A general mechanism of martensitic nucleation: Part II. FCC → BCC and other martensitic transformations

The general mechanism of martensitic nucleation by faulting from groups of existing dislocations, as proposed in Part I, is applied to the fcc → bcc, bcc → fcc, bcc → hcp, and related transformations, including mechanical twinning. Where thermodynamic data are available, the conditions at the observedM _s temperatures are consistent with nucleation from a defect composed of four or five properly spaced lattice dislocations. Examples of nucleation by faulting on the planes predicted are found in published electron microscopy. The faults are observed at the types of sites where the required dislocation groups are expected. These include grain boundaries, incoherent twin boundaries, and inclusion particle interfaces. Having defined the function of a nucleation site, mechanisms of strain induced nucleation and autocatalysis are then considered. This paper is Part II of a three-part series based on a thesis submitted by G. B. Olson for the degree of Sc.D. in Metallurgy at the Massachusetts Institute of Technology in June 1974.     

A general mechanism of martensitic nucleation: Part I. General concepts and the FCC → HCP transformation

Consideration of the martensitic nucleation process as a sequence of steps which take the particle from maximum to minimum coherency leads to the hypothesis that the first step in martensitic nucleation is faulting on planes of closest packing. It is further postulated that the faulting displacements are derived from an existing defect, while matrix constraints cause all subsequent processes to occur in such a way as to leave the fault plane unrotated, thus accounting for the observed general orientation relations. Using basic concepts of classical nucleation theory, the stacking fault energy is shown to consist of both volume energy and surface energy contributions. When the volume energy contribution is negative, the fault energy decreases with increasing fault thickness such that the fault energy associated with the simultaneous dissociation of an appropriate group of dislocations (e.g. a finite tilt boundary segment) can be zero or negative. This condition leads to the spontaneous formation of a martensitic embryo. For the specific case of the fcc → hcp martensitic transformation in Fe-Cr-Ni alloys, the defect necessary to account for spontaneous embryo formation at the observedM _s temperatures may consist of four or five properly spaced lattice dislocations. Such defects are considered to be consistent with the known sparseness of initial martensitic nucleation sites. This paper is Part I of a three-part series based on a thesis submitted by G. B. Olson for the degree of Sc.D. in Metallurgy at the Massachusetts Institute of Technology in June 1974.     

Atomic structure of interphase boundary enclosing bcc precipitate formed in fcc matrix in a Ni-Cr alloy

The atomic structure of the interphase boundaries enclosing body-centered cubic (bcc) lath-shape precipitates formed in the face-centered cubic (fcc) matrix of a Ni-45 mass pct Cr alloy was examined by means of conventional and high-resolution transmission electron microscopy (HRTEM). Growth ledges were observed on the broad faces of the laths. The growth ledge terrace (with the macroscopic habit plane ) contains a regular array of structural ledges whose terrace is formed by the (111)_fcc//(110)_bcc planes. A structural ledge has an effective Burgers vector corresponding to an transformation dislocation in the fcc → bcc transformation. The side facet (and presumably the growth ledge riser) of the bcc lath contains two distinct types of lattice dislocation accommodating transformation strains. One type is glissile dislocations, which exist on every six layers of parallel close-packed planes. These perfectly accommodate the shear strain caused by the stacking sequence change from fcc to bcc. The second set is sessile misfit dislocations (∼10 nm apart) whose Burgers vector isa/3[111]_fcc =a/2[110]_bcc. These perfectly accommodate the dilatational strain along the direction normal to the parallel close-packed planes. These results demonstrate that the interphase boundaries enclosing the laths are all semicoherent. Nucleation and migration of growth ledges, which are controlled by diffusion of substitutional solute atoms, result in the virtual displacement of transformation dislocations accompanying the climb of sessile misfit dislocations and the glide of glissile dislocations simultaneously. Such a growth mode assures the retention of atomic site correspondence across the growing interface. formerly Graduate Student, Kyoto University, Kyoto 606-01, Japan This article is based upon a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18–22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.     

A localized soft mode model for the nucleation of thermoelastic martensitic transformation: Application to the β → 9r transformation

A nucleation model for bcc → 9R martensitic transformation has been developed based on the experimental data from a Cu-Zn-Al alloy. It has been shown through the third order elastic constants values that the C′ = 1/2(C₁₁ - C₁₂) elastic constant in the bcc phase is very sensitive to the homogeneous {011} (011) shear strains. Consequently it has been demonstrated that, around defects like dislocations, mechanically unstable zones are present where C softens dramatically. The C′ constant is related to the {011} (011) type shear which is precisely the homogeneous lattice strain involved in the bcc → 9R transformation. Nuclei can, therefore, develop in such zones without generating any resistive strain energy. Nucleation is postulated to occur in the zones when the reduced critical nucleus size becomes equal to the unstable zone size.     

