Martensitic Transformation In Ti Research Articles

Transformation Behaviors and Microstructure Modifications in Hydrogen-Charged Ti–Ni Shape Memory Alloy

AbstractThe effect of hydrogen charging duration on the transformation behavior, microstructural evolution, and dynamic microstructural changes associated with thermoelastic martensitic transformation in Ti–Ni shape memory alloy was investigated. Compared with the uncharged specimen, the martensitic transformation start (Ms) and reverse transformation finish (Af) temperatures increased with charging time, whereas the martensitic transformation finish and reverse transformation start temperatures remained almost unchanged. In situ SEM results were consistent with these behaviors. Upon cooling, the transformation progressed from the center to the surface in charged specimens, indicating a higher transformation temperature in the center than the surface. The latent heat of transformation decreased with increasing charging time, quantitatively attributed to an untransformed region consisting of hydrogen-induced martensite and a hydrogen-affected layer. The hydrostatic effect from those layers on the interior B2 phase was proposed as the origin of the increased Ms and Af temperatures.

Phase field simulation of martensitic transformation in Ti–24Nb–4Zr–8Sn alloy

A phase field model for cubic to orthorhombic martensitic transformation (MT) at the nanoscale in a β titanium (Ti) alloy Ti–24Nb–4Zr–8Sn (in wt.%) is investigated by finite element simulation. The approach is based on phase field theory, time-dependent Ginzburg-Landau theory, and mechanical equilibrium equations. Partial differential equations (PDEs) were solved using the commercial software COMSOL Multiphysics. The morphology of the product phase exhibits plate-like or needle-like shapes that reduce the elastic strain energy of the system. The simulation result for random initial order parameters is in agreement with previous experimental observations. The final volume fractions of two different orthorhombic martensitic variants are not dependent on the initial conditions.

The effect of quench rate on the β-α″ martensitic transformation in Ti–Nb alloys

While Ti–Nb binary alloys have been extensively studied over the years, there are discrepancies in the literature relating to the β-α″ martensitic transformation that indicate an incomplete understanding. In particular, there are inconsistencies regarding the phase constitution of as-quenched material and the ability for α″ phase to be thermally-induced. To resolve these issues, the β-α″ phase transformation is studied in a Ti–24Nb (at.%) alloy subjected to two different water quench conditions. It is demonstrated that only the fastest quench rates (≳500 °C/s) result in the formation of α″ phase, and these materials exhibit a reversible thermally-induced β-α″ transformation at low temperatures. In contrast, slightly slower water quench procedures lead to a β+ω microstructure, and these samples do not exhibit a thermally-induced β-α″ transformation, even when cooled to −168 °C. Despite this, room temperature mechanical testing results indicate that α″ can be induced by stresses as low as 200 MPa. To explain these results, it is proposed that quenching stresses play a role in the competition between α″ and athermal ω phases at rapid quench rates. The present results are then reconciled with the body of literature and a broader understanding of phase stability in metastable β-titanium alloys.

The microstructure and martensitic transformation of Ti–V–Al–B elevated temperature shape memory alloy tailored by thermo-mechanical treatment

In the present work, the mechanical and strain recovery characteristics of Ti–V–Al–B shape memory alloy were tailored by changing annealing temperatures. Moreover, effect of annealing temperatures on microstructure and martensitic transformation behavior were investigated systematically. At the annealing temperatures of 650 °C and 700 °C, both α" martensite phase and hard α phase coexisted in the annealed Ti–V–Al–B alloys. With the annealing temperatures increasing, amount of α phase gradually decreased and α" martensite phase was dominated. The reduction of α phase resulted in the compositional variation, which further triggered the rising of transformation temperatures. The elongation of annealed Ti–V–Al–B alloys continuously increased with the annealing temperatures increasing; whereas the yield strength gradually decreased. The mechanical properties were closely related to the amount of α precipitates and dislocations. Under optimum annealing condition of 700 °C, the maximum yield strength of 489 MPa can be achieved in Ti–V–Al–B alloy. Meanwhile, it showed a largest recoverable strain of 4.6% in correspondence to 6% applied pre-strain.

