Martensite Nucleation Research Articles

Isothermal martensite formation in an AISI 52100 ball bearing steel

The formation of isothermal martensite from the retained austenite in an AISI 52100 ball bearing steel was investigated. Optical microscopy reveals that there are mainly two types of isothermal martensite formation: the growth of the athermal martensite and the nucleation and growth of new martensite in the retained austenite. X-ray diffraction shows that during the isothermal transformation, the ratio of lattice constantsc/a decreases, and TEM verifies the precipitation of Fe, Cr)3C in martensite. The kinetics of the isothermal transformation in the quenched steel also shows “C” shape characteristic. At the first stage of the isothermal formation, recovery of the athermal martensite occurs with an activation energy of 91.8 kJ mol-1, implying that the diffusion of carbon in athermal martensite results in the precipitation of carbide and the relaxation of the strain energy at the martensite/matrix boundary. In the second stage, the activation energy for the isothermal formation is 130 kJ mol-1; that may be the energy required for the rearrangement of the configuration of dislocations, forming preferred sites for nucleation.

Roles of dislocations and grain boundaries in martensite nucleation

In order to elucidate roles of dislocations and grain boundaries in martensite nucleation, the transformation temperature (Ms) of specimens austenitized at various temperatures and subjected to prestrain has been measured, using Fe-Ni, Fe-Ni-C, and Fe-Cr-C alloys. It is concluded that the plastic accommodation, in austenite, of the shape strain of the transforming martensite is a vital step in the nucleation event. Any factors impeding such plastic accommodation, such as the lack of dislocations, work hardening, and grain refinement, suppress the transformation. Contrary to the general belief, dislocations themselves do not act as favorable nucleation sites. Grain boundaries provide nucleation site, but only certain types of grain boundaries are qualified to be potential nuclei. A quantitative analysis shows that the increasing difficulty for the plastic accommodation with decreasing grain size is the main factor to depress Ms in fine-grained specimens.

Surface segregation analysis of martensite nucleation in model systems

In this work the study of the dependence of the critical temperatureM s for the onset of martensite in noble metal (NM)-based ternary alloys in extended to some new cases of interest, for which experimental data are now available. A recently developed model connecting surface segregation and martensite nucleation is applied, which on the one hand gives information in agreement with experimental data, and on the other allows for a critical comparison between the obtained results and the theoretical predictions in the framework of related predictive theories for surface segregation.

Kinetics of martensitic transformation induced by a tensile stress pulse

The kinetics of martensitic transformation induced by a tensile stress pulse (generated by the reflection of a compressive shock wave at a free surface) in time durations in the microsecond range, were studied in an Fe-32wt%Ni-0.035wt%C alloy. The tensile hydrostatic component of stress interacts with the dilatational strain (~0.04) of the martensitic transformation and increases the M s temperature. Shock waves were produced by normal impact of a projectile on a target in a one-stage gas gun. Impact experiments were conducted by varying either the temperature (−10 to −50°C), or pulse duration (0.22−1.76 μs) at a constant pressure. The martensitic transformation, normally considered to be athermal in Fe-Ni-C alloys, exhibits an isothermal nature in the microsecond regime. The fraction transformed increases with decrease in temperature at a constant pulse duration, and increase in pulse duration at a constant temperature. The mean volume of the lenticular martensite was found to be constant throughout the progress of the transformation, consistent with the autocatalytic spreading of clusters. The activation energies for γ→α' transformation in the Fe-32wt%Ni-0.035wt%C alloy, calculated with a modified version of Pati and Cohen's kinetic equation [ Acta metall. 17, 189 (1969)], range from 38,000 J/mol at −10°C to 25,000 J/mol at −60°C. The activation energies are linearly related to the total driving force (chemical free energy change + mechanical work due to the transformation). The activation volume for the transformation was calculated and found to be equal to approximately 60 atoms (0.7nm 3). This indicates that the martensitic nucleation in this alloy, and under the imposed stress conditions, is interface-mobility controlled.

The kinetics of isothermal martensitic transformation in an Fe-23.2wt.%Ni-2.8wt.%Mn alloy

In this paper the results of a detailed study of the isothermal martensitic transformation kinetics in an Fe-23.2wt.%Ni-2.8wt.%Mn alloy are reported. The transformation kinetics were determined as a function of test temperature, superimposed elastic stress field and prior plastic deformation of the austenite. Extensive quantitative metallographic measurements show that the overall transformation kinetics agree very well with the model that incorporates the vital contribution of autocatalysis. On the assumption that the rate-controlling step in martensitic nucleation is the thermally activated motion of dislocations, the activation energies for nucleation were calculated under an applied elastic stress or in work-hardened austenite. These show good agreement with the experimentally determined activation energies. The present results support the Olson-Cohen suggestion that the critical step in martensite nucleation is the thermally activated motion of the partial dislocations bounding the fault embryo.

