© 2016 Cengage Learning Engineering. All Rights Reserved.

Chapter Learning Objectives Develop the expression for total free energy change during a solid-state phase transformation. Describe the kinetics of nucleation and growth during a solid-state phase transformation. Design an age-hardening treatment for an appropriate alloy. Draw the phase diagram of a low-carbon steel between pure iron & cementite. Predict mechanical properties of hypoeutectoid, eutectoid & hypereutectoid steels. © 2016 Cengage Learning Engineering. All Rights Reserved.

Chapter Learning Objectives Explain how martensite is produced and describe its structure & properties. Explain the relationships between processing, composition, quench rates & annealing to control a material’s microstructure. © 2016 Cengage Learning Engineering. All Rights Reserved.

Chapter Outline Sections 12-1 Nucleation & Growth in Solid-State Reactions 12-2 Alloys Strengthened by Exceeding Solubility Limit 12-3 Age/Precipitation Hardening & Applications 12-4 Microstructural Evolution in Age Hardening 12-5 Effects of Aging Temperature & Time 12-6 Requirements for Age Hardening 12-7 Use of Age-Hardenable Alloys at High Temperatures © 2016 Cengage Learning Engineering. All Rights Reserved.

Chapter Outline Sections 12-8 The Eutectoid Reaction 12-9 Controlling the Eutectoid Reaction 12-10 The Martensitic Reaction & Tempering 12-11 The Shape-Memory Alloys (SMAs) © 2016 Cengage Learning Engineering. All Rights Reserved.

12-1 Nucleation & Growth in Solid-State Reactions Heterogeneous nucleation of a precipitate occurs most easily on surfaces already present in the structure, i.e. grain boundaries and defects Growth: Precipitate growth occurs mainly by long-range diffusion and redistribution of atoms In most cases, the controlling factor is the diffusion step © 2016 Cengage Learning Engineering. All Rights Reserved.

12-1 Nucleation & Growth in Solid-State Reactions Kinetics: Overall rate (‘kinetics’) of a transformation depends on both nucleation & growth Transformation time depends on initial concentration & temperature Avrami equation relates the transformed fraction to time Effect of Temperature: Rate of phase transformation often depends on the undercooling. Temperature increase has opposite effects on growth rate and nucleation rates, both of which influence overall rate of transformation © 2016 Cengage Learning Engineering. All Rights Reserved.

12-1 Nucleation & Growth in Solid-State Reactions © 2016 Cengage Learning Engineering. All Rights Reserved.

12-2 Alloys Strengthened by Exceeding Solubility Limit Alloys can be dispersion-strengthened after the solubility limit is exceeded (Chapter 11) Widmanstätten Structure: The second phase may precipitate out in certain planes & directions parallel to preferred planes & directions. This basket-weave pattern is called the Widmanstätten structure Such structures permit faster growth rates, lower ductility, and good fracture toughness. © 2016 Cengage Learning Engineering. All Rights Reserved.

12-2 Alloys Strengthened by Exceeding Solubility Limit Interfacial Energy Relationships: When precipitates form at an interface/boundary, their shape is influenced by the interfacial energy of the boundary. Precipitate shape is determined by the dihedral angle between matrix-precipitate interfaces © 2016 Cengage Learning Engineering. All Rights Reserved.

12-2 Alloys Strengthened by Exceeding Solubility Limit Coherent Precipitate: In coherent precipitates, the precipitate’s crystal planes are related to/continuous with the crystal planes of the matrix. This helps block more dislocations © 2016 Cengage Learning Engineering. All Rights Reserved.

12-3 Age/Precipitation Hardening & Applications Age/precipitation hardening is produced by a sequence of phase transformations that leads to a uniform dispersion of nanoscale precipitates in a more ductile matrix. It is used to increase yield strengths of many alloys via simple heat treatments. Aluminum, Nickel superalloys, and Titanium alloys are age hardened for use in many applications A disadvantage is that age-hardened alloys can be used over a limited temperature range. At high temperature the precipitates begin to grow and eventually dissolve © 2016 Cengage Learning Engineering. All Rights Reserved.

12-4 Microstructural Evolution in Age Hardening There are 3 steps in the age-hardening heat treatment. © 2016 Cengage Learning Engineering. All Rights Reserved.

12-4 Microstructural Evolution in Age Hardening Step 1: Solution Treatment Alloy is heated above solvus temperature until a homogeneous solid solution is produced, with no phases or microchemical segregation Step 2: Quench Solid solution is rapidly cooled (‘quenched’), so atoms cannot diffuse to nucleation sites. This forms a nonequilibrium super-saturated solid solution Step 3: Age The solution is heated at a temperature below solvus temperature (‘aging temperature’). The second phase precipitates out Eventually a 2 phase equilibrium may be formed © 2016 Cengage Learning Engineering. All Rights Reserved.

12-4 Microstructural Evolution in Age Hardening Nonequilibrium Precipitates during Aging: During aging of many alloys, a continuous series of precursor precipitates forms as intermediates. These nonequilibrium precipitates are coherent precipitates. Alloy strength increases with time as these intermediate coherent precipitates are produced. This is the aged state of the alloy Alloy strength begins to decrease when noncoherent equilibrium precipitate is produced. At this point the alloy is over-aged. © 2016 Cengage Learning Engineering. All Rights Reserved.