Significantly fast spinodal decomposition and inhomogeneous nanoscale martensitic transformation in Ti–Nb–O alloys

The β phase spinodal decomposition during continuous cooling in Ti‒Nb‒O alloys is investigated by the phase-field method. Addition of only a few at.%O to Ti‒23Nb (at.%) alloy remarkably increases the driving force of the β phase spinodal decomposition. During isothermal heat treatment at 1000 K and 1100 K in Ti‒23Nb‒3O (at.%) alloy, the β phase separates into β1 phase denoted as (Ti)1(O, Va)3 and β2 phase denoted as (Ti, Nb)1(Va)3, resulting in the formation of nanoscale concentration modulation. The phase decomposition progresses in 0.3‒20 ms. In Ti‒23Nb‒XO alloys (X = 1.0, 1.2, 2.0), the spinodal decomposition occurs during continuous cooling with the rate of 500 K s‒1, indicating that the spinodal decomposition occurs during water quenching in the alloys. It is assumed that there is a threshold value of oxygen composition for inducing the spinodal decomposition because it does not occur during continuous cooling in Ti‒23Nb‒0.6O (at.%) alloy. The concentration modulation introduced by the β phase decomposition has significant effect on the β→α” martensitic transformation. Hence, it seems that for controlling microstructure and mechanical properties of Ti‒Nb‒O alloys, careful control of heat treatment temperature and cooling rate condition is required.

On the Oxidation Behavior and Its Influence on the Martensitic Transformation of Ti–Ta High-Temperature Shape Memory Alloys

In the present work, the influence of oxidation on the martensitic transformation in Ti–Ta high-temperature shape memory alloys is investigated. Thermogravimetric analysis in combination with microstructural investigations by scanning electron microscopy and transmission electron microscopy were performed after oxidation at 850 °C and at temperatures in the application regime of 450 °C and 330 °C for 100 h, respectively. At 850 °C, internal oxidation results in the formation of a mixed layered scale of TiO2 and β-Ta2O5, associated with decomposition into Ta-rich bcc β-phase and Ti-rich hexagonal α-phase in the alloy. This leads to a suppression of the martensitic phase transformation. In addition, energy dispersive X-ray analysis suggests an oxygen stabilization of the α-phase. At 450 °C, a slow decomposition into Ta-rich β-phase and Ti-rich α-phase is observed. After oxidation at 330 °C, the austenitic matrix shows strong precipitation of the ω-phase that suppresses the martensitic transformation on cooling.

Investigations on Electronic and Magnetic Properties along with Martensitic Transformation in Ti2CoGa1-xCux Alloy

Investigations on Electronic and Magnetic Properties along with Martensitic Transformation in Ti₂CoGa_1-xCu_x Alloy

Phase-field simulation of spinodal decomposition and its effect on stress-induced martensitic transformation in Ti–Nb–O alloys

In this study, the phase-field method was used to simulate the spinodal decomposition of the β phase in Ti–23Nb–XO (atomic percent (at.%)) alloys (X = 1, 2, 3, 4, and 5) at 1073 K and the subsequent stress-induced β → α” martensitic transformation (MT) at 300 K. At 1073 K, the β phase separates into an O-rich β1 phase and an O-lean β2 phase in some Ti–23Nb–XO alloys (X = 2, 3, 4, and 5). The tendency toward phase decomposition increases with an increase in oxygen content. When an external tensile stress of 240–270 MPa is applied to certain Ti–23Nb–XO alloys (X = 1, 2, and 3) along the [0 1 1]β direction at 300 K, a stress-induced β → α” MT occurs. In Ti–23Nb–XO alloys (X = 4 and 5), the β1 → α” MT first occurs at 50–110 MPa and the nanoscale α” phase with a size of around 2 nm is formed, reflecting the (β1 + β2) microstructure. This change is followed by a β2 → α” MT at 660 MPa in Ti–23Nb–4O alloy; however, the β2 → α” MT does not occur even with an external tensile stress of 800 MPa applied along the [0 1 1]β direction in Ti–23Nb–5O alloy. Further, the calculated hysteresis loop of the stress–strain (S–S) curve at 300 K becomes slim as the oxygen content increases. It is therefore concluded that the S–S characteristics of Ti–Nb–O alloys are strongly influenced by the (β1 + β2) microstructure, which gives rise to a nanoscale heterogeneity of the driving force for the β → α” MT.

