composites with metallic matrices

7/31/2019 Composites With Metallic Matrices

1/24

Composite with Metallic

MatricesBy

Dr. H Suresh Hebbar

Asst. Professor

Department of Mechanical EngineeringNITK, Surathkal


2/24

Considerable research into boron fibre reinforcedalloys was carried out in the USA in the early 1970s,

leading to aerospace applications in the Space shuttleand in military aircraft. Problems of chemical reactionbetween boron fibres and the matrix at temperatureabove 600c restricted fabrication techniques todiffusion bonding of plasma-sprayed thin sheets. Morerecently the development of coatings to prevent fibredegradation and of more inert fibres like silicon carbide

and alumina have enabled liquid metal processingroutes to be developed. Nevertheless, most metalmatrix composites (MMCs) are still in the developmentstage or early stage of commercial production.


3/24

In comparison with most polymer matrix

composites, MMCs have certain superiormechanical properties, namely higher transversestrength and stiffness, greater shear andcompressive strengths, and better high

temperature capabilities. There are alsoadvantages in some of the physical attributes ofMMCs such as no significant moistureabsorption properties, non-inflammability, highelectrical and thermal conductivities, andresistance to most radiations.


4/24


5/24

Solid State Processing

Solid state processing involves bringing theparticles or foil into close contact with thereinforcement whence, on the application of asuitable combination of temperature and pressure,

the free energy of the system is reduced by thematrix consolidating to give lower energy solid-solid interfaces. The actual mechanism by whichmaterial transport takes place during consolidation

can differ but it invariably involves diffusion.Methods using foil are usually called diffusionbonding whereas those using particles tend to bereferred to as powder metallurgy.


6/24

Fig. 1 Diffusion bonding (a) starting components: fibre mat and sheetsof foil; form ply (sometimes the ply is consolidated); ( c) plies are

stacked; (d) hot press; (e) and (f) finishing.


7/24

Composites with up to 50% reinforcement can be producedby powder metallurgy but most manufacturers limit thereinforcement to a maximum about 25% because ofdamage to fibres during processing and loss toughness at

high volume fractions. In the case of discontinuous fibresand whiskers some alignment of their axes perpendicular tothe applied takes place during pressing. This leads toanisotropic behaviour improved longitudinal properties; SiC

whisker reinforced aluminium haves in this way.

Powder Metallurgy


8/24

Liquid Processing

Work in recent years has concentrated on the production ofmetal matrix composites by adapting conventional castingtechniques.

The major barriers to this type of process are non-wetting ofthe reinforcement and adverse matrix-reinforcement

reactions due to the high temperatures involved.Various approaches are being pursued to overcome theseproblems, the most promising being precoating thereinforcement with an appropriate material to protect

against any reactions and to enhance wetting.For example, specially graded pyrolitic graphite coatingshave been developed for use on SiC fibres in an attempt toenhance wettability at the expense of some small loss in

mechanical properties.


9/24

Alternatively the matrix composition may be modified to aidprocessing. It has been reported that it is possible to

produce aluminium castings reinforced with FP Alumina fibreby addition of lithium to the aluminium melt. This facilitateswetting by the formation of an Li2O.5Al2O3 spinel at the fibreinterface, with no apparent degradation of composite

properties. However there are still major problems withsome metals; at the present, liquid state processing is notused for titanium and its alloys because of their highreactivity.


10/24

The simplest liquid state technique, referred to as meltstirring, is to mix the particulate, whisker or discontinuousfibre reinforcement with the molten metal and cast in theconventional manner. Even with stirring, uniform mixing isdifficult to achieve because of differences in density betweenthe molten matrix and reinforcement, although mixing isimproved by allowing the melt to cool to a more viscous two-phase solid-liquid state for stirring.

This modification of the melt stirring technique is known ascompo casting or rheocasting. (To confirm that it is easier tomix particles uniformly into a fluid the more viscous the fluidtry mixing sand (denser than water) or grass seeds (lighterthan water) into water and then into more viscous custard!)There is a limit of approximately 20 vol.% since effectivedispersion of the reinforcement becomes difficult above this

level.

compo casting:


11/24

The melt stirring process is limited to conventional castingalloys and the use of rheocasting is even more restricted tothose alloys that have a wide solidification range over which

the 'mushy' solid-liquid state exists.

Even when the process is carried out under pressure thepre-form cannot be too densely packed or infilitration isincomplete; consequently there is an upper limit of about 30

vol. % reinforcement. The pressure has to be chosen sothat it is sufficient for infiltration but not so high that itdamages the fibres or distorts the preform.

The pressure is applied mechanically in the techniqueknown as squeeze casting, which is suitable for theproduction of small net shaped components.

The pressure (70-100 MPa for aluminium alloys) is applied

by means of a ram and forces the metal into the preform.


