Martensite Nucleation Research Articles

Martensitic Transformations in TiNi Alloys with Ti3Ni4 Precipitates

The thermoelastic martensitic transformations in Ti 4.>Ni5, and Ti48Nij2 alloys after isothermal aging have been studied by means of transmission electron microscopy and dilatometry. The presence of TUNi4 precipitates and the difference in precipitates size affect the martensite nucleation and martensitic transformation temperatures and sequences in TiNi alloys. Highly coherent and densely dispersed TUNi4 particles ( B19'. When sizes of Ti-,Ni 4 particles run as large as several microns, they provide nucleation sites of the B19'-martensite. Transformation sequence is B2-»B19' as in solution treated specimen.

Formation of α-Martensite Crystals with Habits (hhl) in Cryston Model

The mechanisms of formation of martensite with habit planes of the (hhl)-type are discussed for Fe-based alloys which undergo a γ-α (fcc-bcc) martensitic transformation. The relaxation of the lattice is interpreted in the framework of the cryston model. A cryston is the combination of dislocations of one or two slip systems which play the role of quasiparticles among the dislocation association on the mesoscale level. A number of cryston models of simple shear are presented. A possibility of the limiting transition to the cryston wave model is noted. Using the lath martensite as an example, it has been shown that using the cryston model allows to interpret morphological features of separated martensite crystals such as non-equivalence of parallel faces of lath crystals, deflection of orientation relationships from the ideal ones, the high residual density of dislocations. It is emphasized that the choice of prismatic dislocation loops as probable centers of martensite nucleation is a promising idea.

Martensitic nucleation mechanism

A sort of special dislocation configuration was deformation-induced in an Fe−Ni-V-C alloy byin-situ elongation tests of TEM. The coolingin-situ observations, as well as the SADPs from the region of the special dislocation configurations, proved that they are martensitic nuclei. In martensitic transformation, a nucleus changed into a small martensitic sub-plate, and a group of parallel sub-plates that formed from a group of parallel nuclei made up a big martensitic plate. Martensitic transformation involved opposite shear between adjacent martensitic nuclei. By using the reduced-cell method, the crystallographic structure of observed martensitic nuclei was indexed as a face-centered orthogonal (FCO) lattice, which was explained by the nucleation mechanism proposed by the present authors. The crystallographic analysis confirmed that the defect faulting involved in martensitic nucleation took place among three close packed planes, instead of between two adjacent planes as an ordinary stacking fault.

Influence of small uniaxial strains on the martensitic phase transition in alkali-metal systems

A neutron scattering investigation of the influence of a uniaxial elastic deformation on the martensitic phase transformation in the alkali-metal systems Li and ${\mathrm{Li}}_{10\mathrm{}\mathrm{at}.%}$Mg is described. The experimental results show that a small but symmetry-breaking elastic deformation of the bcc matrix drastically promotes the phase transformation. The deformation further induces a significant reduction of the otherwise large transformation hysteresis in a virgin crystal and leads to a memory effect; i.e., the hysteresis remains small when the applied elastic strain is released. Moreover, the applied deformation of about 0.1% of the lattice parameter induces a softening of the transverse acoustic ${\mathrm{TA}}_{1}$[110] phonon branch of several percent. The results are compatible with a description of martensitic nucleation in alkali metals as being triggered by strain fluctuations.

Three-dimensional field model and computer modeling of martensitic transformations

A three-dimensional (3D) continuum stochastic field kinetic model of martensitic transformations which explicitly takes into account the transformation-induced elastic strain is developed. The model is able to predict the major structural characteristics of martensite during the entire transformation including nucleation, growth and eventually formation of internally twinned plates which are in thermoelastic equilibrium with the parent phase. No a priori constraints are made on the possible configurations and sequences of structural patterns formed by orientation variants of the martensite. 3D computer simulations are performed for a generic cubic → tetragonal martensitic transformation in a prototype crystal which is elastically isotropic and elastically homogeneous. The simulations predict that (i) nucleation of martensite in a perfect crystal occurs collectively to accommodate the coherency strain, e.g. the critical nuclei are formed by two internally twinned orientation variants; (ii) the ultimate structure consists of plate-like martensite and retaining parent phase. The martensitic plates consist of twin-related platelets of two orientation variants and the habits of the plates meet the invariant plane requirement. These simulation results are in good agreement with experimental observations.

