Martensite Nucleation Research Articles

Conception of New Phase Dislocation-Based Nucleation at Reconstructive Martensitic Transformations

The role of dislocations and the dynamical mechanism controlling the structural reconstruction in the process of nucleation and wave growth of new phase unit crystals at martensitic transformations in metallic systems are discussed. It has been established that near some rectilinear dislocations with lines and Burgers's vectors typical for the original phase, there are areas where an elastically deformed state is characterized by package of particularities unambiguously corresponding to the well definite morphological attribute set (habit, orientation relationship, macroshear) of the martensite crystal. The distinctiveness of these areas for martensite nucleation is caused by character of strains reducing the magnitude of interphase energetic barrier. The elastic model of the dislocation-based nucleation center of martensitic crystal, allowing to select the dislocations being the most probable for nucleation and to make a martensitic crystal with the morphological attribute specific collection corresponding to each a dislocation, has been proposed. Such dislocations for certain Fe-, Cu- and TiNi-based alloys are indicated. New results for titanium nickel are presented in more detail.

Pretransformational amplitude-dependent internal friction in CuAlNi single crystals undergoing martensitic transformation

In CuAlNi single crystals, amplitude-independent and amplitude-dependent internal friction due to phase transformation exhibits different temperature dependence in the transformation temperature range during direct transformation. An ADIF{sub pt} maximum is observed near the M{sub s} temperature, suggesting that at least part of the ADIF{sub pt}, is a pretransformational phenomenon, which may be associated with the nucleation of martensite at the dislocations in {beta}-phase. The hypothesis that both AIIF{sub pt} and ADIF{sub pt} may be separated into premartensitic and phase transition components, may allow one to explain rather complicated experimental observations in a coherent fashion.

Isothermal formation of martensite in a 12Cr-9Ni-4Mo maraging stainless steel

The present paper is concerned with the nature of the martensite, which provides the basis for the maraging treatment. Rather than forming martensite during cooling, 1RK91 develops martensite when held at a constant temperature in a range from room temperature and below. Isothermal martensite formation showing C-curve kinetics was found to occur in the maraging steel 1RK91, the nose temperature being about {minus}40 C. The kinetics was found to be enhanced for higher austenitizing treatment temperatures, presumably through a combination of larger grain size and a larger number of quenched in nuclei for isothermal martensite transformation. Experiments involving different cooling rates showed that fast cooling enhanced the transformation kinetics. Based on this observation it is suggested that quenched-in vacancy clusters provide suitable strain embryos for isothermal martensite nucleation.

Computer simulation of martensitic transformation in Fe-Ni face-centered cubic alloys

The F.C.C. → B.C.C. martensitic transformation in Fe-Ni alloys containing between 20 and 40 at.% Ni was studied using molecular dynamics simulations. The interatomic interactions were represented using the embedded atom method pseudomonoatomic potentials, which were shown to represent correctly the relative stability of the F.C.C. and B.C.C. structures as a function of the nickel content. The simulation results clearly demonstrate the important role that crystal defects, such as the free surface or grain boundary, play in the formation of martensite. This is consistent with the experimental observations, which suggest that the martensitic nucleation is of a heterogeneous nature. The simulation results further indicate the need for lattice invariant deformation, such as twinning and slip, for the continued growth of martensite.

On the heterogeneous nucleation of martensite

Martensitic nucleation near inhomogeneities is modeled using linear elasticity. The coherent strain energy due to the formation of a martensite embryo decreases when the inhomogeneity is elastically stiffer than the matrix and vice versa. A maximum reduction of 20% in strain energy is calculated for the case when the embryo is formed near a free surface. The results are consistent with the experimental observations of preferential nucleation of martensite at a free surface. A possible explanation for the nature of the “preexisting martensite embryo” in the Kaufman and Cohen model of a nucleation site is also proposed: the dislocation loop in the parent phase is itself the site for the embryo such that it will transform into martensite during transformation. The calculated critical characteristics of this embryo are in good agreement with the model of Chen and Chiao and their experimental results.

Deformation microstructures and the shear strain rate of type 304 stainless steel sheet in cylindrical deep drawing of warm working

A deep-drawing test was conducted under a single temperature or two temperatures on both sides of type 304 stainless steel sheet, 0.8 mm thick. The different results produced by these processes are described. A lower rate of shear straining is generated at 100 °C in the two-temperature process than in the single-temperature process, and a much larger rate is produced with room-temperature working. Their microstructures at the failure point and the flange portion nearing the die throat were studied by optical microscopy and TEM. An intermixed structure of lath and blocky α′-martensites appeared at the failure point in cold working. By warm working at a single temperature, homogeneous shear in a single direction and coarser slip bands were generated. Heterogeneous shear straining on both sides of the specimen occurred in the two-temperature process, and the microstructure of the failure point was observed as twin-twin intersections. The microstructures generated in the flange portion corresponding to the individual process, appeared to be blocky martensite nucleation and growth at the shear band intersections, twin-twin band or twin-slip band intersections, respectively. The strengths of these microstructures at different positions are compared with each other and the drawability is explained.