Slip transfer and dislocation nucleation processes in multiphase ordered Ni-Fe-Al alloys

Directionally solidified (DS) alloys with the nominal composition Ni-30 at. pct Fe-20 at. pct Al having eutectic microstructures were used to study slip transfer across interphase boundaries and dislocation nucleation at the interfacial steps. The slip transfer from the ductile second phase, γ(fcc) containing ordered γ′(L1₂) precipitates, to the ordered β(B2) phase and the generation of dislocations at the interface steps were interpreted using the mechanisms proposed for similar processes involving grain boundaries in polycrystalline single-phase materials. The criteria for predicting the slip systems activated as a result of slip transfer across grain boundaries were found to be applicable for interphase boundaries in the multiphase ordered Ni-Fe-Al alloys. The potential of tailoring the microstructures and interfaces to promote slip transfer and thereby enhance the intrinsic ductility of dislocation-density-limited intermetallic alloys is discussed.     

A thermodynamic analysis of the phase equilibria of the Fe-Ni system above 1200 K

A quasi-subregular solution model is used to describe the thermodynamic properties of the liquid phase; values of the solution parameters are obtained from extensive and consistent thermochemical data reported in the literature. For the fcc and bcc phases, the same model is used to account for the nonmagnetic part of the Gibbs energy and the magnetic contribution is taken from the previous paper. Again, the values for the quasi-subregular solution parameters for the fcc phase are obtained from extensive and consistent thermochemical data reported in the literature at high temperatures. The values of the solution parameters for the bcc phase are obtained from the thermodynamic values of the liquid and fcc phases and the known phase boundary data. The calculated phase equilibria are in good agreement with the available data. Based on the thermodynamic data, the metastablel + γ andl + δ phase boundaries as well as theT ₀ (γ + l) andT ₀(δ +l) curves are calculated.     

Enhancement of plastic flow during massive transformations

An enhancement of plastic flow under an applied tensile stress has been observed during a relatively slow γ(fcc)→ α_m (bcc) massive transformation in a Fe 2.0 at. pct Cr alloy. As the rate of transformation increases with undercooling the flow rate also increases; but the time during which the enhanced flow can take place becomes progressively shortened. Enhanced plastic flow was not observed during a β(bcc) → ξ_m (hep) massive transformation in Ag-Al and Cu-Ga alloys, apparently because the duration of the trans-formation was too short to yield a measurable effect. The elongation behavior observed during the γ → α_m reaction indicates that an enhanced flow is associated with the move-ment of the incoherent γ/α_m interfaces that are active during a massive transformation and that the rate of enhanced flow increases with increased interface velocity.     

Influence of heat treatments on microstructures in an Fe-Co-V alloy

The microstructural changes in an Fe-Co-V alloy (composition by wt pct: 2.97 V, 48.70 Co, 47.34 Fe and balance impurities, such as C, P and Ni) resulting from different heat treatments have been evaluated by optical metallography and transmission electron microscopy. Results indicate that, on air cooling or quenching into iced-brine from the high temperature single-phase γ (fcc) field, vanadium can be retained in a supersaturated solid solution (α₂) which has bcc structure. For the range of cooling rates employed, a portion of the material appears to undergo the γ-α₂ transformation massively and the remainder martensitically. Also antiphase boundaries are observed in the air-cooled samples. On annealing in the two-phase α₁ + γ field, α₂ decomposes into vanadium-rich subgrains (γ) and vanadium-poor subgrains (γ₁), and only the former undergo the γ → α₂ transformation during air cooling or iced-brine quenching. The α₁t subgrains in a sample, slowly quenched in quartz, show superlattice dislocations and antiphase boundaries, whereas both the transformed and untransformed areas exhibit (100) superlattice reflections. There is, however, no evidence of long-range order in the specimens quenched into iced-brine. The two-phase annealing sequence followed by a 2 h anneal at 600°C and air cooling results in precipitation within the vanadium-rich, transformed subgrains. Also there is evidence of long-range order in both types of subgrains.     

Microstructure of a pressure-cast Fe3AI intermetallic alloy composite reinforced with zirconia-toughened alumina fibers

A composite of Fe-28Al-2Cr-lTi (at. pct) reinforced with 20-@#@ μm diameter zirconia-toughened alumina fiber, PRD-166, was pressure cast and examined by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). A new phase, tentatively identified as Fe₂AlZr, with an fcc crystal structure and a lattice parameter of 1.18 nm was occasionally found at fiber/ matrix interfaces. It was proposed that the phase formed by the eutectic reaction L → Fe(Al) + Fe₂AlZr. The Zr in the compound became available as a result of the dissolution of ZrO₂ from the fiber into the molten alloy. The matrix contained a high density of dislocations resulting from a difference in the coefficients of thermal expansion between the matrix and fiber. It was proposed that dislocations which formed at high temperatures in either A2 or B2 states were incompatible with the low-temperature DO₃ state. Geometrically necessary antiphase boundaries have been proposed to provide compatibility between dislocations formed in either the A2 or B2 states and the DO₃ state.