOC for 1/2 hour, water quenched, and variously cold swaged

or drawn up t o 96% RA with no r e su l t an t defects and no intermediate annealing

treatments. The elastic modulus is low--between 14 and 16 x 1 0

on preferred or ientat ion.

been determined by hardness measurements t o be 0.16.

microscopy on the cold worked material [SI shows dis locat ion tangles growing in

densi ty and i r r egu la r i ty as deformation increases.

transformation induced phases are seen.

an i r regular ce l l s t ruc ture of approximately 0.05 t o 0.08 1.1 diameter.

6 p s i , depending

The strain-hardening exponent is also low, having

Transmission electron

Few twins are found and no

The most heavily worked material has

Results f r o m unaged specimens tested i n uniaxial tension support t h e above

-1/4 analysis. In Figure 3a, yield s t rength plot ted as a function of (1 - CW) ,

- 10 -

the measure of the e f f ec t ive c e l l s i ze , agrees w i t h the Petch hardening equation.

A t deformations up t o 96.2% RA, the l i m i t of t h e present invest igat ion, l i t t l e

tendency f o r spontaneous recovery is exhibited.

i n s imi la r ly heavily cold-drawn chromium [SI, t h a t t he subgrain s i z e decreases

rap id ly up t o 98% RA but s t a b i l i z e s thereaf te r .

I t has been observed, however,

Aging the highly deformed material between 350 and 5 O O O C produces a f i n e ,

uniformly dispersed a prec ip i ta te .

of thermomechanical treatments of two d i f f e ren t heats of material are presented

i n Table I and Figure 4.

occurs by void growth and duc t i l e shear as i l l u s t r a t e d in F igu res 5 and 6.

Representative t e n s i l e data f o r a va r i e ty

In even the very high s t rength condition, fa i lure

The overa l l r e s u l t s from heat #3 show an increase of about 20% over previ-

ously obtainable t e n s i l e s t rengths and were r igorously examined t o assure t h e i r

va l id i ty . Testing equipment was carefu l ly ca l ibra ted and monitored; specimens

were repeatedly measured using a 0.017-inch blade micrometer and two measuring

microscopes; data for the high s t rength aged specimens were reproduced in three

separate t e s t series.

the as-machined surface a t 425OC i n a i r has no adverse e f f e c t s on mechanical

properties.

aged t o high s t rength shows observable necking with wolf-ear fractures.

some specimens t h i s precluded the measurement of RA's as seen i n Table I.

The order of machining and aging proved irrelevant--aging

While the drawn w i r e exhib i t s cup-cone f r ac tu re , the swaged w i r e

On

The enhanced propert ies of the swaged material f r o m heat #3 are due t o a

combination of the mode of deformation and a l loy chemistry. The maximum flow

strength is highly sens i t ive t o chemistry, w i t h material f r o m heat $3 showing

higher s t rength l eve l s both as cold worked and aged. The r e s u l t s of chemical

analyses are shown in Table 11--heat # 3 is consis tent ly lower i n B s t a b i l i z e r

concentrations and higher i n the a s t a b i l i z e r , A l . To determine the e f f ec t of

- 11 -

TABLE I

Representative Tensile Data

Heat #2 ST 1 / 2 hr. at 85OoC, cold drawn to 96% RA.

_ - Hours Aged UTS ( k s i ) RA(%) 1--< - r , ?$. at 425OC

/L, I 58 1 3 1 2

7

0 195 8 2 67

1 6 272 1 6 274

Heat #3

Hours aged

ST 1/2 hr. at 85OoC, cold drawn to 96% RA.

at 425OC UTS(ksi) M(%) 0 0 6

1 2

210 210 2 95 3 06

5 1 55 4 6

Heat #3 ST 1/2 hr. at 85OoC, swaged to 96.2% RA, resolution treated.

UTS(ksi) RA(%)

65 135 1 4 0 -

Heat #3

Hours Aged

ST 1 / 2 hr. at 85OoC, swaged to 96.2% RA.