On the nucleation and propagation of phase transformation fronts in a NiTi alloy

In this paper we present an experimental methodology for simultaneous full field monitoring of the deformation and thermal changes in NiTi during mechanically unstable regimes associated with the pseudoelastic material response. The deformation history is established by photographically recording surface changes of a brittle coating as austenite-martensite phase transition fronts traverse the specimen. Temperature changes are monitored by infrared thermal imaging. Synchronized sets of optical and thermal images and the corresponding stress histories are presented for uniaxial experiments conducted at two relatively slow end-displacement rates. The results help clarify the interactions among the prevailing heat transfer conditions, the loading rate and the fundamental sensitivity of transformation stress to temperature. It is shown that, during loading, nucleation of martensite in an austenitic region is a distinct event requiring a higher stress than the stress required subsequently to continue the transformation. By contrast, the nucleation stress of austenite in a martensitic region during unloading is lower than the stress required to continue the transformation. This distinction between the nucleation and propagation stresses, coupled with the local temperature change caused by the latent heats of the two transformations, govern the number of nucleations of a new phase. It is also shown that coexisting transition fronts tend to propagate at the same speed which is inversely proportional to the number of fronts in the specimen.

Initial stages of γ 2 precipitation in an aged Cu-Al-Ni shape memory alloy

Effect of ageing on martensitic transformation in a low-temperature Cu-Al-Ni shape memory alloy was investigated by calorimetry and transmission electron microscopy. Shifts of the transformation temperatures were followed and the microstructural observations with special attention to the early stages of precipitation of the γ 2 phase were done on samples annealed for 30 min between 175 and 325 °C. The most remarkable structural features in the as-quenched β 1 phase are prismatic dislocation loops with Burgers vectors in 〈100〉 directions, which were formed by coalescence of excess vacancies. Most probably, some off-stoichiometric Al atoms segregated to the plane of the loops during quenching, thus predetermining the places where the first coherent precipitates of the equilibrium γ 2 phase form upon ageing. Stresses and concentration changes in the close vicinity of the precipitates can facilitate the nucleation of martensite and, consequently, raise the M s temperature. However, an unambiguous explanation of the observed increase of the A s temperature has not been found. In the later stages of precipitation, rows of relatively large γ 2 particles arranged along 〈100〉 directions may act as effective obstacles to the motion of the phase interfaces and impede the martensite transformation.

Micromechanics-based modeling of two-way memory effect of a single crystalline shape-memory alloy

A micromechanics-based Helmholtz free energy is obtained. Then, in the framework of irreversible thermodynamics, a kinetic relationship, a martensitic nucleation criterion and the reorientation criterion of martensitic variants are obtained. These relationships are valid for a three-dimensional proportional or non-proportional mechanical loading or a combination of mechanical and thermal loading. The two-way memory effect is simulated and compared quantitatively with experimental results. This simulation is based on the assumption that mechanical dissipation of active variants decreases during cycling (training).

Study of the Faults Stackings in the γ(f.c.c.)-ε(h.c.p.) Martensitic Transformation

The shape memory effect exhibited by Fe-Mn-Si based alloys is due to the γ(f.c.c.)-e(h.c.p.) martensitic transformation. The martensitic transformation induced by traction at room temperature in a Fe-16Mn-9Cr-5Si-4Ni (%mass) is here studied by optical microscopy, scanning electron microscopy, transmission electron microscopy and scanning tunnelling microscopy. The martensitic microstructure and the fine structure of the faults stackings are both studied to clarify the nucleation and growth mechanisms of martensite. The band structure of the martensite is pointed out. Inside a grain, the monopartial nature of the martensite have been demonstrated from the elementary plate to all the martensite bands. From all the observations, the pole mechanism appears to be the main mechanism of martensite nucleation.

Nucleation of Martensite in CuZnAl Alloys as Investigated by Young's Modulus and Acoustic Emission Measurements

Young's modulus (E) has been measured during cooling/heating cycles carried out between a fixed upper temperature T2 (T 2 =340 K) and a gradually decreasing lower temperature T 1 (T 1 =320, 310, 300, 290, 285, 281 and 279.5 K). The E(T) curves show closed thermal hysteretic loops, which are needle-like for T 1 >Ms while they assume a downward bent shape for T 1 <Ms. The occurrence of thermal hysteresis in the cooling/heating E(T) curves is interpreted as evidence of the formation, far above Ms, of static martensite embryos, which are stabilized by preexisting lattice defects. The formation, on cooling and the delayed disappearance, on heating, of these static embryos is believed to be responsible for the needle shaped E(T) loops. Acoustic emission is found to occur only during the heating stages of partial thermal cycles carried out above Ms, suggesting that only the disappearance of a stabilized embryo is associated with an abrupt release of the stored energy.

