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    PHASE TRANSFORMATIONS INMETALS AND ALLOYS 8546Glyn Meyrick and Gordon W. PowellDepartment f Metallurgical Engineering, The OhioState University, Columbus,OhioINTRODUCTIONUnderstanding a phase transformation involves an appreciation of the reasonsfor its occurrence and of the modeor modesby which it takes place. The formerwill not be explored herein beyond noting that a system can spontaneouslyundergo a change in phase (or phases) if by so doing its free energy is reduced.Provided a reduction in free energy ensues, the product of the transition need notbe that pertaining to the equilibrium state. A total change mayyield directly theequilibrium state or traverse a path composedof several tranformations involv-ing metastable phases. This, combinedwith the fact that a given change can beaccomplished by more than one kinetic mechanism generating modal competi-tion, endows the study of phase transformations and their consequences with thecomplexities that render it so intriguing. Phase transitions have received exten-sive investigation because of their inherent fascination and because they play aparticularly important role in property control for material applications.

    As can be seen from someof the more recent surveys of the field (1-3) itcustomary to group together changes that exhibit common haracteristics in aneffort to classify phase transformations in a systematic manner. Criteria involvedin classification are predominantly of morphological character but also include,or imply, mechanistic processes. This process has led to the establishment ofaccepted group names: 1. continuous precipitation, 2. massive transformations,3. discontinuous precipitation, 4. martensitic transformations, 5. bainitic trans-formations, 6. order-disorder transformations, and 7. spinodal decomposition.Because of an initial restriction on the length of this review, order-disordertransformations and spinodal decomposition will not be considered here, butthey have been discussed by Wayman4) in a pievious volumeof this series.

    Continuous precipitation is characterized by the formation of grain boundaryand intragranular particles of the newphase, which generally has a structure and

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    328 MEYRICK POWELLcomposition different from that of the metastable matrix. The growth of the newphase is controlled by long range diffusion through the matrix, whose averagecomposition changes continuously as the reaction proceeds. The reaction at theinterface between the parent and product phases is presumed to be relativelyfast.

    If the new phase forms by the massive mode, it has the same composition asthe parent phase and the reaction is accomplished by the rapid motion of a highenergy, incoherent boundary. Thus this solid state reaction does not involve longrange diffusion and the transfer of atoms from the parent to the product phaseis assumed to be effected by the uncoordinated, random jumps of individualatoms across the interphase boundary. Although Phillips (5) was apparently thefirst investigator to recognize this modeof transformation as a unique type ofsolid state reaction, the term massive was first applied by Greninger (6). It isappropriate term in the sense that it is descriptive of the relatively largemorphological units which in some cases form by this mode of transformation.

    The discontinuous modeof transformation results in the formation of a two-phase mixture at an advancing, incoherent boundary. The atom transport andatom rearrangement required to produce the two-phase mixture are assumed totake place in the advancing boundary and also, if volumediffusion is significant,within a region immediately adjacent to the advancing boundary. The bulk ofthe metastable matrix remains essentially unchanged until traversed by theinterface. Generally, the compositions and crystal structures of the two phasesare different from that of the metastable matrix. The two phases are oftenarranged in the form of a lamellar aggregate.

    The martensitic mode of transformation yields platelets of an oftentimestransitional phase by a displacive reaction which is diffusionless, producessurface relief effects, and is reversible in somealloy systems (7). In addition,definite crystallographic relationships exist between the product and parentphases, and the physical plane of the martensitic platelet is usually parallel to anirrational lattice plane (habit plane) of the metastable matrix. Since the formula-tion of th~ phenomenological theory of displacive reactions by Wechsler,Lieberman & Read (8) and Bowles & Mackenzie (9), much of the researchbeen concerned with the crystallographic and geometrical aspects (lattice rela-tionships, habit plane, shape deformation, and inhomogeneous deformationwithin the martensite) of martensitic transformations.

    The bainitic modeof transformation is the subject of considerable controversyas demonstrated quite clearly by a recent debate on this reaction (10). Some olidstate reactions which occur in ferrous and nonferrous alloys and whose productshave distinctively different morphologies have been labeled bainitic. In the caseof hypoeutectoid steels, lower bainite has some of the characteristics (latticerelationships, surface relief, inhomogeneous ubstructure) of martensite, but theslow edgewise growth of the platelets of bainite is controlled by the rate ofdiffusion of carbon in the austenite matrix.Whereas a classification scheme based upon clearly distinguishable modes oftransformation has obvious merits, it can also be disadvantageous if adhered to

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    PHASE TRANSFORMATIONS N METALS AND ALLOYS 329too rigidly. It is to be emphasized hat a phase transition can involve a structuralchange, a compositional change, or both. All of these can, in principle, beaccomplished by diffusion processes, while part can be achieved by displaeiveprocesses. Both processes could be operative in a given transformation and mightor might not be practically separable. Thus, although transformations exist that,by common greement, belong within a particular group, others exhibit charac-teristics of several groups. It is moreprofitable to regard this main classificationas establishing general guidelines rather than providing a series of separatecompartments into which all transformations can be unequivocally placed.

    The objective of this review is primarily to summarize on the basis ofexperimental observations the current knowledge of the morphological andgrowth characteristics of the various modes of transformation. It does notconsider in any great detail mathematical models of these reactions. Each of thevarious modes of transformation will be considered under one of three broadgroups of solid state reactions: 1. diffusional transformations, 2. displacivetransformations, and 3. hybrid (mixed mode) transformations.DIFFUSIONAL TRANSFORMATIONSMassive TransformationsAs noted in the introduction, a massive transformation is accomplished by therapid motion of a high energy, incoherent boundary which converts the parentphase into a more stable phase of the same composition. Any nonmartensiticpolymorphic ransformation in a pure metal is obviously a degenerate or limitingcase of a massive transformation. Thus Owen& Gilbert (11) suggested that the"r-to-a transformation which occurs in pure iron at cooling rates less than5500C/sec is a massive reaction. These observations were made on bulkspecimens. Differences are to be expected for microscopic specimens; forexample, the a-to--/transformation in iron whiskers exhibits characteristics of amartensitic transformation and is well described by the phenomenological heory(12). Bibby & Parr (13), using iron containing less than 0.0017% C,concluded that the transformation at cooling rates less than 30,000C/sec ismassive. At higher cooling rates the "r-to-a transformation is martensitic. Typicalmassive microstructures obtained by cooling pure iron (0.001% C) at slow ratesare shown n Figure 1; in general the grain size decreases with an increase in thecooling rate. This modeof transformation does not 15roduce surface tilts (13-15),a fact consistent with the concept that the boundary between the transformedand untransformed region is displaced by the random, noncooperative move-ment of the atoms at the boundary; Transmission electron microscopy has shownthat the internal structure of the massiveferrite grains consists of a random rrayof dislocations and that neighboring massive grains are separated by high angleboundaries (14, 15) (Figure 2). The dislocations maybe the result of transforma-tion-lnduced stresses and quenching stresses.The motion of the austenite-ferrlte boundary in pure iron at undercoolings onthe order of 25-50C has been investigated by Eichen & Spretnak 06) and

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