UTS ( k s i ) RA(% 1

I ' '- 0 2 6

1 2 12 1 2 30

2 24 3 08 3 1 8 324 3 1 6 32 2 329

60 1 9

1 6 1 7 1 2

-

-

- 12 -

a l loy chemistry, a series of s i x ingots is desired, decreasing t h e a l loy ing addi-

t i o n s of C r , V , and Mo i n 5% s t eps from t h e nominal T i : 1 0 Cr, 7 V, 3.5 Mo,

3 A1 t o T i : 7.5 C r y 5.25 V, 2.75 Mo, 3 A l .

TABLE I1

Chemical Analysis

A 1 v Cr Mo Fe cu N 0 H

Heat #2 2.73% 6.74% 9.80% 3.39% 0.07% N i l 0.011% 0.116% 0.0136%

H e a t #3 3.05 6.99 8.86 2.91 0.07 N i l 0.016 0.146 0.0163

Average fracture s t rengths , applying the Bridgman correction f o r t r i a x i -

a l i t y [7], have been calculated for t h e various classes of aged material and

appear i n Table 111.

cant ly higher than t h e o thers , indicat ing t h e importance of t h e processing mode.

Furthermore, as seen in Figure 3b, t h e f r ac tu re s t rength of t h e aged material

The fracture s t rength of t h e swaged material is s i g n i f i -

follows t h e modified Petch equation with t h e same slope as t h e unaged material.

Thus t h e aging temperature is below t h e recovery temperature and the cellular

barriers t o d is loca t ion motion appear unchanged. As seen in comparing Figures

3a and 3b, aging and t h e mode of deformation control t h e f r i c t i o n stress, u

o r uf , i n each case and have l i t t l e effect on t h e d is loca t ion pinning term

o r Kk.

posit ion and mechanical processing and independent of heat treatment.

YO K'

Y 0

In t h e aged material, uf i s r e l a t i v e l y constant for a given a l l o y com-

For t h e

l imi t ing case of no work hardening, t h e y i e ld stress and the UTS are t h e same,

and -1/4 u + K'(1 - CW)

U yo Y = Y = 9

f 1 - R A 1 - R A

- 13 -

where RA is t he reduction of area.

and combinations of s t rength and d u c t i l i t y can be manipulated within a wide

range by varying the amount of cold work and t h e aging treatment.

Thus a t rade-off between UTS and RA exists,

TABLE I11

Specimen Class

96.2% Swaged: Heat #3

96% Drawn: Heat #3

95% Drawn: Heat #2

80% Drawn: Heat #2

63% Drawn: Heat #2

Average Fracture Strength ( k s i 1

356

309

2 94

271

265

Effects of cold work on polygonized and a-B s t ruc tu res

Specimens were prepared by solut ion t r e a t i n g a t 85OoC, water quenching,

cold r o l l i n g 65% a t room temperature, polygonizing a t 7OOOC f o r 20 minutes, and

aging a t 375OC.

ness was less f o r t h i s material, and it showed earlier overaging (Figure 7).

Solutionizing a t 75OOC i n t h e a-8 f i e l d produced a rec rys t a l l i zed duplex s t ruc-

ture with continuous a on t h e f3 grain boundaries and smal l equiaxed grains of

i so la ted a within the f3 grains.

higher aged hardness and more rapid aging kinetics than t h e all-B material

(Figure 8 ) .

Contrary t o a similar Soviet invest igat ion C3,41, maxi" hard-

After 50% cold reduction t h i s material showed

Effects of cold-reduction temperature

As previously indicated 151, there is no s ign i f i can t hardness var ia t ion

following cold r o l l i n g t o 80% RA a t temperatures between - 6 O O C and 100°C.

tensive t e s t i n g confirms t h i s (Figure 9) .

effect f r o m varying the r o l l i n g reduction between 5% and 20% per pass.

Ex-

Furthermore, t he re appears t o be no

Aghg

- 14 -

kinetics and peak hardness remain unaffected.