Electron microscope observation of the early stages of thermoelastic martensitic transformation in a TiNiCu alloy

In situ observations by electron microscopy were made on nucleation and growth of a thermoelastic martensite in a Ti39.5Ni10.0Cu(at.%) alloy. It has been found that a stress field near defect aggregates plays an important role in the martensite nucleation. The habit plane of the martensite plates in the early stages of formation was determined to be the (334) plane of the parent cubic lattice by trace analysis. The evperimentally determined habit plane was in good agreement with the theoretically predicted one. The atomic configuration along the habit plane was considered on the basis of the evperimentally determined lattice deformation. It has been found that the macroscopic (334) habit plane is composed of (112) steps.

Effect of Nucleation on Transformation Toughening

A simple model is presented to account for the effect of martensitic nucleation in ZrO2 particles on theoretical predictions of toughness enhancement during transformation toughening. The model allows for shear and volumetric strain contributions during nucleation, whereas only volumetric strain contributes to toughness enhancement. It is predicted that toughness enhancement results for both stationary and growing cracks. The predicted toughness enhancement exceeds the theoretical prediction for the case when only volumetric strain is allowed to operate. The important material property is identified to be the ratio of volumetric to shear strain during nucleation of the martensitic product in the transformation from tetragonal to monoclinic symmetry of the ZrO2 particles.

Overview no. 45: On the nucleation of the martensitic transformation in zirconia (ZrO 2)

The kinetics of the martensitic tetragonal ( t)→monoclinic ( m) transformation in ZrO 2 are nucleation-controlled, as they are in metals and alloys, and classical and non-clasical models for martensitic nucleation are considered in this paper. Transmission electron microscopy studies of the several types of transformation-toughened ZrO 2-containing ceramics now available do not reveal any obvious defects that could act as classical heterogeneous nuclei. This is true for t-ZrO 2 as a coherent precipitate phase, as an incoherent dispersed phase, as a fine-grain matrix phase, or as an incoherent precipitate phase in internally-oxidized alloys. On the other hand, nucleation via a Clapp-type localized soft mode mechanism at strain concentrations is consistent with all observations. Nucleation is always stress-assisted, even when the transformation occurs spontaneously. In these latter cases, the stresses arise from thermal expansion mismatch or thermal expansion anisotropy and are enhanced by the stress concentrations provided by particle or grain facets and corners.

Theory and experiment of martensitic nucleation in ZrO2 containing ceramics and ferrous alloys

An in situ TEM experiment on martensitic nucleation was performed using sub-micron ZrO2 particles which were initially defect free. Extrinsic, spontaneous and heterogeneous nucleation was induced by introducing dislocation loops with a strong shear component and Hertzian contact stresses into selected particles. The critical characteristics of these defects which were required to trigger barrierless nucleation, in terms of the loop radius, Burgers vector, contact stress, and contact area were in good agreement with an analytic elasticity model. Along with dislocation walls, these defects which have a short range stress field can also account for intrinsic nucleation in ZrO2 containing ceramics and ferrous alloys. The essential features of nucleating defects, which may be classified into intrinsic or extrinsic ones depending on their origins, and short range or long range ones depending on their stress fields, are analyzed and contrasted in view of their experimental evidence.

Statistics of martensitic nucleation

Nucleation statistics, incorporating the particle size effects and the temperature effects, of martensitic transformations in ZrO2-containing ceramics and in Fe-Ni alloys was analyzed. A phenomenological potency distribution of nucleating defects, of the form of (excess driving force)2, was found. Using a simple model of long range nucleating defects, of the type of a dislocation wall, a theoretical potency distribution was predicted. The model was in good agreement with the experimental data and with the phenomenological form, over a broad range of excess driving force. A universal potency distribution for intrinsic, pre-existing defects thus became apparent Experimental evidence was found, in literature and in new data reported here, that both the size effects and the universal potency distribution will be violated when nucleation is extrinsically induced, especially in very small particles or in strain-induced transformations. An essential distinction in the origin of nucleation, as reflected in nucleation statistics, can thus be drawn for classification of martensitic nucleation.

The effect of variables on martensitic transformation temperatures

A formulation is proposed for the factors which have an effect on the start of martensite. They should be subdivided depending on whether they affect the equilibrium temperature T o ( M s < T o : necessary condition) or the additional undercooling ΔT m = T o − M s (sufficient for the onset of transformation). T o depends on chemical composition, degree of order, hydrostatic stress, ΔT m on nucleation and propagation of martensite inside the matrix. This, in turn, is affected by an external shear stress, and the resistance to shear, i.e. hardening mechanisms caused by point defects, dislocations, or particles. The course of M s as the function of the period of isothermal aging provides an example for a different change of T o and ΔT m .

