thermal sensitivity

8/21/2019 Thermal sensitivity

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Microstructure-Properties: I Sensitivity of Properties to

Thermal History?

27-301

Lecture 2, August 28th

Fall, 2002

Prof. A. D. Rollett


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Materials Tetrahedron

Microstructure Properties

ProcessingPerformance


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Objective

•The objective of this lecture is to define amaterial property and explain how materialproperties are dependent on microstructure.A brief introduction is given to the predictionof microstructure development based onphase relationships.

•Part of the motivation for this lecture is to

prepare the class for a Lab on the sensitivityof mechanical properties to microstructure.


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ExampleStrength

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References

•Materials Principles & Practice, ButterworthHeinemann, Edited by C. Newey & G. Weaver.

•Mechanical Metallurgy, McGrawHill, G.E. Dieter, 3rdEd.

•Hull, D. and D. J. Bacon (1984). Introduction toDislocations. Oxford, UK, Pergamon.

•Courtney, T. H. (2000). Mechanical Behavior ofMaterials. Boston, McGraw-Hill.


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ExampleStrength

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Martensite

What is a Material Property?

•AMaterial Property is some quantifiable behavior ofa material.

•For a property to be amaterial property, it shouldbe a characteristic of the material, not the

configuration in which it is used.•Example: theload carrying capacityof a beamdepends on the cross-section of the beam,therefore isnota material property.

•The yield strengthis a material property because itis the same no matter how the material is tested.


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ExampleStrength

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Martensite

Properties & Microstructure

•Why are [some] properties dependent on microstructure?•Many properties are controlled by the propagation ofdefectswithin the material.

•The defect propagation is an example of a mechanismthat controls the property.

•Example: yield strengthmeasures the resistance to plasticflow, which is controlled by the mechanism of dislocationmotion.Dislocationsare line defects whose motion ismore sensitive to precipitates, grain boundaries etc. than

to the lattice. The latter constitutesmicrostructure, aspreviously discussed.


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ExampleStrength

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Martensite

Phase Relationships

•Physical metallurgy was built on the exploitation ofphase relationships in order to optimize acombination of properties.

•Although the emphasis has tended historically to be

on mechanical properties, other properties candominate the design requirements. Oxidationresistance is often a limiting factor in manyapplications.

•In this course, we will explore several differentproperties such as strength, fracture toughness,thermal conductivity and electrical conductivity, andtheir sensitivity to microstructure.


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ExampleStrength

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Thermal History

• The basic idea behind annealing is to take advantage ofvarying solubilities of dissolved elements and the tendency of

super-saturated solutions (at low temperatures) to precipitateas a finely dispersed second phase.

• Typical recipe:

–1. Anneal at high temperature to dissolve a solute;–2. Suddenly decrease the temperature (quench) to maintain thesolid solution;

–3. Re-heat the material to a temperature at which the material isa 2-phase mixture (at equilibrium) such that precipitation of the

solute occurs.• Why?! Most materials are too soft (metals) or too brittle(ceramics) in the pure, single phase condition. Precipitation

develops useful properties.


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ExampleStrength

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Martensite

Thermal History: Example

•To illustrate the principles involved in heat treatment,and the great variety of microstructures that can beproduced, we will perform experiments on a steel.

•Carbon can be readily dissolved at high

temperatures where the iron is present in the fccform (austenite). Lowering the temperature changesthe iron to the bcc form with far lower solubility ofcarbon.

•The combination of phase transformation andprecipitation of carbon provides many alternateapproaches to heat treatment.


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ExampleStrength

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Microstructure of medium-C steel

Given a steel with 0.8% C (by weight), agreat variety of microstructures can beproduced.

History Strength Ductility Hardness (MPa) (% elongation) Hv

1) Slow cooled (normalized) 930 15 250

2) Quenched from 1030K (water) nil 0 800

3) Quenched from 1030K, tempered (annealed) 820K 1160 25 350

4) Quenched from 1030K

tempered at 970K 570 40 170

Matls. Princ. & Practice, fig. 5.2


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ExampleStrength

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Hardness

• Hardness is closely related to strength: we use it because it providesa quick and convenient estimate of strength.

• Hardness is measured by forcing a small object with a well definedshape (pyramid or sphere) under a constant load into the surface ofthe material to be tested (for a fixed amount of time).

• The size of the indent is measured

after the indentation is complete:smaller indents correspond to higherhardnesses.

• Softer materials require lower loadsand larger indentors andvice versa for stronger materials.

• The thickness of the material tested must be several multiples of theindent size in order to avoid a variation with thickness.