Multi-step aging

The effect of multi-step aging on hardness and t e n s i l e propert ies was inves-

t iga ted . It is expected t h a t a primary low-temperature aging treatment enhances

the rate of p rec ip i t a t e nucleation,yielding a f i n e , uniformly dispersed second

phase of e i t h e r w as found i n Beta-I11 C81 or, more l i k e l y , B1 t B2 as observed

i n T i :

s i b l e t o p re fe ren t i a l ly nucleate and g r o w a a t a higher aging temperature.

16 V, 2.5 A 1 [9]. Upon t h e intermediate B2 sites it should then be pos-

Duplex aging a t 375OC/425OC was investigated through hardness measurements

as i l l u s t r a t e d in Figure 10. A decrease i n peak hardness is noted, and aging

k ine t i c s are accelerated. S i m i l a r results have been previously noted i n aging

a t 300°C/3750C and 42SoC/475OC c 5 1

A l imited number of multi-step aging experiments on t e n s i l e specimens f r o m

t he remaining small amount of material f r o m heat #3 were a l s o conducted. Results

appear i n Table I V .

d u c t i l i t y equal t o those obtained by conventional aging a t 425OC.

Only specimens #l and #3 show combinations of s t rength and

The combina-

t i o n of 261 k s i and 55% RA is a pa r t i cu la r ly outstanding r e s u l t . The reasons

f o r t he otherwise poor response include retrogression a t t h e higher temperatures--

the primary p r e c i p i t a t e is smaller than the c r i t i ca l s i z e a t t h e elevated tem-

perature--and coarsening of t h e p rec ip i t a t e during t h e secondary aging treat-

ments. An exhaustive study of multi-step aging should, however, produce some

enhancement of proper t ies and prove economically des i rab le i n sharply reducing

required aging times.

- 15 -

TABLE N

Specimen No.

1

2

3

4

5

6

Stress

h r . a t : 245OC

1

1

1

Multi-Step Aging

Heat #3 swaged 96.2% RA

UTS ( k s i 1

2 61

296

290

307

31 8

323

%RA

55

6

20

4

7

6

STRESS CORROSION

corrosion tests were conducted i n four-point bending of 0.14 inch

th ick s t r i p specimens of TS6 and 6-120VCA and i n uniaxial tension of 0.050 diam-

eter t e n s i l e specimens of TS6 in solut ions of H20-3% N a C l and CH30H-1% HC1 a t

25OC.

97%, aged up t o e ight hours a t 425OC, and t e s t ed a t 185,000 p s i in the salt

so lu t ion ; nor on unaged specimens t e s t ed a t 150,000 p s i in the methanol-HC1

solut ion.

corrosion suscep t ib i l i t y .

TS6 has low aluminum content, contains an appreciable amount of the isomorphous

beta s t a b i l i z e r s molybdenum and vanadium, and t h e specimens w e r e not precracked.

No failures were observed after 300 hour tests on TS6 cold worked 70% or

These conditions are thus presumably below the threshold for stress

Such results f o r t h s salt solut ion are expected s ince

Data fkom tests between 160,000 p s i and 190,000 p s i i n methanol-HC1 appear

i n Figure 11.

nature of the phenomenon; nevertheless, t h e s ign i f i can t t rends can be ident i f ied .

TS6 shows decreasing s e n s i t i v i t y t o stress corrosion with increasing amounts of

Some scatter is observed i n the da t a due t o t h e statistical

- 16 -

cold work and increasing s e n s i t i v i t y as aging time increases.

pated s ince suscep t ib i l i t y t o stress corrosion is suppressed by homogeneity.

The presence of a small amount of water a l s o i n h i b i t s t he process--TS6 swaged

96.2%, aged twelve hours a t 425OC and t e s t ed i n methanol-1%HC1-1%H20 a t 185 k s i

f a i l e d after t h i r t y minutes in cont ras t t o e ight minutes for s imi la r ly pre-

pared specimens t e s t ed in methanol-l%HCl.

This is antici-

Specimens t e s t ed in tension fa i l somewhat earlier than s imi la r ly t r ea t ed

specimens t e s t ed in bending since t h e bend t e s t is less severe--the t e n s i l e

stress i n the outer f i b e r being reduced after cracking begins, as opposed t o

being increased when t e s t ed in tension. In all cases, TS6 shows superior prop-

erties compared with s imi la r ly t r ea t ed $-120VCA specimens.