Effect of pre-strain on the kinetics of isothermal martensitic transformation

Abstract A detailed study of the effect of pre-strain on the isothermal martensitic transformation kinetics in an Fe-23·2wt% Ni-2·8wt% Mn is reported. The activation energy for isothermal martensitic nucleation in pre-strained samples is derived by analysing the nuleation and overall transformation kinetics data in terms of a phenomenological model. A critical analysis of the literature data on the activation energy for martensitic nucleation in solid-solution-strengthened alloys at subzero temperatures, enables us to estimate the contribution of Mn and Ni to the thermal component of the interfacial frictional work. With knowledge of the athermal and thermal contributions to the interfacial frictional work due to these solute atoms, it is possible to make a quantitative estimate of the interfacial friction owing to dislocation forest hardening.

Effects of aging on pseudoelastic behaviour in Cu-15.0 at.%Sn alloy single crystals

The effect of aging on pseudoelasticity in a shape memory alloy was studied using single crystals of a CuSn alloy, in which the martensitic transformation behaviour is fairly susceptible to an aging treatment. The aging effect on the nucleation of martensite was investigated in the specimens aged in the parent phase. The effect on the growth of martensite was studied by strain aging during tensile testing, and also by the analysis of strain rate dependence of pseudoelasticity. The former effect is attributed to the embryo exhaustion due to the diffusion of Sn atoms, originally proposed by Kennon. On the other hand the latter is explained from the pinning of a martensite/parent interface by the diffusion of Sn atoms.

Effects of aging on pseudoelastic behaviour in Cu-15.0 at.%Sn alloy single crystals

The effect of aging on pseudoelasticity in a shape memory alloy was studied using single crystals of a CuSn alloy, in which the martensitic transformation behaviour is fairly susceptible to an aging treatment. The aging effect on the nucleation of martensite was investigated in the specimens aged in the parent phase. The effect on the growth of martensite was studied by strain aging during tensile testing, and also by the analysis of strain rate dependence of pseudoelasticity. The former effect is attributed to the embryo exhaustion due to the diffusion of Sn atoms, originally proposed by Kennon. On the other hand the latter is explained from the pinning of a martensite/parent interface by the diffusion of Sn atoms.

Kinetics of F.c.c. → b.c.c. heterogeneous martensitic nucleation—II. Thermal activation

A critical analysis of the experimental data for the activation energy for f.c.c. → b.c.c. martensitic nucleation in FeNi, FeNiC, FeNiCr and FeNiMn alloys has been performed employing the theory of dislocation discrete-obstacle interaction. It is shown that the normalized activation energy Q ( T)/ μ( T) vs normalized driving force Δg n /Δ g ̂ data for isothermal martensitic nucleation follows the general form proposed by Kocks, Argon and Ashby [in Thermodynamics and Kinetics of Slip, Progress in Materials Science, Vol. 19. Pergamon Press, Oxford (1975)] for the kinetics of conventional slip deformation in solid solution strengthened alloys. Curvature of the Q( Δg) function defines a temperature dependent activation volume for isothermal martensitic nucleation. The analysis is consistent with rate control by the interaction of solute atoms with the thermally-assisted motion of the martensitic interface. The contribution of a solute to the thermal component of interfacial friction is found to scale with its athermal contribution. The analysis incorporates kinetic data for the effects of applied elastic stress and plastic pre-strain, providing a quantitative estimate of the interfacial friction from dislocation forest hardening.

Kinetics of F.C.C. → B.C.C. heterogeneous martensitic nucleation—I. The critical driving force for athermal nucleation

Employing available experimental data for athermal f.c.c. → b.c.c. martensitic transformation in binary, ternary and multicomponent Fe-base alloys, a model is developed and tested for the critical driving force at the M s temperature. Incorporating the theory of solid solution hardening, we describe the composition dependence of the athermal frictional work for martensitic interface motion governing the kinetics of barrierless heterogeneous nucleation. The available data suggests that the composition dependence of the athermal frictional work is of the same form as that for slip deformation. We have evaluated the athermal strengths of 14 alloying elements Al, C, Co, Cr, Cu, Mn, Mo, N, Nb, Ni, Si, Ti, V and W from the experimental data. Except for Al, Ni and Co, the athermal strengths of the common substitutional alloying elements are similar in magnitude, while the interstitial solutes C and N exert a stronger influence. Previously proposed superposition laws are used to account for the presence of multiple solutes having different athermal strengths. With an improved assessment of the magnetic parameters of alloy systems, the model predicts M s temperatures within ±40 K for M s > 300 K where thermal contributions to the frictional work can be neglected.