12/24

Fig 2. Squeeze casting (a) insert preform into die cavity; (b) meter in aprecise quantity of alloy; (c) close die and apply pressure; (d) remove ram;

(e) extract component.


13/24

Applied pressure to the melt effectively removes problemof size.

Major advantage of using a gas is that it results in a morerapid process and therefore any fibre capable ofwithstanding contact with molten metal for relatively shortperiods of time may be used.

Application of pressure by gas rather than by mechanicalprocesses also reduces the extent of fibre breakage andmisalignment.

Use of applied gas pressure


14/24

Fig. 3 Liquid melt infiltration under gas pressure; (a) insert preform and

close die; (b) evacuate air (c) apply gas pressure and maintain duringsolidification.


15/24

Deposition process has considerable potential is sprayco-deposition which is a modification of the Osprey

deposition process.

It involves atomizing a melt and introducing thereinforcement particles into the spray affine metaldroplets

The metal and the reinforcement particles are co-deposited on to a substrate.

The atomised metal exists as discrete droplets for short

times, of the order of a few milliseconds, and the rapidsolidification leads to a matrix with a fine microstructureand reduces the possibility of extensive chemicalreaction.

Deposition process


16/24

Fig. 4 Diagram of spray-co-deposition production of SiC particulate

reinforced metal. (Source: Wills, 1988)


17/24

Control of atomization and of the particle feed enablesan MMC with a uniform distribution of particles and ofacceptable density (typically >95% theoretical density)to be produced at a reasonable rate.

This technique has been mainly used for SiC particulatereinforcement of aluminium alloys.

Some form of secondary processing is usuallyemployed after co-deposition.


18/24

Unidirectional solidification of eutectic alloys can lead to atwo-phase microstructure where one of the phases is in a

lamellar or rod-like morphology and aligned approximatelyparallel to the direction of heat flow

Unidirectional solidification is generally achieved by meansof induction heating; the induction coil is moved up a bar of

the eutectic alloy at a controlled rate.The parameters which are most important in determining the

microstructure are:

the thermal gradient G at the liquid-solid interface

the growth rate RG,

the velocity at which the solid-liquid interface advances up

the bar.

In situ process


19/24

Fig. 5 Production of in situ composite by unidirectional solidification: (a)eutectic phase diagram; (b) diagram showing induction heating used forunidirectional solidification; (c) alignment of the two-phasemicrostructure; (d) scanning electron micrograph of g(Ni3A1)-(Mo)composite. (Source: Funk and Blank, 1988).

M l ifil d


20/24

The superconducting constituent of these composites is theintermetallic compound Nb

3Sn, or occasionally V

3Ga. For

an intermetallic superconductor to be commercially viable itmust be possible to produce it in long lengths with uniformproperties and, for complex reasons associated withsupercondueting performance, it must be in the form of

filaments or a thin tape.

Multifilamentary superconductors

Nb3Sn is extremely brittle and cannot therefore be produced by conventionalmetal working processes, such as extrusion and wire drawing.


21/24

This process is known as the bronze route.a) The first stage in the process is to drill holes in a block of

bronze and insert rods of niobium.b) The bronze block with the embedded niobium rods isswaged down to reduce the cross-section of the niobium.

c) Sectioning of the swaged composite and rebundling are

carried out to increase the number of niobium filaments inthe cross-section. The rebundled composite is placedinside a copper can whose inner surface has a thincoating of tantalum or niobium. The presence of thecopper in the final product increases the stability of thesuperconductor by minimizing local temperature rises.The canned, rebundIed composite is drawn down to givea final niobium filament size of a few microns


22/24

d)The composite wire is then heat treated in thetemperature range 600-800 C to allow diffusion of tinfrom the bronze into the niobium, and hence to formNb3Sn by a solid state reaction.

The niobium is not completely reacted and typically there

will be a layer of Nb3Sn of less than 2 m thick on a coreof unreacted niobium.

The exact thickness will depend on the extent ofinterdiffusion and is therefore determined by thetemperature (through its effect on Dd) and time of heattreatment in accordance with equation -------.


23/24

Fig. 6 Schematic diagram of the production of multifilamentarysuperconducting composite by the bronze route: (a) holes drilled inbronze block and niobium rods inserted; (b) swaging; (c) sectioning,rebundling, canning and final reduction; (d) heat treatement.


24/24

The niobium or tantalum diffusion barrier prevents the tin

from the bronze entering the copper.

The Nb3Sn has the A15 crystal structure and the layer is

fine grained (grain size of less than 0.2 m.

The superconducting properties are a function of the layer

thickness and the grain size.

The major use of multifilamentary composites is as

windings for superconducting magnets. The mechanical

properties are important as the composite is stressed

during assembly of the magnet and during service.

composites with metallic matrices

Documents