Localised surface segregation and martensite nucleation in noble metal based ternary alloys

A recent model connecting martensite nucleation with localised surface segregation phenomena is extended to dilute ternary alloys based on β-CuZn and β-AuCd. Experimental data about composition induced variations of the critical temperature for the onset of martensitic transformation are organised in a coherent fashion by the atomistic Valence Electron Localisation Degree variation model. An analysis of the surface segregation properties of the original and effective alloys involved in micro nucleation events of martensite is performed by a model relying upon charge transfer effects. It is concluded that martensite nucleation may be interpreted in terms of preferential, localised surface segregation events.

Martensitic Nucleation in Small ZrO2 Particles

AbstractSmall ZrO2 particles were studied by in-situ transmission electron microscopy and the types of events which led to martensitic nucleation were recorded. These metastable particles were single crystals of several hundred nanometers size and were supported on a carbon thin film but were otherwise free from any constraint. At room temperature, they are more than 1200°C below the equilibrium temperature. After dislocation loops were injected in-situ into the originally defect-free particles, a critical loop size was found beyond which the particle transformed spontaneously. It was also observed that Hertzian contacts between two particles led to martensitic nucleation and spontaneous transformation. These results were satisfactorily rationalized by ab-initio theoretical analysis.

Compaction and Sintering Phenomena in Titanium—Nickel Shape Memory Alloys

AbstractTi–Ni specimens of the equiatomic composition have been processed by conventional powder metallurgy techniques. The effects of compaction pressure, sintering temperature, and powder particle size on dimensional changes of sintered compacts are reported. During sintering, anisotropy of dimensional change occurs, with expansion in the radial and contraction in the axial direction. Densities decrease during sintering. From metallographic evidence it is suggested that these observations are connected with the difference in interdiffusion rates of Ti and Ni and the segregation of powder particles in the green compacts. The sintered compacts demonstrate a well defined shape memory behaviour, with martensite nucleation being enhanced by the presence of pores. PM/0358

HVEM &lt;I&gt;in situ&lt;/I&gt; Observation of Isothermal Martensitic Transformation under Applied Stress

An attempt was made to observe in situ early stages of the isothermal martensite formation by a high voltage electron microscope, using Fe–Ni–Mn and Fe–Ni–Mn–C alloys. Observed transformation characteristics are different, in many respects, from those in bulk specimens, but some experimental facts which characterize the isothermal martensitic transformation have been obtained; especially, an observation of the generation of dislocations prior to the martensite nucleation provides direct evidence supporting a rate controlling mechanism proposed by Kajiwara for the isothermal martensitic transformation. It is considered that, in the case of the fcc/bcc (or bct) martensitic transformation, observation of the nucleation stage in thin foil experiments is intrinsically impossible due to the small thickness of specimens even on very favorable experimental conditions.

Statistical Aspects of Martensitic Nucleation

AbstractAnalysis of the Cech-Turnbull small-particle martensitic transformation experiments in terms of heterogeneous nucleation theory defines an exponential nucleation-site potency distribution. Sensitive acoustic-emission detection of martensitic nucleation events shows that the same form of distribution describes the behavior of bulk polycrystals, and the influence of heat treatment on the distribution amplitude can be identified in this way. On the assumption of a random distribution of pre-existing nucleation-site orientations, the effect of applied stress on the effective potency distribution has been calculated, thus accounting for an observed nonlinear stress dependence of the transformation kinetics. The model also predicts a transformation “yield locus” for multiaxial stress.

Martensitic nucleation and growth via molecular dynamics

Martensitic nucleation and growth via molecular dynamics

Martensitic nucleation and growth via molecular dynamics

Martensitic nucleation and growth via molecular dynamics

Nucleation of recrystallization in a Co- Cr- Mo alloy

Incoherent twin boundaries and twin-twin intersections have been identified as nucleation sites for recrystallization in a Co-Cr-Mo alloy. The recrystallized grain is a region of hep and has a twin orientation with respect to the fee matrix in its early stages of growth. This crystallographic relationship is discussed in terms of a martensitic embryo nucleation model for the fee → hcp transformation.

Continuous observation of isothermal martensite formation in Fe-Ni-Mn alloys

The isothermal formation of martensite in Fe-23Ni-3.9Mn alloy was continuously observed at low temperature, using a cold stage of optical microscope. Various aspects of the dynamic features of the isothermal transformation have been found; such as 1. (1) the isolated martensite nucleation, 2. (2) the intensive slip in austenite concurrent with martensite formation, and 3. (3) the lack of the autocatalytic effect in the nucleation events. It seems that these characteristic features are important factors for the mode of the martensitic transformation to be isothermal.