Martensitic Transformation In Alloys Research Articles

Relaxation Rotation during Martensitic Transformation in Alloys with Thermoelastic and Nonthermoelastic Martensite

Relaxation Rotation during Martensitic Transformation in Alloys with Thermoelastic and Nonthermoelastic Martensite

Relaxation Rotation during Martensitic Transformation in Alloys with Thermoelastic and Nonthermoelastic Martensite

It is shown that application of the crystallographic theory of martensitic transformation instead ofthe phenomenological one allows proper understanding of the mechanism of martensitic transformation incrystallographic analysis and computing, in addition to the previous characteristics, a new important characteristic,that is, the relaxation rotation of martensite crystallites to obtain the invariant plane. It is demonstratedthat the angle of relaxation rotation is most strongly dependent on the crystallographic systems of the original and final phases, as well as on the lattice parameters of these phases. It has been discovered that scatteringof the texture peaks of martensite, determined as the integral half-width of the texture maximum, is in linear proportion to the angle of relaxation rotation of martensite crystals.

Influence of Severe Plastic Deformations on Martensitic Transformation in Alloys of Fe-Cr-Ni System

The article investigates influence of large plastic deformations created in the Bridgman anvils on the mechanical properties and microstructure of metastable alloys of the iron-chromium-nickel system of the austenitic-martensitic class. It is found that the application of deformation in the Bridgman anvils with a true logarithmic deformation e = 6-7 leads to formation of the α-phase (deformation martensite) with significantly higher mechanical properties compared to martensite formed during deformation by linear rolling. The authors reveal the differences in the microstructure of the alloy after deformation in the Bridgman anvils and deformation by rolling, which explain the discovered effect.

Spinodal decomposition and martensitic transformation of the high manganese Mn‒xCu alloys fabricated by additive manufacturing

Mn‒Cu-based shape memory alloys have great potential owing to their excellent shape memory properties, high damping capacity, low production cost, and ease of fabrication. In this study, a selective laser melting (SLM) technique was used to design and manufacture a series of high-manganese Mn‒xCu (x=10–40 wt.%) alloys by blending the elemental powders. Spinodal decomposition and martensitic transformation of the alloys during SLM and heat treatment (HT) processes were investigated and correlated with the tensile, damping, and shape memory properties of the alloys. The results showed that γ‒(Mn, Cu) was the main constituent phase in all of samples subjected to SLM only (hereafter as-SLMed) and the SLMed + HTed Mn‒Cu alloys. The effect of the chemical composition and manufacturing process on the phase structural evolution of the Mn‒Cu alloys were monitored. The spinodal decomposition, martensitic transformation and α-Mn precipitation reactions occurred in the as-SLMed Mn‒xCu samples with x = 15–20 wt.%, x < 30 wt.%, and x < 15 wt.%, respectively. The tensile properties of the as-SLMed Mn‒xCu alloys followed a general downward trend with increasing Mn content, while the corresponding damping capacity and shape memory recovery strain showed the opposite trend. HT can promote spinodal decomposition and martensitic transformation to form an equiaxed γ‒(Mn, Cu) grain, parallel twin plate structure, tweed microstructure, and α-Mn precipitate in the alloys. The SLMed + HTed Mn‒30% Cu alloy displayed a better combination of tensile properties (764, 540 MPa, and 12.5%), damping capacity (tan δ = 0.072 at a strain amplitude of 900 × 10−6), and one-way and two-way (εsme = 1.6% and εtw = 0.7% under the pre-bending strain of 10%) shape memory properties. In addition, the performance of the as-SLMed Mn‒xCu (x = 15–20 wt.%) and SLMed + HTed Mn‒xCu (x = 30 wt.%) alloys is comparable with and even superior to those of multi-component and commercial Mn‒Cu-based alloys, respectively.

First-principles characterization of reversible martensitic transformations

Reversible martensitic transformations (MTs) are the origin of many fascinating phenomena, including the famous shape memory effect. In this work, we present a fully ab initio procedure to characterize MTs in alloys and to assess their reversibility. Specifically, we employ ab initio molecular dynamics data to parametrize a Landau expansion for the free energy of the MT. This analytical expansion makes it possible to determine the stability of the high- and low-temperature phases, to obtain the Ehrenfest order of the MT, and to quantify its free energy barrier and latent heat. We apply our model to the high-temperature shape memory alloy Ti-Ta, for which we observe remarkably small values for the metastability region (the interval of temperatures in which the high-and low-temperature phases are metastable) and for the barrier: these small values are necessary conditions for the reversibility of MTs and distinguish shape memory alloys from other materials.