12-5 Effects of Aging Temperature & Time Properties of age-hardenable alloys depend both on aging temperature & time. Using lower temperature and higher aging times leads to increased maximum strength, which is also maintained over a longer time. The properties are also more uniform if aging proceeds slowly. Some alloys can age at room temperature (‘natural aging’). It requires long times, but peak strength is higher and there is no over-aging © 2016 Cengage Learning Engineering. All Rights Reserved.

12-5 Effects of Aging Temperature & Time © 2016 Cengage Learning Engineering. All Rights Reserved.

12-6 Requirements for Age Hardening Not all alloys can be age hardened. 4 conditions must be met: Alloy system must have decreasing solubility with decreasing temperature Matrix should be soft and ductile, and precipitate hard and brittle The alloy must be quenchable A coherent precipitate must form Many important alloys meet these conditions, such as those of Ni, Cu, Al, Mg, Fe, Cr and Ti. © 2016 Cengage Learning Engineering. All Rights Reserved.

12-7 Use of Age-Hardenable Alloys at High Temperatures We cannot use age-hardenable alloys at temperatures which enable overaging or at which the second phase redissolves in matrix. There are also issues with welding age-hardened alloys. In the heat affected zone (HAZ), the area closest to the welding beam loses all age hardening, while the areas slightly further away overage, weakening the welded region. © 2016 Cengage Learning Engineering. All Rights Reserved.

12-7 Use of Age-Hardenable Alloys at High Temperatures © 2016 Cengage Learning Engineering. All Rights Reserved.

12-8 The Eutectoid Reaction The eutectoid is a solid-state reaction in which one solid phase transforms into two. One of the most important uses of the eutectoid reaction is to control the microstructure and properties of an alloy. © 2016 Cengage Learning Engineering. All Rights Reserved.

12-8 The Eutectoid Reaction

12-8 The Eutectoid Reaction Solid Solutions: Iron goes through 2 allotropic transformations during heating or cooling. On cooling, it transforms initially into a BCC structure called delta-ferrite, and then into an FCC structure called austenite. On further cooling, it reverts to the BCC structure again, now called alpha-ferrite or just ferrite. Both ferrites and austenite are solid solutions of interstitial carbon in iron. Compounds: A stoichiometric compound Fe3C or cementite is formed when solubility of carbon in iron is exceeded. It is extremely hard & brittle, and present in all steels. © 2016 Cengage Learning Engineering. All Rights Reserved.

12-8 The Eutectoid Reaction Heating an alloy containing eutectoid composition of 0.77% C above 727o Celsius produces a structure containing only austenite grains. When austenite cools to 727o Celsius, the eutectoid reaction begins, and 2 phases form. The ferrite contains 0.0218% C and the cementite 6.67% C. Pearlite: The lamellar structure of ferrite and cementite formed in the iron-carbon system is called pearlite. Pearlite is a microconstituent and provides dispersion strengthening. © 2016 Cengage Learning Engineering. All Rights Reserved.

12-8 The Eutectoid Reaction Primary Microconstituents: Hypoeutectoid steels contain less than 0.77% C, and hypereutectoid steels above 0.77% C. In hypoeutectoid steels, ferrite is the primary microconstituent, whereas in hypereutectoid it’s cementite. © 2016 Cengage Learning Engineering. All Rights Reserved.

12-8 The Eutectoid Reaction © 2016 Cengage Learning Engineering. All Rights Reserved.

12-9 Controlling the Eutectoid Reaction Controlling the Eutectoid Amount: Increasing the carbon content of the steel increases cementite & pearlite, hence increasing the strength until a certain peak is reached. © 2016 Cengage Learning Engineering. All Rights Reserved.

12-9 Controlling the Eutectoid Reaction Controlling the Austenite Grain Size: Pearlite grains/colonies grow at grain boundaries of austenite Reducing grain size/increasing number of austenite grains can increase pearlite colonies More, smaller pearlite grains increase strength Controlling the Cooling Rate: Increasing the cooling rate during the eutectoid reaction reduces diffusion, making finer lamellae Finer pearlite results in increased strength © 2016 Cengage Learning Engineering. All Rights Reserved.

12-9 Controlling the Eutectoid Reaction Controlling the Transformation Temperature: The solid-state eutectoid reaction is very slow, and the austenite may cool below the eutectoid temperature before the transformation begins Lower transformation temperatures give finer, stronger structures, but require longer times The information for this phenomenon is contained in the time-temperature-transformation (TTT) diagram. Two kinds of micro-constituents are formed in the transformation: pearlite above 550o C, and bainite below that Bainite is a cementite precipitate as discrete rounded particles in a ferrite matrix. © 2016 Cengage Learning Engineering. All Rights Reserved.

12-9 Controlling the Eutectoid Reaction © 2016 Cengage Learning Engineering. All Rights Reserved.

12-9 Controlling the Eutectoid Reaction © 2016 Cengage Learning Engineering. All Rights Reserved.

