Martensite Nucleation Research Articles

Nucleation barrier of fcc(g)?hcp(e) martensitic transformation in Fe-based alloys

Based on the dislocation theory and Olson’s stacking fault model, a model describing the nucleation of an hcp( e ) martensite embryo at low-angle grain boundary is proposed with the influence of external stress field taken into account. The dependences of temperature ( T ), shear stress ( τ ) and dislocation density at grain boundary on the martensite nucleation in FeMnSi based alloy, as an example, are numerically simulated. It has been shown that there exist the subcritical and critical embryos during the course of e-phase nucleation. The free energy difference between them is just the energy barrier of embryo growth. Depending on T and τ , the characteristic embryo sizes may vary in wide ranges and decrease with increasing σ and decreasing T . The energy condition of martensitic transformation at M s and critical shear stress ( τ c ) is discussed from the viewpoint of kinetics and thus the TEM observed result that stacking fault energy is not zero at M s temperature is reasonably explained. Besides, it is predicted that the high dislocation density at grain boundary can promote the nucleation of fcc→hcp transformation in Fe-based alloys.

Fcc/Hcp martensitic transformation in the Fe-Mn system: Part II. Driving force and thermodynamics of the nucleation process

This article, which continues the series started by Cotes et al. (Metall. Mater. Trans. A, 1995, vol. 26A, p. 1957–69), presents a study of the energetics of the heterogeneous nucleation of hcp martensite in the fcc matrix of the Fe-Mn system. A major goal of the work is the exploration of the various qualitative and quantitative consequences of applying the Olson and Cohen approach (OCA) to the most reliable information on the Fe-Mn system. To this end, an extensive assessment of the quantities involved in the OCA is performed. The selected Gibbs energy and the lattice parameter descriptions for fcc and hcp are based on recent experimental studies. Explicit calculations are presented of the key quantities in the OCA, including those quantities that have not yet been measured. In particular, a probable range of values is established for the surface energy parameter σ, which plays a crucial role in the treatment of the nucleation size. On these bases, values are derived for the probable number of atomic planes (n C) of the embryo, which, according to the OCA, is at the condition of spontaneous growth. Predictions of the Mn content upon n C are also reported. The present attempt to calculate in detail the energetics of the fcc −hcp martensitic transformation also revealed the lack of thermophysical data for Fe-Mn alloys. The need for experimental studies of various relevant quantities is emphasized.

A physically based model for the isothermal martensitic transformation in a maraging steel

Isothermal transformation from austenite to martensite in steel products during or after the production process often show residual stresses which can create unacceptable dimensional changes in the final product. In order to gain more insight in the effects influencing the isothermal transformation, the overall kinetics in a low Carbon-Nickel maraging steel is investigated. The influence of the austenitizing temperature, time and quenching rate on the transformation is measured magnetically and yields information about the transformation rate and final amount of transformation. A physically based model describing the nucleation and growth of martensite is used to explain the observed effects. The results show a very good fit of the experimental values and the model description of the transformation, within the limitations of the inhomogeneities(carbides and intermetallics, size and distribution in the material and stress state) and experimental conditions.

Dilatometric study of SME and TWSME in TiNi alloy

TWSME and SME, the B2-phase aging and B19'-martensite texture have been studied by methods of dilatometry, TEM and X-ray in Ti-50.5 at.% Ni alloy after rolling in austenitic state and after tension. TWSME of austenitic type takes place after rolling and after tension at 623 K. TWSME of martensitic type takes place after tension at room temperature. We showed that Ti 3 Ni 4 particles precipitated in Ti-50.5 at.% Ni alloy at 623 K, and they dissolved at 773 K. This precipitation increased martensitic transformation temperatures and deformation of TWSME. Dissolving of particles decreased transformation temperatures and deformation of TWSME. The increase of TWSME deformation during precipitation is due to oriented arrangement of particles relatively to deformation and oriented nucleation of martensite on particles. After tension at room temperature specimens contain B19'-martensite with axial texture [001] and have negative thermal expansion coefficient to this direction. We showed that (020) and (111) interplanar spacings of B19'-phase decreased, but (002) and (111) interplanar spacings increased with temperature decrease.