First principles calculations on the effect of interstitial oxygen on phase stability and β–α″ martensitic transformation in Ti–Nb alloys

The effect of oxygen on phase stability and β–α″ martensitic transformation in Ti–Nb alloys has been studied using first principles calculations. Three stable atomic configurations of Ti–Nb (Ti-12.5, 16.6, and 25 at.% Nb) systems, which can transform from β-phase to α″-phase without changing the local atomic position of Nb atoms, were first identified using the cluster expansion method. Phonon calculations indicated that these structures were stable. Next, the martensitic transformation behavior of Ti–Nb–O system was studied using these structures. We observed a significant lattice distortion around oxygen atoms occupying octahedral interstitial sites that resembles a bcc type of stacking. Our results conclusively revealed that while the oxygen interstitials can oppose the atomic shuffle required for martensitic transformation, they can also cooperatively stabilize the β-phase even at 1 at.% oxygen concentrations by inducing local elastic shear strains. Interestingly, the canceling of these fields can stabilize the β-phase by suppressing the β to α″ transformation which decreases the martensitic start temperature (Ms). Our study revealed that the reduction in Ms is higher at lower Nb concentration. The stabilization of β-phase increases with oxygen concentration.

In situ scanning electron microscopy study of the thermoelastic martensitic transformation in Ti–Ni shape memory alloy

The thermoelastic martensitic transformation from the B2 to B19′ structures in the Ti–Ni shape memory alloy was observed by in situ scanning electron microscopy in order to investigate the self-accommodation microstructure. Nucleation of the V-shaped habit plane variant cluster (2HPVC) triggers the formation of the hexangular HPVC (6HPVC) that is formed by six habit plane variants in the grain interior. The triangular morphology (3HPVC) is a derivative of 2HPVC. The autocatalytic transformation spreads from the 6HPVCs to the grain boundary and the reverse transformation proceeds like a rewind of the forward transformation. 2HPVC and 6HPVC are the most favorable morphologies for nucleation and growth, respectively, as explained by the analysis of the kinematic compatibility at junction planes.

In-situ investigation of stress-induced martensitic transformation in Ti–Nb binary alloys with low Young's modulus

Microstructure evolution, mechanical behaviors of cold rolled Ti–Nb alloys with different Nb contents subjected to different heat treatments were investigated. Optical microstructure and phase compositions of Ti–Nb alloys were characterized using optical microscopy and X-ray diffractometre, while mechanical behaviors of Ti–Nb alloys were examined by using tension tests. Stress-induced martensitic transformation in a Ti–30at%Nb binary alloy was in-situ explored by synchrotron-based high-energy X-ray diffraction (HE-XRD). The results obtained suggested that mechanical behavior of Ti–Nb alloys, especially Young's modulus was directly dependent on chemical compositions and heat treatment process. According to the results of HE-XRD, α″-V1 martensite generated prior to the formation of α″-V2 during loading and a partial reversible transformation from α″-V1 to β phase was detected while α″-V2 tranformed to β completely during unloading.

Crystallography and morphology of antiphase boundary-like structure induced by martensitic transformation in Ti–Pd–Fe alloy

The antiphase boundary (APB)-like structure of both 9R and B19 martensites in the Ti–Pd–Fe alloy was investigated by means of transmission electron microscopy. Some APB-like structures with curved and wide contrasts along the (001)9R basal plane are observed in 9R martensitic plates. The atomic displacement on the APB-like structure reflects the atomic movement stemming from the microdomains formed as a pre-martensitic transformation. The displacement vector of the APB-like structure in the B19 martensite can be expressed as R=〈1/30−1/2〉B19. The density of APB-like contrasts increases by the substitution of Fe for Pd in Ti–Pd–Fe alloy.

Thermodynamic Basis of Non-equilibrium Phase Transformations of bcc β-Phase in Ti–Mo System

This study reports the experimental identification of transformation products of high temperature bcc β phase in Ti–xMo (x = 1, 7, 15, 25 wt%) alloys and aims to understand the transformations by thermodynamic modelling. The high temperature bcc β phase had undergone martensitic transformation in Ti–1Mo and Ti–7Mo alloys, resulting in acicular martensitic structure within large β grains. X-ray diffraction (XRD) analysis confirmed the martensite to be hcp (α′) and orthorhombic (α″) in Ti–1Mo and Ti–7Mo alloys respectively. Combined analysis of XRD and transmission electron microscopy (TEM) suggested the formation of fine plates of α″, omega (ω) and bcc β phases in Ti-15 and Ti-25 Mo alloys. Calculation of enthalpy of formation supported the stability of solid solution phase over the amorphous phase in the entire concentration range of Mo.