CONCLUSIONS

One means of optimizing high s t rength and high d u c t i l i t y in metals can be

characterized on t h e bas i s of f ive criteria: a metastable, s ing le phase fcc or

bcc s t a r t i n g material; a low strain-hardening exponent and good workabili ty; a

low stacking fault energy and high recovery temperature; a hardening mechanism

ac t ive below t h e recovery temperature; and a low s e n s i t i v i t y of flow stress t o

temperature. By appl icat ion of these criteria, Feznomechanical processing of cl the B-Ti a l l o y TS6 has resu l ted in a material with an ultimate tens i le s t rength 1- in excess of 320,000 p s i with 15% RA, exhibi t ing d u c t i l e behavior at elastic

s t r a i n s greater than 2%. Equivalent propert ies i n other metals on a modulus

basis would be 230,000 p s i f o r aluminum alloys, 360,000 p s i for copper alloys,

and 680,000 p s i for steels.

resent ing d u c t i l e behavior in metals a t elastic s t r a i n s i n excess of 2.3%, they

While t h e present results are extraordinary, rep-

are no t , we k l i e v e , unique t o t h i s spec i f i c a l loy , for it is noted t h a t a com-

bination of 270 k s i UTS and 34% RA i n a s imi la r ly t r ea t ed Beta I11 has been --I

- 17 -

r recent ly reported &These r e s u l t s are somewhat i n fe r io r t o the propert ies

w e have obtained i n TS6, but they support our contention t h a t t h e extraordinary

response t o t h e thermomechanical treatments we have investigated is a micro-

s t r u c t u r a l effect not unique t o a spec i f i c a l loy . Rather they represent the

behavior of a pa r t i cu la r u l t r a - f ine f ibrous microstructure previously exploited

only i n heavily drawn p e a r l i t e .

The response of TS6 t o a wide var ie ty of thermomechanical treatments has

Combinations of s t rength and d u c t i l i t y can be manipulated been invest igated.

within a wide range by varying the amount of cold work and the aging treatment.

Polygonization p r i o r t o aging results in lower maxi” hardness and more rapid

kinet ics .

temperature has no effect on r e su l t an t hardness between -6OOC and 100°C.

s t ep aging of TS6 produced lower peak hardness and accelerated aging response.

Raising the UTS t o 261 k s i while re ta in ing 55% RA is extremely a t t r a c t i v e and

demonstrates the wide range of property manipulation ava i lab le in t h i s material

as w e l l as the need f o r continued research on the in te rac t ion and optimization

of mechanical working and heat treatment. Mechanical proper t ies are strongly

dependent upon the mode of deformation during cold working--swaging producing

an apparently l a rge r amount of redundant work than drawing--and s l i g h t varia-

t i ons i n a l l o y chemistry.

Extensive t e s t i n g confirms the observation t h a t t he cold reduction

M u l t i -

Stress corrosion tests i n H20-3%NaC1 and CH30H-1%HC1 indicate the thresholds

for suscep t ib i l i t y t o be above 185 k s i and 150 k s i for the respect ive e n v h n -

ments.

treated 6-120VCA.

In both cases TS6 shows superior performance compared with s imi la r ly

A scale-up of ingot melting and processing techniques has been accomplished.

Ingots of TS6 weighing twenty-eight pounds have been prepared using a Cr-Mo

master al loy. A hot forging study has been conducted and a two-inch diameter s A

- 18 -

I L d has been extruded a t 18OOOF. This material has been successful ly press-

swaged t o 20% RA and cold r o l l e d t o 90%; it shows s l i g h t l y more rapid k i n e t i c s

and overages at somewhat lower hardness l e v e l s than previous one-pound heats.