Nucleation model for hcp martensite: Application to Co thin films

The martensitic face-centered cubic (fcc) to hexagonal close-packed (hcp) transformation was studied in cobalt thin films on silicon substrates in order to understand effects of film constraints, texture and stress state. The martensitic transformation was observed in situ with the substrate curvature method. The transformation manifests itself as a stress drop revealing a positive work. However, with increasing tensile stress, the start of the martensite transformation is shifted to lower temperature. Thus, stress obstructs the transformation in spite of the positive sign of the transformation work. A dislocation-based model for the martensite nucleation is presented and applied to Co thin films. A distinction is made between a microscopic shape change and a macroscopic shape change. The model predicts the stress-induced delayed nucleation due to the microscopic shape change. The positive work results from the macroscopic shape change. Within experimental error, model and experiments agree well.

The kinetics of lath martensitic transformation

The multicomponent solution thermodynamics, a model for barrierless heterogeneous martensitic nucleation, a model for the composition and temperature dependence of the shear modulus, and a set of unique interfacial kinetic parameters are integrated to model the kinetics of lath martensitic transformation. It is shown that Ms in multicomponent alloys can be predicted within ±40K. The geometric partitioning of austenite grain and the dependence of average lath volume on the fraction transformed is established by a careful analysis of lath microstructure. The main contribution of lath martensitic transformation is due to autocatalytically generated defects.

Martensitic transformation in nanostructured Fe-Ni alloys

In Fe-Ni thin films prepared by magnetron sputtering at room temperature, the bcc structure can exist stably in a larger range of Ni-content than that in Fe-Ni equilibrium diagram. Furthermore, the experiment verified that the bcc structure forms directly from collision of atoms during the sputtering process, rather than the product of fcc→bcc martensitic transformation. The starting temperature for bcc→fcc transformation in thin films is near to that of bulk Fe-Ni alloys. Theoretical calculations show that the nucleation barrier of martensitic transformation and the critical size of the martensitic embryo increase with decreasing grain size within the nanometer scale, implying that the martensitic transformation in nano-sized grains would be suppressed. Moreover, the autocatalytic tendency of martensitic transformation is significantly weakened within a transformed grain or within its neighboring grain due to the reduction of probability of martensite nucleation or the drop of the stress field at grain boundary produced by martensitic transformation.

Heterogeneous nucleation of martensite on precipitates and the martensitic-transformation kinetics in shape memory alloys

Heterogeneous nucleation of martensite on precipitates and the martensitic-transformation kinetics in shape memory alloys

Kinetics of the martensitic transformation in low-alloy steel studied by means of acoustic emission

The kinetics of the martensitic transformation in three carbon steels (C60, C70 and C80) have been studied using the acoustic emission (AE) technique. It is demonstrated that the volume fraction of martensite f as a function of time t during cooling can be derived from the measured AE power ℧ 2 , since ℧ 2 ∝ df /dt . An analysis of results shows that the fraction data as a function of temperature T can be described by the Koistinen and Marburger (KM) equation with high accuracy. This indicates that the nucleation of martensite takes place heterogeneously and that the average volume of martensite crystals is constant over the extent of the transformation. The change in kinetics with carbon content is attributed to the amount of dislocations created in the neighboring austenite, which influences the degree of autocatalysis. Also, the acoustic emission energy generated per unit volume of martensite has been found to scale with the change in dislocation density in the formed martensite as the carbon content is varied.