Elastically driven metamagnetic-like phase transformations of shape memory alloys

A theoretical model of metamagnetic-like (ferromagnetic–paramagnetic and ferromagnetic–antiferromagnetic) phase transitions is developed for the interpretation of experimental results obtained recently for the Ni–Mn–Co–X (X = In, Sn, Ga) shape memory alloys. The conditions of elastically driven (caused by the martensitic transformation of alloy) metamagnetic-like phase transitions are determined. These conditions are: high magnetic susceptibility of paramagnetic/antiferromagnetic phase; large (but real for some alloys) volume change during the martensitic transformation; and large value of volume magnetostriction caused by the metamagnetic-like phase transition. The magnetoelastic mechanism is proposed for the explanation of magnetic field influence on the martensitic transformation. The elastically driven ferromagnetic–paramagnetic phase transition is considered in more detail and the results of corresponding magnetic measurements are described.

Effects of second phases on mechanical properties and martensitic transformations of ECAPed TiNi and Ti–Mo based shape memory alloys

TiNi and Ti-based shape memory alloys were processed by equal channel angular pressing (ECAP) at 673-773 K along Bc route to obtain ultrafine grains for increasing the strength of parent phase and improving the functional properties. The effects of both thermodynamically stable and metastable second phases on the mechanical properties and martensitic transformations of these alloys were investigated. It is found that thermodynamically stable Ti2Ni phase has no effect on martensitic transformation and superelasticity of Ti-rich TiNi alloy, thermodynamically stable α phase is harmful for ductility of Ti–Mo–Nb–V–Al alloy, but metastable Ti3Ni4 phase is effective for R phase transformation, martensitic transformation and superelasticity of Ni-rich TiNi alloy. The mechanisms of the second phases on the martensitic transformations and mechanical properties were discussed.

Structurally inhomogeneous fct-martensite in alloys based on the Mn – Cu system: structure, strain mechanism and strength characteristics

The conditions of occurrence of high-temperature fcc →fct martensitic transformation in alloys of the Mn – Cu system containing two isomorphic nanocrystalline γ1 and γ2 phases with high and low manganese content, respectively, are considered. The structure formed as a result is studied. The mechanisms of straining of the inhomogeneous fct-martensite and the factors determining its strength properties are determined.

Model of the B2 → R martensitic transformation in alloys with a B2 superstructure

Various B2 → R structural transformations that can occur through an intermediate R0 structure belonging to the space group D3d3 have been considered. Subgroups with a hexagonal lattice (group D3d3) related to the symmetry breakdown of the R0 structure due to atomic displacements with wave vectors belonging to the two-arm star \(K_{13}^H = \left\{ {k_1^H = \frac{1} {3}\left( {b_1^H + b_2^H } \right),k_2^H = - k_1^H } \right\}\), where b1H and b2H are the reciprocal-lattice vectors of the hexagonal system, have been calculated. A crystallographic model of the possible transformations of the B2 structure into the R martensite, which has a single (common for all transformations) order parameter, has been suggested.

Physical nature of martensite transformations in B2-type Ti compounds and TiNi-based alloys

Thermoelastic martensite transformations in alloys based on titanium nickelide are a complex hierarchical process occurring at different structural scale levels. The feasibility and temperature of these transformations are determined in many respects by peculiarities of the electronic structure of TiNi-based alloys. In the review, we analyze the nature of structural instability of the high-temperature B2-phase in TiNi-based alloys and B2-type Ti intermetallic compounds with resort to data of our experimental electronic structure research and energy-band structure calculations. It is shown that the concept of martensite transformations in TiMe intermetallic compounds as having their origin in tendency of titanium to polymorphic transformation in the lattice of the compounds makes it possible to formulate criteria to control the temperature of martensite transformations. Electronic peculiarities responsible for precursor effects in the alloys are analyzed.

Crystal-structural features of pretransition phenomena and thermoelastic martensitic transformations in alloys of nonferrous metals

The unusual and practically important phenomena and physical properties that are caused by reversible martensitic transformations occurring mainly in alloys and compounds of nonferrous metals have been analyzed. The main types of martensitic transformations, their crystal-geometrical features and structure-morphological characteristics, the nature and structural mechanisms of reversibility, and the phenomenon of thermoelasticity have been considered. Structural mechanisms of atomic rearrangements upon martensitic transformations have been discussed, and premartensitic phenomena and their relation to martensitic transformations have been analyzed.

Effect of a magnetic field on the reverse and forward martensitic transformations in the Fe-33% Ni alloy

Effect of a magnetic field on the reverse and forward martensitic transformation in an iron-nickel alloy with the temperature of the start of the α-γ transformation that is close to the Curie temperature of the γ phase has been studied. The result obtained is explained by the mutual compensation of two mechanisms of the effect of a magnetic field on the reverse martensitic transformation in this alloy. The role played by the magnetic state of austenite in the realization of the corresponding kinetics of the martensitic transformation in the iron-based alloys with a mixed kinetics is discussed.