12-9 Controlling the Eutectoid Reaction © 2016 Cengage Learning Engineering. All Rights Reserved.

12-10 The Martensitic Reaction & Tempering Martensite is a phase formed by diffusionless solid-state transformation. The growth rate in martensitic transformations is so high that nucleation becomes the controlling step. It results from an athermal transformation, depending only on temperature, not on time. Martensite in Steels: In steels with under 0.2% C, quenching causes the FCC austenite to transform into a nonequilibrium supersaturated BCC martensite. In higher carbon steels, austenite transforms into a BCT martensite. © 2016 Cengage Learning Engineering. All Rights Reserved.

12-10 The Martensitic Reaction & Tempering Properties of Steel Martensite: Martensite in steels is very hard & brittle, has a fine grain structure and even finer substructure within grains Low carbon martensites have a ‘lath’ structure, while high carbon martensites have a ‘plate’ structure. Tempering of Steel Martensite: Martensite is not an equilibrium phase, so it doesn’t show up on the phase diagram. Heating martensite below the eutectoid temperature precipitates out the ferrite and cementite phases. This is called tempering. Tempering reduces strength, increases ductility © 2016 Cengage Learning Engineering. All Rights Reserved.

12-10 The Martensitic Reaction & Tempering © 2016 Cengage Learning Engineering. All Rights Reserved.

12-10 The Martensitic Reaction & Tempering © 2016 Cengage Learning Engineering. All Rights Reserved.

12-10 The Martensitic Reaction & Tempering Martensite in Other Systems: In iron alloys with zero/negligible carbon, martensite can form by transforming from FCC to BCC. The martensitic reaction also occurs in certain ceramics and polymers Martensite and martensitic reaction are general terms, and the specific of the transformation and obtained properties vary in different systems In titanium allows, martensite is actually weaker than the original structure, and tempering strengthens rather than weakens the alloy. © 2016 Cengage Learning Engineering. All Rights Reserved.

12-11 Shape-Memory Alloys [SMAs] The shape-memory effect is a unique property possessed by some alloys which undergo the martensitic reaction. These alloys can be processed by a sophisticated thermomechanical treatment into a martensite, then deformed into a desired shape The metal can then be deformed into a second shape, but upon increasing temperature or stress, reverts to its original shape. Such alloys are often used in biomedical devices SMAs also exhibit a superelastic behavior, with recoverable strains up to 10%. © 2016 Cengage Learning Engineering. All Rights Reserved.

Summary Solid-state transformations have a profound effect on a material’s structure & properties, and can be controlled by proper heat treatments. These heat treatments are designed to provide an optimum distribution of 2 or more phases in the microstructure. Dispersion strengthening permits a wide variety of structures & properties to be obtained. © 2016 Cengage Learning Engineering. All Rights Reserved.

Summary Such transformations require both nucleation & growth of new phases from original structure. Understanding phase transformation kinetics helps design heat treatments for desired microstructure. Appropriate phase diagrams help us select necessary compositions and temperature. Age/precipitation hardening is a powerful method for controlling optimum dispersion strengthening in alloys. © 2016 Cengage Learning Engineering. All Rights Reserved.

Summary In age hardening, a very fine widely dispersed coherent precipitate is formed via heat treatment that includes: Solution treating to produce single-phase solid solution Quenching to retain that phase Aging to permit a precipitate to form For age hardening to occur, the phase diagram must show decreasing solubility of solute in solvent as temperature decreases. © 2016 Cengage Learning Engineering. All Rights Reserved.

Summary The eutectoid reaction can be controlled to permit one solid to transform into 2 different solids. Eutectoid reaction kinetics depend on nucleation of new solid phases & diffusion of different atoms in the material to allow growth of new phases. The most widely used eutectoid reaction occurs in producing steels from iron-carbon alloys. Pearlite/bainite can be produced as a result of the eutectoid reaction in steel. Also, primary ferrite or cementite may be present, depending on the alloy’s carbon content. © 2016 Cengage Learning Engineering. All Rights Reserved.

Summary Factors influencing mechanical properties of the material produced by eutectoid reaction include: Alloy composition (amount of eutectoid microconstituent) Grain sizes of original solid and any microconstituents Interlamellar spacing Cooling rate during phase transformation Undercooling (temperature at which transformation occurs) © 2016 Cengage Learning Engineering. All Rights Reserved.

Summary A martensitic reaction occurs with no long-range diffusion. The best known example is in steels: Amount of martensite formed depends on the transformation temperature (athermal reaction) Martensite is very hard & brittle, hardness primarily being determined by carbon content. Amount & composition of martensite are the same as of the austenite from which it forms. Martensite can be tempered, which produces a dispersion-strengthened structure. In steels, tempering: Reduces strength & hardness Improves ductility & toughness © 2016 Cengage Learning Engineering. All Rights Reserved.

Summary Since optimum properties are obtained via heat treatment, structure & properties of the material may change when used at elevated temperatures, causing overaging/overtempering during service. Shape memory alloys (e.g. Ni-Ti) are smart materials which can remember their shape & exhibit superelastic behavior. © 2016 Cengage Learning Engineering. All Rights Reserved.