Heterogeneous nucleation of martensite at dislocations and the martensitic-transformation kinetics in shape memory alloys

The martensite phase formation in elastic fields of isolated screw and edge dislocations, as well as in planar clusters of like-sign dislocations and in a two-dimensional network of opposite-sign edge dislocations, is quantitatively analyzed within the theory of smeared martensitic transitions. The heterogeneous nucleation of martensite at dislocations is shown to increase the characteristic temperature of the martensitic transition and its temperature smearing.

Controlling factor for nucleation of martensite at grain boundary in Fe-Ni bicrystals

A favourable nucleation of martensites at a grain boundary was investigated using Fe-Ni bicrystals containing a <211> symmetric tilt boundary with various tilt angles, focusing on the martensite-start temperature ( Ms), the morphology of martensites and the variant selection. Near the grain boundaries, some variants with the habit plane almost parallel to the boundaries were preferentially selected and their variants changed depending on the tilt angle. In addition, the equivalent variants were symmetrically adjoined at the boundary to maintain the compatibility of shape strain of martensites across the boundary. Such characteristic nucleation can be regarded as a self-accommodation across the boundary, which is called cooperative nucleation (CN). The CN effectively reduces the strain energy due to the formation of martensites compared with the independent nucleation within a grain resulting in an increase in Ms. The characteristic variant selection of martensites can also be explained from the minimisation of strain energy.

Electron irradiation-induced changes of martensitic transformation characteristics in a TiNiCu shape memory alloy

A narrow hysteresis TiNiCu shape memory alloy was irradiated by 1.7 MeV electrons by electron electrostatic accelerator. The irradiation temperature was below the martensite finish temperature. The transformation temperatures and the latent heat of phase transformation were measured by a differential scanning calorimeter (DSC). The results indicated that the transition temperatures were raised and the hysteresis was increased by the irradiation. Thus, the martensite was stabilized by the irradiation. While the enthalpy and entropy differences of the martensitic and parent phases were reduced slightly, the driving force of martensite nucleation was raised for 40% and the stored elastic energy of martensite decreased for 30% after the irradiation by electrons to a dose of 11.1×1020 m−2. Their changes were rather substantial for the electron irradiation-induced stabilization of martensites from a thermodynamical analysis.

Dislocation-disclination model of heterogeneous martensite nucleation in transformation-induced-plasticity steels

A dislocation-disclination model is proposed, describing the heterogeneous nucleation of an embryo of hcp martensite at a tilt grain-boundary segment containing some extrinsic dislocations. The total energy gain due to hcp embryo nucleation is analyzed in detail, and the existence of both the equilibrium and critical embryo sizes under varying external conditions (temperature and shear stress) is shown. Depending on the external conditions, these characteristic embryo sizes may vary in wide ranges. So, the equilibrium size increases while the critical size decreases as the external shear stress increases and the temperature decreases. It is also demonstrated that a critical external stress exists which induces athermal embryo nucleation when the nucleation-energy barrier disappears and the terms of equilibrium and critical embryo sizes lose their significance. The critical external stress has been studied, depending on the temperature and characteristic parameters of the grain boundary where the fcc-to-hcp martensite transformation takes place. We have shown, in particular, that the critical external stress increases in direct proportion to both the grain-boundary misorientation angle and temperature.

In situ study of martensitic transformation and nucleation and propagation of cracks in Cu-Ni-Al shape memory alloy

The stress induced martensitic transformation and the relationship between it and the nucleation and propagation of cracks in the Cu-Ni-Al shape memory alloy were investigated through in situ tensile tests by SEM and TEM. The results indicated that the stress concentration ahead of the crack tip could induce formation of stacking faults and different types of martensites. Transmission electron microscope observations showed that the martensites could transform from one type to another type and even reversely to parent during loading. The microcracks nucleated along the martensite/parent interface and intersections between two martensites. When the crack propagated a certain distance, the stress concentration ahead of the crack tip was large enough to result in formation of slip bands, in this condition the microcrack nucleated along slip bands more easily.