In view of t he d i f f i c u l t y of loca t ing a sa t i s f ac to ry processor t o accomplish

the 95% cold reduction, separate i n d u s t r i a l contract ing t o develop the capacity

f o r heavy cold deformation of la rge sect ion sizes is recommended i f large-scale

t appl icat ions are t o be -g

ACKNOWLEDGMENT

The authors g ra t e fu l ly acknowledge t h e use of equipment and technica l

ass i s tance provided by the Wyman-Gordon Co. through Messrs. J. Coyne and

W. Couts and the Bethlehem Steel Co. through D r . G . Miller and Messrs. C. Foster

and R. Peeling during t h e scale-up program.

.

- 1 9 -

BIBLIOGRAPHY

1. J. D. Embury, R . M . F i s h e r : Acta Met., 1966, vol. 1 4 , p. 147.

2. J. D. Embury, A . S. Keh, R. M . Fisher: Trans. TMS-AIME, 1966, vol. 236,

p . 1252.

3. R . M . Lerinman, K. I . Khvostyntsev, et a l . : Fiz. Metl. Metalloved., 1966,

vol. 22 , p. 591.

4. R. M. Lerinman, G. V . Murzayeva: Fiz. Metal. Metalloved., 1968, vol. 25,

p. 924.

5. D. H. Avery, J. Greene, R. Wallace: Thermomechanical Processing B

Titanium. Army Materials and Mechanics Research Center, AMMRC CTR 72-2,

February 1972.

6. A , Ball, F. P. Bullen, F. Henderson, H. L. Wain: Phi l . Mag. 1970, vol. 21 ,

p . 701.

7. P. W. Bridgman: Trans. ASM, 1944, vol. 32, p. 553.

8. L. A. Rosales, A. W. Sommer, e t a l . i n Titanium Science and Technology.

New York: Plenum, 1973, vol, 3, p. 1813.

9. E. L. Harmon, A. L. Woiano: Trans. ASM, 1971, vol. 53, p. 43.

1 0 . V. C . Peterson, F. H. Froes, R. F. Malone in Titanium Science and

Technology. New York: Plenum, 1973, vol. 3, p. 1969.

m :rr/ z

a LT

I I 1 I I I I I I I 1 I I I I I I l l I I I 1 I I l l

A I N l T l A L S O L N . TREAT.

V NO. S.T.

0 HEAT N0.I 0 HEAT N0.2 BOTH HEATS SOLN. TREAT. AT 85OOC R O L L E D AT ROOM TEMP. TO A TOTAL 8 0 % AT 10-20% PER PASS.

3 7 5 O C AGE 425°C ---

- V A

A V

H E A T NO. 5 R O L L E D 90% at RT, AGED a t 4 2 5 ° C

I

hJ 0

I

- 21 -

GRAIN S I Z E

do CELL S I Z E

GRAIN S I Z E A S A FUNCTION OF COLD DEFORMATION

\ .-.

\ \

'\

\ \ \ \ -- -c

0 100 % c w

FIGURE 2, GRAIN SIZE AND SUBGRAIN SIZE AS A FUNCTION OF

SUBCELL SIZE AT 0% cou) WORK, THE SOLID PERCENT COLD WORK, I S THE HYPOTHETICAL

LINES INDICATE THE REGIONS OVER WHICH EACH STRUCTURE I S WMINANT,

- 22 -

200 -

FIGURE 3, a: .2% Y I E L D STRESS OF U N A G E D S W A G E D M A T E R I A L FROM HEAT tj3 A S A F U N C T I O N OF T H E AMOUNT O F C O L D W O R K PRIOR TO TESTING.

100

0 -$ ( I -CW) O/o c w

-

I I

b: AVERAGE F R A C T U R E S T R E N G T H OF DRAWN AND AGED M A T E R I A L FROM HEAT #2.