Internal Friction Study on Manet Steel : Effects of Cooling Rate from Austenitic Domain

Snoek type Q -1 peaks observed in Fe-Cr-C alloys are related to jumps between equivalent octahedra or tetrahedra with n Cr atoms at the corners, with n up to 6 or 4. The effects of high T and cooling treatments are considered on the characteristics of the C-Cr n associates and in consequence of martensite.The results are interpreted in terms of C-Cr n associates acting as embryos for the nucleation of martensite with stability depending on tetrahedrality or octahedrality of the associates.

Modelling of austenite stability in low-alloy triple-phase steels

A model for the stability of dispersed austenite in low alloy triple-phase steels has been developed. The model was based on the dislocation dissociation model for classical heterogeneous martensitic nucleation by considering stress effects on the nucleation site potency distribution. The driving force for martensitic transformation has been calculated with the aid of computational thermodynamics. The model allows for the effects of chemical composition of austenite, mean austenite particle size, yield strength of the steel and stress state on austenite stability. Chemical enrichment in C and Mn, as well as size refinement of the austenite particles lead to stabilization. On the contrary, the increase in the yield strength of the steel and triaxiality of the stress state lead to destabilization. The model can be used to determine the microstructural characteristics of the austenite dispersion, i.e. chemical composition and size, for optimum transformation plasticity interactions at the particular stress state of interest and can then be useful in the design of low-alloy triple-phase steels.

High-resolution transmission electron microscopy investigation of the face-centered cubic/hexagonal close-packed martensite transformation in Co-31.8 wt pct Ni alloy: Part 1. Plate interfaces and growth ledges

The face-centered cubic/hexagonal close-packed (fcc/hcp) martensite transformation in a Co-31.8 wt pct Ni alloy was studied by high-resolution transmission electron microscopy (HRTEM). High-resolution transmission electron microscopy was used to study the structure and properties of growth ledges, the tips of martensite plates, and martensite nucleation sites. The HRTEM image simulations were performed in order to determine the effects of both beam and crystal tilt on the experimental images. In the investigation, it was determined that the fcc/hcp martensite transformation in Co-Ni occurs by the passage of Shockley partial dislocation ledges (b=1/6〈112〉) along every other (111) plane in the fcc matrix. The hcp martensite thickens by the lateral movement of ledges across the fcc/hcp interface. Although superledges were observed, the majority of the ledges were two (0002) planes and this is the basic ledge height. Image simulations show that both beam and crystal tilt can have a marked effect on HRTEM images of fine hcp martensite plates. The effects of tilt must be minimized in order to unambiguously resolve the interfacial structure.

Thermodynamic Interpretation of the Martensitic Transformation in Ultrafine γ-Fe(N) Particles

Face centered cubic (fcc) γ-Fe (5.9–10.4 at.%N) ultrafine particles ranging in size from 30 to 100 nm were prepared at room temperature by laser induced pyrolysis of Fe(CO)5 and NH3. It is found that these austenitic particles did not transform to martensite when subsequently quenched to 77 K or 4.2 K, which is greatly lower than the martensite-start temperature of the bulk alloy with the same composition. Many planar defects, which can serve as the favorable martensitic nucleation sites in bulk austenite alloy, have been observed in these particles. Taking into account the size effect of the ultrafine particles on the martensitic transformation, a modification for the conventional martensitic transformation has been made to interpret the stabilization behavior of the ultrafine γ-Fe(N) particles. It is indicated that the strengthening of the particles due to the reduction of the particle size and refinement of grain size may play important roles in both inhibiting the homogeneous nucleation and martensitic transformation occurred in the ultrafine particles.

Investigation on the martensitic transformation in γ-Fe(n) nanoparticles

gamma-Fe(5.90-10.9 at. % N) nanoparticles ranging in diameters from 30 to 100 nm were prepared by laser-induced pyrolysis of mixtures of Fe(CO)(5) and NH3. During the quenching from high temperature to room temperature and even down to 4.2 K, no martensitic transformation occurred in these particles. Based on the effects of surface tension and refinement of grain size on the yield strength of the nanoparticles, the dependencies of nucleation driving force and the martensite-start temperature on the particle size was revealed, from a viewpoint of thermodynamics of martensitic transformation in the gamma-Fe(N) nanoparticles. It is suggested that the yield strength increment due to the surface tension and to the very fine crystallite grains is probably the predominant aspect responsible for inhibiting the nucleation of martensite and drastic decrease of the martensite-start temperature of the nanoparticles. High pressure of 0.5 - 4.0 GPa were adopted to induce the martensitic transition of the gamma-Fe nanoparticles. It is found that the extent of martensitic transformation increased with the increasing pressures, most individual gamma-Fe particles were partially transformed to martensite after pressurization.