Effects of precursor matrix events on subsequent nucleation

A precursor transformation may actually increase the chemical driving force for subsequent nucleation of certain phases while decreasing the free energy of the system. Spinodal decomposition of Fe-C austenite has been proposed as a means of providing C-poor regions in which subsequent nucleation of ferrite or martensite is easier. However, the C-C interaction potential in austenite precludes such a reaction. Statistical fluctuations have also been proposed as a means of providing C-poor regions in austenite. However, the activity coefficient of C in austenite varies only slightly with C content, so that the scale of these fluctuations is far too small to provide attractive sites for martensite or ferrite nucleation. Statistical concentration fluctuations observed above the τ solvus in Al-Cu alloys bear a striking resemblance to Guinier Preston (GP) zone critical nuclei which will form only at temperatures that are hundreds of degrees lower. The role played by these fluctuations in subsequent critical nucleus formation is as yet unclear. A proposal that solute segregation to dislocations increases the driving force for nucleation is shown to be rendered invalid by the requirement that the chemical potentials in the system be uniform.

Elastic model of a dislocation center for martensite nucleation

The possibility of spontaneous nucleation of a crystal of new phase when the original structure is metastable is usually connected with the catalyzing effect of defects playing the role of nucleation centers. In the case of the {gamma}{r_arrow}{alpha} martensite transformation in iron alloys, even individual dislocations can act as such defects, based on analysis of long-range elastic fields of isolated linear dislocations in a linearly elastic anisotropic continuum, the authors established the existence of a correlation between the geometric characteristics of the elastically deformed state in the vicinity of 60-degree and 30-degree dislocations and the structure and morphological characteristics of {alpha}-martensite observed in massive iron alloy samples. These results suggest that the dislocation affects the pathway of the martensite reaction and allows the authors to say that the specific characteristics of heterogeneous nucleation of new phase for the martensite mechanism of the {gamma}{r_arrow}{alpha} transformation involves singling out a single structural rearrangement variant which is suitable from the standpoint of adapation of the transforming lattice to the characteristic features of the elastically deformed state created by the dislocation. The possibilities for such adaption are limited by the crystallography of the transformation and the reactions of the surrounding austenite occurring when regularmore » connections exist with the morphological characteristics of the martensite crystal, and are not necessarily compatible with the individual features of the elastic field of each dislocation. Considering this, the authors can introduce the concept of a dislocation center for nucleation of a martensite crystal about the region of the dislocation where conditions are realized which are favorable for the formation of a nucleus of martensite crystal of a certain shape and orientation, and they can develop an elastic model corresponding to this concept.« less

On the mechanism of two way shape memory effect obtained by stabilised stress induced martensite

Two way shape memory effect (TWME) can be obtained by suitable thermomechanical processing which involve repetitive training routines. J. Perkins found that TWME take place as a result of a macroscopic non-uniform residual stress field, concluding that plastic deformation was necessary to get TWME. K. Enami et al found that complex dislocations arrays are generated by thermomechanical cycling during the training procedure. TWME results obtained by the above training methods are not as good as would be expected, because during the thermal cycles, above and below A[sub f] and M[sub f] respectively, new dislocations are generated which interact with the dislocations that control TWME so giving a loss of TWME. A different training method has been developed by J.M. Guilemany et al based on the stabilization of stress induced martensite variants (SSIM). This method has been derived from the observation made by J. Perkins and R.O. Sponholz who found that retained (not stabilized) martensite acts as a nucleation site of thermal martensite. Thus, during cooling the stabilized martensite would grow or influence the nucleation and growth of the thermal martensite giving TWME. The effect of training temperature, time and stress on TWME obtained by SSIM has been studied.

On martensitic phase nucleation with surface effects

C ontinuum treatments of martensitic phase transitions are capable of accounting for a variety of surface effects caused by the interaction of coexisting phases of a material. Such phenomena are thought to play a critical role in determining the size, shape and stability of nucleated embryos as well as to affect the conditions under which nucleation occurs. These issues are examined within a purely mechanical context with the interaction modeled by traction and energy fields defined on the interface between phases. A class of isotropic, hyperelastic materials is introduced that is capable of modeling the dilatational component of martensitic phase transformations. Such materials are considered in a noninertial, axisymmetric setting that provides a means of exploring a variety of surface effects. Here nucleation events are modeled as deterministic, temporal shocks that are global in spatial extent, and a criterion for nucleation is suggested that is based on the energy available to create an embryo composed of a new phase. The model presented does more than capture the desired surface effects. It shows how they are related to specific assumptions regarding the constitution of interfaces. Three different types of interface are presented that serve to illustrate this.