I .o I

1.5 2.0 I I

3 2 0

30 0

2 8 0 w r n Y

5 260 v) I- 3

24 0

220

TENSILE PROPERTIES OF SWAGED AND DRAWN TS 6

I I I I I ' I " [ I I

0 1 5 IO 30 HRS. AGED A T 425°C

FIGURE 4, TENSILE PROPERTIES OF MGED AND DRAW TS6,

- 60

- 5 0

- 4 0

Q cr .30 s

20

IO

0

I

h) 0

I

- 24 -

TENSILE AXIS

FIGURE 5, LONGITUDINAL SECTION OF NECKED SPECIMEN, SWGED %,E wcw AGED 6 m, 8 425°C MAG: 2Ooox

UTS = 328 KSI

PRIOR TO FRACTURE, SPHERICAL VOIDS ARE GENERAL TWOUGHOUT CENTER SECTION,

- 25 -

FIGURE 6, WOLF'S EAR FRACTURE SURFACE

AGED 12 HR, 8 425°C

CARBON REPLICA: 72OOX

SWAGED 96% RAcw

as = 320 KSI

v, 55 v) W z a: n

a I 5 0 0

1 1 LLI

x 0 0 a:

3 45

40

POLYGONIZED STRUCTURE. HEAT N 0 . 2 , SOLN. TREATED A T 8 5 O O C FOR 2 5 MIN.,WORKED AT ROOM TEMPERATURE TO 6 5 % , POLYGONIZED AT 7OOOC FOR 2 0 MIN., R O L L E D TO 40%, AGED A T 375OC.

---

- - HEAT NO.1, SOLN. TREATED A T 8 5 O o C , ROLLED TO 40°/o,AGED

AT 375OC.

/ /

0 0

/ 0

/ 0

0 /

- 0

1 I I I I I I I I I I I I I I l l f I I I I l l 1

0 1 IO IO0 1000

FIGURE 7 , C~MPARISON OF THE EFFECTS OF THERMOMECHANICAL PROCESSING ON POLYGONIZED AND

A G I N G TIME (Hrs.)

SOLUTION TREATED Ts6,

60

: 55 W Z

CL

1 0

-I -I W

n

a 50

3 0 45 0 Lx

(

40

HEAT NO. I , SOLUTION TREATED IN THE a + ,6 FIELD AT 750° C, C O L

HEAT NO.1, SOLUTION TREATED IN THE A L L + F I E L D A T 8 5 O o C y COLE R O L L E D TO 60% A T R O O M T E M P E R A T U R E y AGED A T 3 7 5 O C .

--- ROLLED T O 50% AT ROOM TEMPERATURE, AGED A T 3 7 5 O C .

-

/ /

I I I I I I I I I I I I I I I I l l I

0 1 10 100 A G I N G TIME (Hrs.)

A L 1000

FIGURE 8, h A R I S O N OF THE EFFECTS OF THEMECHANICAL PROCESSING ON a -t 8 AND 8 Ts6,

60

cn 55 tn w z 13 c 2 50 V -.l -.l w

$45 V 0 E I

40

--O-- H E A T N O . I , A S - R E C E I V E 0 , R O L L E D A T R O O M T E M P . ( I 5 - 2 O 0 / o / P A S S ) T O 8 6 % , A G E D A T 371

--a- H E AT N 0. I , A S .- R EC E I V E 0, R O L L E 0 AT-I 0 5 O F ( I 0 - 2 0 O h / P A S S I T O e 4 O/o, A G E D A T 375 O C

--O-- H E A T NO. 2 , S 0 L N . T R E A T . 8 5 O 0 C R O L L E D A T R O O M T E M P . ( 2 ° / o / P A S S ) T 0 8 O o / o , A G E D A T 37

*-s-* H E A T N O . 2 , S O L N . T R E A T . 8 5 O 0 C R O L L E D A T - 1 0 5 ° F ( 5 0 / o / P A S S ) To B O % , A G E D A T 3 7 5 O C

’P /

I I I I I I I I I I I I I I 1 I l l I I I I I I I

0 1 I O 100 1000 A G I N G T I M E ( H r s . )

FIGURE 9, CQMPARISON OF THE EFFECTS OF mu) REDUCTION TEMPERATURE ON THE RESPONSE TO THERMOMECHAN ICAL PROCESS I NG 8

6 0

5 5 u) W z c1 [r a 50

0 -I -I W 3 Y 45 0 0 rK

40

HEAT NO. 2, SOLN. TREATED AT 850°C, ROLLED A T ROOM TEMPERATURE - - --- TO 8O%, AGED 24HRS. AT 375"C, THEN CUMULATIVELY AGED AT 425°C (DUPLEX AGED).