Possibility of the martensitic transformation triggered by thermal fluctuation

Motivated by the recent experimental results of martensitic transformation of nanometer-sized particles, we theoretically analyze the possibility of martensitic transformation triggered by thermal fluctuation in nanometer-sized particles and reconsider an earlier presented heterogeneous nucleation model. Our results show that homogeneous nucleation of martensite is possible in iron-based alloys.

Heterogeneous nucleation of martensite near free surface

A theoretical model is proposed which describes heterogeneous nucleation of an embryo of hcp-martensite near a free surface at a tilt grain boundary segment containing some extrinsic dislocations. The total energy gain due to the hcp-embryo nucleation is analysed in detail depending on internal (the distance from free surface and orientation of the grain boundary plane) and external (the temperature and shear stress) conditions. It is shown that the main characteristics of the hcp-embryo nucleation (the energy barrier and critical size) depend strongly on the distance from the free surface.

Model for nucleation of isothermal martensite

Kaufman and Cohen's model for thermally activated growth of preexisting embroys of isothermal martensite was defined with a series of obstacles but they only considered jumps in one direction. Jumps in both directions across obstacles were recently considered and the activation energy for the net reaction was described. That nucleation model is now developed further.

Effect of grain boundary character on the martensitic transformation in Fe–32at.%Ni bicrystals

The effect of grain boundary character on the martensitic transformation was examined in two types of Fe–32at.%Ni bicrystals containing a 90°<211> tilt or a 90°{211} twist boundary focusing on the martensite-start temperature (Ms) during cooling, the morphology of the martensite and the variant selection. The Ms of bicrystals with a tilt boundary was higher than that of single crystals, while bicrystals with a twist boundary showed no significant difference from that of single crystals. Coarse lenticular martensites formed symmetrically in neighbouring grains around the tilt boundary. In contrast, tiny martensites were uniformly distributed in bicrystals with a twist boundary, and in single crystals. In the vicinity of the tilt and twist boundaries, some variants with the habit plane almost parallel to the boundaries were preferentially selected among 24 variants. Moreover, since the equivalent variants in neighbouring grains at the tilt boundary are selected, strain compatibility of the shape strain of martensite across the tilt boundary is satisfied, resulting in an increased Ms. The effect of pre-strain on the heterogeneous nucleation of martensite at the boundaries was also investigated.

In-situ study of martensitic transformation and nucleation, propagation of crack in CuNiAl shape memory alloy

The stress-induced martensitic transformation and its relation with crack nucleation and propagation in CuNiAl shape memory alloy were investigated through in-situ tensile tests by SEM and TEM. The results indicated that the stress concentration ahead of the crack tip could induce formation of stacking faults and different types of martensites. TEM observations showed that the martensites could transform from one type to another and even reversely to the parent during loading. The micro-cracks nucleated along the martensite/parent interface and intersection between two martensites. When the crack propagated a certain distance, the stress concentration ahead of the crack tip was large enough to result in formation of slip bands, and in this condition the microcrack nucleated along slip bands more easily.

A continuum description of dislocations in elastically nonlinear media: martensitic embryo formation

Candidate sites for heterogeneous nucleation of martensite are constructed from two dislocation structures placed in a nonlinear, nonlocal elastic continuum. The resulting static strain fields of the dislocation arrays give rise to mesoscopic embryos of the product phase, which exhibit athermal behavior as the effective temperature of the system is changed. A high-potency dislocation array gives rise to a strained classical (sharp boundary) martensitic embryo at high temperatures, corresponding to the limit of metastability for the martensitic phase. This embryo grows rapidly with decreasing temperature, interacting with the system boundary before the transformation can occur. A low-potency dislocation gives rise to a more diffuse, nonclassical structure, which increases in strain amplitude and size with decreasing effective temperature, becoming metastable before finally triggering the transformation of the full system. Transformation occurs in the low-potency case when force is 61% of the driving force at the lattice instability, and the strain embryo achieves 80% of the transformation strain.