Homogeneous martensitic nucleation in FeCo precipitates formed in a Cu matrix

The martensitic transformation on subambient cooling has been monitored in defect-free nanocrystalline f.c.c. FeCo particles that have been coherently precipitated in a Cu matrix; such a CuFeCo system was chosen for study because the f.c.c. → b.c.c. transformational driving force in FeCo alloys is exceptionally large. Transformation is found to occur at a driving force of ∼ 10kJ/mol, a factor of 7 higher than the known critical driving forces for heterogeneous nucleation in bulk alloys. The observed transformation characteristics are entirely consistent with classical homogeneous coherent nucleation, whereas heterogeneous nucleation and homogeneous semicoherent nucleation can be ruled out in this case. An observed variation in transformation temperatures is explained by the experimentally-determined differences in Co content (and, hence, in transformational driving force) among the FeCo precipitates in relation to their distribution of particle sizes. The role of thermal activation in the homogeneous nucleation process is demonstrated by applying a high magnetic field to impose an increment of driving force at low temperatures.

Distributed-activation kinetics of Heterogeneous Martensitic Nucleation

Martensitic nucleation under normal circumstances is a heterogeneous process, and the heterogeneity can be explained by the potency distribution of defects that provide the nucleation sites, including both preexisting and autocatalytically generated defects. The potency distribu- tion of the preexisting defects has been shown earlier by small-particle studies to follow an exponential function, whereas that of the autocatalytic defects is found in the present work to obey a Gaussian function. A major distinction between these two distributions is that the number density of thepreexisting defects increases monotonically with decreasing defect potency, while the number density of theautocatalytic defects is distributed about a mode, giving a saturation level of its cumulative distribution. As a result of these potency distributions, the nucleating sites of both origins exhibit distributions in their activation energies for nucleation, and a distributed- activation kinetic model is now proposed to take these variations into account for martensitic transformations. The features of this model are tested with considerable success in experiments on an Fe-32.3Ni alloy, leading to calculations of the entire course of martensitic transformation curves (including the athermal, anisothermal, and isothermal contributions), the changing shapes of time-temperature-transformation (TTT) diagrams for a range of Fe-Ni compositions, and the grain-size dependence of “bursting” behavior.

A constitutive model for transformation plasticity accompanying strain-induced martensitic transformations in metastable austenitic steels

We propose a constitutive model which describes the transformation plasticity accompanying strain-induced martensitic transformation in nonthermoelastic alloys. The model consists of two parts: a transformation kinetics law describing the evolution of the volume fraction of martensite and a constitutive law defining the flow strength of the evolving two-phase composite. The Olson-Cohen model for martensite volume fraction evolution is recast in a generalized rate form so that the extent of martensite nucleation is not only a function of plastic strain and temperature, but also of the stress state. A selfconsistent method is then used for predicting the resultant stress-strain behavior. The model describes both the hardening influence of the transformation product, and the softening influence of the transformation itself, as represented by a spontaneous transformation strain. The model is then implemented in a finite element program suitable for analysis of boundary value problems. Model predictions are compared with existing experimental data for austenitic steels. We present results from a few simple analyses, including tensile necking, illustrating the critical importance of stress state sensitivity in the evolution model.

Martensitic Nucleation—Revisited

Some recent findings concerning heterogeneous martensitic nucleation are highlighted in this memorial tribute to Professor Zenji Nishiyama. It is now possible, with appropriate experimental observations, to express the distributions of nucleation-defect potencies (or equivalently, the activation energies) of both preexisting and autocatalytically-generated defects. Based on this treatment, the full course of a martensitic transformation on cooling or on isothermal holding can be derived

Mechanism of Isothermal Martensitic Transformation

The mechanism of isothermal martensitic transformation has been studied, using the most typical alloy, Fe-Ni-Mn, of this type of martensitic transformation. The martensitic nucleation rate was investigated by measuring effect of applied stress on the transformation rate. The volume fraction of the martensite transformed was measured in situ by a magnetic detector with the coil covering the specimen. The transformation at liquid nitrogen temperature was studied in detail. The most important feature of observed results is that there is a critical stress level above which the initial transformation rate is drastically increased