HEAT NO. I , SOLN. TREATED AT 8 5 O o C , ROLLED AT ROOM TEMPER- - - ATURE TO 8O%, AGED A T 375OC.

HEAT NO. 2 , SOLN. TREATED AT 850°C,ROLLED ------- --- AT ROOM TEMPERATURE T O 80%,AGED AT 375°C.

I I I I I I I I I I I I 1 1 1 1 I 1 I I I I I I

0 1 IO 100 A G I N G TIME (Hrs.)

1000

FIGURE 10, GWARISION OF ME EFFECTS OF SINGLE AND TWD STEP AGING TREATMENTS,

KSI ir W m E200 LT W t- 3 0 + 180 4 v, cn W fx m

160

- -- 8 8 Y o R E D . O V I 1 1 I I I I

16 1

STRESS CORROSION IN T S 6

CH,OH-HCL 4 POINT BENDING: HR. AGED AT 425°C

70YoRED. 2 0- 12 a--

97% RED. 0 o---

- - 24

2 0 - 8 16

-

\ - \

% m

'\*,

A\,& '

- idby\ A* -\ -- . - Q \ U N I A X I A L TENSION: -

SWAGED 96.2% R A 0 A- 12 A----

tv 120 V C A 4 POINT BENDING:

0 20 40 6 0 80 100 TIME TO FAILURE ( m i d

FIGURE ll, STRESS CORROSION PROPERTIES OF TS6 IN CtkjlH-HCL,

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A i r Force Mater ia l s Laboratory, Wright-Patterson A i r Force Base, Ohio 45433 ATTN: AFML (LAE), E. Morrissey

AFML (LC) AFML (LMD) , D. M. Forney

National Aeronautics and Space Administration, Washington, D . C . 20546 ATTN: Mr. B. G. Achhammer

M r . G. C . Deutsch - Code R R - 1

National Aeronautics and Space Administration, Marshall Space F l i g h t Center, Hun t sv i l l e , Alabama 35812 ATTN: R - P & E - M , R. J. Schwinghamer

SEE-ME-W, Mr. W.' A . Wilson, Building 4720

Panametrics, 221 Crescent Street , Waltham, Massachusetts 02154 ATTN: M r . K . A. Fowler

Lockheed-Georgia Company, Mar ie t ta , Georgia 30060 ATTN: Advanced Composites Information Center, Dept. 72-14 - Zone 402

Direc tor , Army Mater ia l s and Mechanics Research Center, Watertown, Massachusetts 02172 ATTN: AMXMR-PL

AMXMR- PR MXMR-CT AMXMR-E AMXMR - E?! AMXMR-AP

Unclas s i f i ed

6 R E P O R T D A T E

September 1973 ba CL?%&Y- 31 -"c"-*ijj 6Np

7 b NO O r R E F S 7C. T O T A L NO O F P A C E D

30 10 9.. 0 R l G l N A T O R . S R F P O R T N U M E I t S l

b. PROJECT N O .

lW564603D385 e. AMCMS Code 554C.12.62000 I

1 1 . SUPPLEMENTARY NOTES

AMMRC CTR 73-30

kb. OTnER REPORT NO151 (Any 0lh.r nurnbr. l b ; umy be # 8 E { # ~ . d thls roporf)

12. SPONSORING MILITARY ACTIVITY

Army Materials and Mechanics Research Cent Watertown, Massachusetts 02172

d. I 10. OISTRIBUTION OTATEYENT

Unclassified Security Classification

q 4 I E I 1 0 1 0 )

Titanium Alloys

Thermomechanical Processing

Yechanical Properties

3eformation

>

Security Classification

q 4 I E I 1 0 1 0 )

Titanium Alloys

Thermomechanical Processing

Yechanical Properties

3eformation

L I N K A

R O L E - W T

L I N K B

R O L E - w r - L I N K c

R O L E - W T -

.

Unclassified Sccuritv CImamlficmtlon