An atomistic simulation study of the effect of crystal defects on the martensitic transformation in Ti - V bcc alloys

An atomic level analysis of the effect of crystal defects such as free surfaces, dislocations and grain boundaries on the martensitic transformation in bcc Ti, Ti - 15V (atom %) and Ti - 25V (atom %) has been carried out using the embedded atom method (EAM) interatomic potentials to quantify the atomic interactions and molecular dynamics to study the evolution of atomic positions with time. The presence of crystal defects has been found to have a profound effect on the martensitic transformation in these alloys. The defects can either facilitate the bcchcp martensitic transformation, the transformation which is typically observed in these alloys, or promote formation of the martensitic phase with a face centred orthorhombic (fco) structure. In the case of free surfaces, the atomistic simulation results revealed the role these defects and their crystallographic characters play in the heterogeneous nucleation of martensite and their effect on the progress of martensitic transformation and the crystal structure of the martensitic phase. In Ti and Ti - 15V, where the hcp phase is thermodynamically more stable than the bcc phase, the presence of dissociate a/2 [111] edge dislocations was found to facilitate the bcchcp transformation. In Ti - 25V where the bcc phase is more stable, these dislocations give rise to the formation of dormant hcp martensitic embryos. These embryos are likely to be activated under stress or during cooling giving rise to the bcchcp transformation. The grain boundaries were found to promote formation of the fco martensite accompanied by significant reduction of the mismatch stresses in the grain boundary region.

THE OPTIMAL ORIENTATION OF MARTENSITIC NUCLEATION UNDER DIFFERENT STRESSES

The total-energy change is systematically evaluated in this paper when the thin ZrO2(T) ellipsoidal plate at different orientation transforms into ZrO2(M) under extension, compression and shear stresses. The optimal orientation of Martensitic uncleation is determined by using the oriterion of minimum energy. The numerical results show that the shear strains resulting from the transformation affect obviously the orientation of Martensitic nucleation and the critical stresses inducing transformation are very different.

The Effect of Nitrogen on Martensite Formation in a Cr-Mn-Ni Stainless Steel

The influence of nitrogen (0 to 0.27 wt%) on martensite formation in an experimental low-nickel stainless-steel alloy (Fe-17Cr-7Mn-4Ni) has been investigated. The alloys containing 0.1 wt% or more nitrogen are fully austenitic at room temperature ; those containing less nitrogen consist of a mixture of austenite, martensite and δ-ferrite. The alloys containing less than 0.2 wt% nitrogen are metastable and undergo a transformation from austenite to martensite on deformation. Transmission electron microscopy investigations suggest that, within the nitrogen range considered in this investigation, the addition of nitrogen causes an increase in stacking fault energy which in turn inhibits the nucleation of martensite. As the low-nitrogen alloys (less than 0.2 wt% nitrogen) undergo deformation, e-martensite (with the [110] γ and [1210] e zone axes parallel) is observed at the intersection of stacking faults. With increasing strain, the presence of α'-martensite is observed in conjunction with the e-martensite, and only α'-martensite is observed at very high strains. Both the Nishiyama-Wasserman and Kurdjumov-Sachs orientation relationships are observed between austenite and α'-martensite. The transformation to martensite during deformation causes a significant variation in room-temperature mechanical properties, despite the overall narrow range in composition considered.

Potency of Grain Boundaries as Martensitic Nucleation Sites

The potency of grain boundaries as martensitic nucleation sites has been investigated by measuring the Ms temperatures of bicrystal specimens in an Fe-32%Ni alloy. The average Ms temperature of single-crystal specimens was 88K, while that of polycrystalline specimens was 154K. The free surfaces did not have high potency as nucleation sites, and the higher Ms temperature in the polycrystalline specimens was attributed to the existence of certain kinds of grain boundaries. The Ms temperatures of five kinds of bicrystal specimens having one of annealing twin boundaries and high-angle random boundaries were equivalent to that of the single-crystal specimens and lower than that of the polycrystalline specimens. These results indicate that both the annealing twin boundaries and the high-angle random boundaries do not have high potency as martensitic nucleation sites, and that the most potent nucleation site which defines the Ms temperature of the polycrystalline specimens is attributed to other kinds of grain boundaries.

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.