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ExampleStrength

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Martensite

Hardness Test Methods


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Correlations between methods

• Deformation during a hardnesstest is confined to the regionaround the indent. Strains arehigh enough (as high as 20%)

that significant work hardeningcan occur. The size of the

indent is therefore dependenton the shape of the indentorand the work hardeningcharacteristics of the material.

• Conversion charts arenecessary.

• Reference: D. Tabor,Hardnessof Metals, Oxford, 1951.


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Hardness: summary

• Hardness testing is admittedly crude as a probe of materialproperties because it integrates the stress-strain response over asignificant range of (plastic) strain.

• Arule-of-thumb is that the hardness of a material is approximately 3times its yield strength (when measured in the same units).

• Hardness is extremely useful because it can be performed on almostall materials. This is the case because there is a net hydrostaticcompression under the indentor which avoids brittle fracture (in all

but the most brittle materials).

• Hardness can be measured over a wide range of temperature so itcan give a measure of creep resistance.

• The technology of scanning probe microscopy has been adapted tohardness measurement, leading to the widespread use ofnanohardness testing.


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Medium-C steel, contd.

•In order to explain the variations in microstructure andconsequently the properties, it is necessary to considerthe history of the material in each in relation to its phaserelationships, which in this case means the Fe-C diagram.

•The large range of properties observed for steels is

because several mechanisms exist for the decompositionof austenite (fcc iron) into ferrite (bcc iron) and ironcarbide, Fe3C.

•In the second course (27-302) we will examine the factorsthat control the response of materials to thermo-mechanical processing and the development ofmicrostructure, especially multi-phase structures.


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Fe-Fe3C phase diagram

Case 1: slow cool


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Medium-C steel: case 1

•For case 1, slow cooling (normalizing) allows the material toapproach equilibrium, i.e. a piece of iron and a piece of ironcarbide (=cementite), in contact, with compositionscorresponding to the ends of the tie line (at roomtemperature).

•The actual structure is pearlitic, i.e. cooperative growth offerrite and cementite in a lamellar form. 0.8w/o happens to bethe eutectoid composition, so no other phase is nucleated.

•The strength and ductility are both moderate. The finelamellar structure resists dislocation motion quite effectively

but ductility is intermediate between ferrite (high) andcementite (low).


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Case 2: rapid cool

Martensite formation temperature


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• For case 2, rapid cooling by quenching into water means thatthe previous process of diffusional decomposition into ferrite

and cementite does not have time occur. Instead, anothermechanism intervenes and the austenite decomposes to

martensite.

• Martensite has the same composition as the austenite.Therefore its formation doesnotdepend on diffusion and thephase transformation is sometimes referred to as amilitary

one. The significant change in shape yields a distinctiveappearance with laths or lamellae.

• The high hardness and nil ductility is a consequence of thehighly strained nature of the (tetragonal) martensite: it is very

difficult to move dislocations through the new phase.

Therefore cracks propagate before plastic deformation canoccur.


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Martensite


Case 3: rapid cool + re-heat to 820K



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•For case 3, rapid cooling (quenching) followed by re-heating leads to first martensitic decomposition of theaustenite, followed by decomposition of the martensite tocementite.

•The second heating is known astemperingbecause it

alleviates the extreme character of the martensite. Thecementite is finely dispersed (to be discussed later).

•The strength is very high because a very fine cementite isformed with high resistance to dislocation flow. Ductility ismoderate and higher than for pearlite because the finelydispersed cementite does not initiate fracture as easily.


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Case 4: rapid cool + re-heat to 970K



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• For case 4, rapid cooling (quenching) followed by re-heatingleads to first martensitic decomposition of the austenite,

followed by decomposition of the martensite to cementite.

• The second heating is known astemperingbecause italleviates the extreme character of the martensite. The

cementite is less finely dispersed than in case 3 because theannealing/tempering temperature is very close to the eutectoidtemperature where diffusion of carbon takes place rapidly.

• The strength is low because the cementite has coarsened andresistance to dislocation flow is low. Ductility is high because

the well dispersed but coarse cementite does not initiatefracture easily, compared to pearlite.


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Issues, new ideas, so far

•The following new ideas or concepts have beenintroduced.

1.Strength

2.Hardness

3.Ductility4. Martensite, militarynon-diffusional transformations

5. The Fe-C phase diagram (not completely new)

6.Diffusional transformations, decomposition

7. Pearlite8.Tempering

Properties

Processes


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Strength

• Strength is very basic to the value of astructuralmaterial. Wemeasure it in terms of force per unit area:σ = F/A

• Strength means resistance to irreversible deformation or, if youprefer, the upper limit of elastic stress that is safe to apply to amaterial.