Martensitic transformation in alloys of the Cu – Al – Co system

Alloys of the Cu - Al - Co system with shape memory effect in martensitic transformation are studied. The temperatures of the occurrence of martensitic transformation and the phase compositions of heterogeneous cast and hardened alloys are determined.

Nanoscopic size effects on martensitic transformations in shape memory alloys

The effect of the grain size and transverse film size in nano-and micrometer ranges on the parameters of martensitic transitions in shape memory alloys is theoretically considered in the framework of the theory of diffuse martensitic transitions. A quantitative analysis of the size effects is performed including not only the thermodynamic aspect of the martensitic transformation but also its kinetic aspect, which is particularly sensitive to structural and size factors. This complex approach makes it possible to explain the following three basic facts associated with the influence of a decreased grain size or transverse film size on the parameters of the martensitic transition in shape memory alloys: a decrease in the critical (characteristic) transition temperature, an increase in the transition temperature smearing, and the existence of a critical grain size or film thickness below which the martensitic transformation in alloys is blocked.

Stabilization of high-temperature shape memory effect in functional Ni–Al–Co martensitic alloys

Resistometry and electron microscopy were used to study Ni 65Al 35 and Ni 56Al 34Co 10 (at.%) alloys prepared by the method of accelerated melt spinning. Ribbon samples were studied during and after different heating–cooling cycles or different isothermal holding conditions at temperatures of up to 780 °C. A method was proposed for improvement of the heat resistance of the supersaturated β solid solution and stabilization of the reversibility of the high-temperature martensitic transformation in alloys of the Ni–Al system. This method consists in a partial replacement of nickel by cobalt, accelerated melt spinning, and short-time stabilization annealing in the low-temperature range of the B2 austenite.

Martensitic transformation in Ni–Fe–Ga alloys

Fe-based ferromagnetic shape memory alloys have attractive mechanical properties as compared with the classical Ni 2MnGa ones, because of their enhanced ductility. This opens new possibilities regarding technical applications as magnetic sensors or actuators. In this work, we present a multiple technique study of the martensitic transformation in alloys with nominal composition Ni 55− x Fe 19+ x Ga 26 ( x = 0, 1, and 2) transforming close to room temperature. Magnetic characterisations, performed in a SQUID magnetometer and by AC methods show the Curie temperature ( T C ≈ 300 K) increasing with Fe content while the martensitic temperature ( T M ≈ 240 K) decreases, in agreement with DSC and resistivity measurements. In addition, several Curie temperatures are found by the magnetic measurements, indicating the coexistence of different phases in the alloy. Neutron diffraction study suggests the L2 1-ordered cubic phase for austenite and 14 M structure for the martensitic phase. The transformation is observed from the temperature evolution of the diffraction peaks.

Ferromagnetism in the austenitic and martensitic states ofNi−Mn−Inalloys

The magnetic and structural transformations in the Heusler-based system ${\mathrm{Ni}}_{0.50}{\mathrm{Mn}}_{0.50\ensuremath{-}x}{\mathrm{In}}_{x}$ are studied in the composition range $0.05\ensuremath{\leqslant}x\ensuremath{\leqslant}0.25$. While the cubic phase is preserved in the range $0.165\ensuremath{\leqslant}x\ensuremath{\leqslant}0.25$, we find the presence of martensitic transformations in alloys with $x\ensuremath{\leqslant}0.16$. In a critical composition range $0.15\ensuremath{\leqslant}x\ensuremath{\leqslant}0.16$, the magnetic coupling in both austenitic and martensitic states is ferromagnetic. Magnetic field-induced structural transitions are also found in the $x=0.16$ alloy, whereby the structural transition temperature shifts by $42\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in a field of $50\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$.

Acoustic Emission in Martensitic Transformations in Alloys under Mechanical Loading

In the alloys undergoing thermoelastic martensitic transformations under mechanical loading, accumulation and recovery of deformation is observed. The acoustic emission accompanying the martensitic transformations is a reflection of peculiar features of deformation-induced behavior of an alloy in the course of its transformation. The anomalous acoustic effect is correlated with the buildup of reversible deformation but is not so with that of the irreversible deformation.

Effect of Copper and Heat Treatment on the Martensitic Transformation in Alloys of the Ti – Ni – Cu System

Effect of Copper and Heat Treatment on the Martensitic Transformation in Alloys of the Ti – Ni – Cu System

Structure and properties of high-temperature alloys with the effect of shape memory in the system Cu−Al−Nb

Martensite transformation in alloys of the system Cu−Al−Nb occurs at temperatures much above 200°C. The hysteresis of the reversible martensite transformation is 108–128°C, which is less than for alloys of the system Cu−Al−Ag [3]. Alloys of the system Cu−Al−Nb possess a high effect of shape memory and an elevated ductility and strength. The degree of recovery of shape for an alloy with 2.56% Nb exceeds 90% at σГ MPa and δ20=12.7%