•Strength is highly dependent on microstructure because it isproportional to the difficulty of moving dislocations through thecrystal lattice.

• Engineers are often taught strength as being related to (chemical)composition. Materials engineers studystrengtheningmechanisms and therefore understand how to control strength.

• Strength is typically measured in a tension test, but we willexamine this test when we discussductility.


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Types of Strength

•Later in the course, we will study stress and strength astensor quantities. For now, we will treat them as scalarquantities, i.e. a single number.

•There are differentmodes of loading materials:

–Yield Strength: ambient conditions, low strain rate

–Dynamic Strength: ambient conditions, high strain rate–Creep Strength: high temperature strength, low strain rate

–Torsion Strength: strength in twisting

–Fatigue Strength: alternating stresses

•The strength value is highly dependent on the loading mode.

•Each type of strength is controlled by a variety ofstrengthening mechanisms.


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Yield strength

•Ayield strength is boundary between elastic andplastic flow.

σ=0 σelastic plastic

Example: tensile stress

σ= σyield


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Microstructure Development

•In the example that we picked, the great variety ofmicrostructures (which you will see in the Lab) werederived from a variety of different thermal histories.

•A basic skill expected of materials scientists/engineers isthat they be able to relate the microstructure observed to

the history of the material and its phase relationships.•Although we will work with equilibrium (and near-

equilibrium) phase diagrams, non-equilibrium behavior isincreasingly important.

•In what follows, very brief summaries of the characteristicsof the different types of phase transformations are given.


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Diffusional Phase Transformations

•Solid state phase transformations are as critical assolidification in determining microstructure.

•Most useful transformations require diffusion tooccur. Why?

•Consider the example of the decomposition ofaustenite to form ferrite and cementite: a high-carbon solid solution of C inγ -Fe must decomposeto form a low-C solution inα-Fe and a carbide.

•Carbon must therefore diffuse so as to partition between the product phases.


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Example

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Martensite

Pearlite

•In our example of pearlite formation, cooperativegrowth of ferrite and cementite lamellae provides anefficient mechanism for the redistribution of the solute(carbon), i.e. a short diffusion path.

•The properties of the pearlite include moderately high

strength because the cementite lamellae are effectivebarriers to dislocation motion.

•Ductility is only moderate despite good work hardening

because the yield strength is high and so not much

strain is required before the necking limit is reached.


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Military/displacive transformations

• If a phase transformation can be accomplished with only localrearrangements of the atoms (and no long range diffusion)

then it is known as a displacive, military, or martensitictransformation*.

• In most cases, the transformation is from a high temperature,

higher symmetry crystal structure to a low temperature, lowersymmetry structure.

• Typically, high cooling rates are required in order to avoiddiffusional transformations: as a result, large undercoolings(thermodynamic driving forces) are obtained and the

transformations proceed extremely rapidly.

*Massive transformations are also characterized by lack of long range

diffusion but the lack of coordinated atom movements at the interface

means that they are classified as diffusionless civilian transformations.


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Fe-C Martensite properties

•In the classical example of the Fe-C martensite, the lowtemperature form is tetragonal because the carbonatom moves to a particular tetrahedral interstice in what

would otherwise be a bcc structure.

•This martensite exhibits very high resistance to

dislocation motion.

•Martensitically transformed steels are very hard andhave very low resistance to fracture, i.e. they are brittle.

•The transformation is accompanied by large shear

strains and the crystallography has been a fascinatingtopic of study for many years.


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Tempering

•If a martensitically transformed steel is re-heated to atemperature that is high up in the ferrite+cementiteregion, further changes in microstructure occur.

•Separation of the high-carbon martensite intoferrite+cementite lowers the free energy of the system.

•The high density of interfaces provides a high density of

nucleation sites for the cementite.

•Therefore the cementite grows as particles, either laths

(low temperature) or spheroids (high temperatures).


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Tempering temperature

•The details of the tempering process are complex(see p417 et seq. in P&E).

•In essence, as the cementite grows and depletes thematrix of carbon, so the strength decreases and the

ductility rises.•For certain histories, significantly better combinationsof strength and ductility can be obtained than in thepearlite structure. This processing route is, however,more expensive than normalizing, i.e. slow cooling

from the austenite range.


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Summary

•The concept ofmaterial propertyhas beenexplored.

•An illustration of the dependence of structuralproperties on microstructure has been given.

•Basic mechanical properties have been defined andillustrated with respect to practical methods formeasurement.

•The importance of phase relationships and phase

transformations for microstructural development hasbeen illustrated in the Fe-C system.

thermal sensitivity

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