Molecular dynamics investigation on the deviation from stoichiometry in martensitic transformation

Abstract How the martensitic transformation in Ti–Ni alloy depends on the deviation from the stoichiometry is investigated by use of the molecular dynamics based on a simple model potential.

Enhancement of athermal α″ martensitic transformation in Ti–10V–2Fe–3Al alloy due to high-speed hot deformation

This work examines the origin of the athermal α″ martensitic transformation in Ti–10V–2Fe–3Al alloys arising from high-speed hot deformation on the basis of chemical thermodynamics and the change in stored energy introduced by deformation. The value of the martensitic transformation starting temperature (Ms) is strongly affected by various microstructural factors. This work reveals that a high accumulation of dislocation density, greater than 1015 m−2, arising from high-speed deformation, resulted in an enhancement of athermal α″ martensitic transformations.

Determining transformation temperatures of equiatomic TiNi shape memory alloy by dynamic mechanical analysis test

Abstract Strain variation induced by martensitic transformation of Ti 50 Ni 50 shape memory alloy (SMA) is investigated by dynamic mechanical analysis (DMA) test. The extent of the positive and negative strain variation is mainly associated with the softening and hardening rate of storage modulus E 0 , respectively. The occurrence of the strain variation is related to DMA instrument design since the stress accommodation cannot keep up with the drastic change of the E 0 softening/hardening rate during transformation. Strain variation is very sensitive and can rapidly respond to the E 0 change, thus martensitic transformation temperatures of SMAs associated with a multi-stage transformation can be determined precisely from the onset/finish of the strain variation.

Influence of annealing on martensitic transformations in porous TiNi-based alloys produced by self-propagating high-temperature synthesis

Abstract Porous TiNi alloys produced by self-propagating high-temperature synthesis (SHS) are characterized by complex phase composition and the nonuniform distribution of the Ni concentration. Thus, the small volume fraction of the alloy is able to undergo a martensitic transformation that results in a poor demonstration of shape memory effects and pseudoelasticity. To improve the functional properties of porous TiNi alloys it is necessary to modify the crystalline structure of the alloy and to increase the volume fraction of the alloys undergoing phase transitions. The main focus of the present work is the influence of annealing on the crystalline structure and kinetics of martensitic transformation in porous TiNi alloys of different compositions. The mixtures of Ti–45 at.% Ni, Ti–48 at.% Ni, Ti–50 at.% Ni and Ti–52 at.% Ni were used for the preparation of TiNi alloys by SHS. All samples were subjected to annealing at temperatures of 300, 400 and 500 °C for 30 min, 60 min, 120 min and 240 min. It was found that annealing practically did not influence the crystalline structure and kinetics of martensitic transformation in Ti–45 at.% Ni alloy. At the same time an increase in a duration of annealing resulted in increase in the volume fraction of the Ti 3 Ni 4 particles and influenced strongly the kinetics of phase transitions in the TiNi alloys with the Ni concentration from 48 at.% to 52 at.%.

Analysis of Ti–Ni–Hf shape memory alloys by combinatorial nanocalorimetry

The martensitic transformation in Ti–Ni–Hf thin films with ultra-fine grain structure has been analyzed as a function of composition using a high-throughput array of nanocalorimeters. The martensite–austenite transformation temperature is significantly lower than in bulk Ti–Ni–Hf, but increases linearly with Hf content at a rate comparable to bulk Ti–Ni–Hf. The response to high-temperature cycling (22°C<T<850°C) changes with Ni concentration. For Ni⩽47 at.%, the transformation temperature increases during high-temperature cycling because precipitation of (Ti1−x, Hfx)2Ni enriches the surrounding matrix in Hf; for Ni⩾47.7 at.%, precipitation of the same phase gradually suppresses the transformation. Low-temperature cycling (22°C<T<450°C) causes the transformation temperature to initially decrease and then stabilize. Relaxation of internal stresses by dislocations generated during thermal cycling is suggested as the active mechanism. Thermal cycling stability of the films is improved compared to previous studies on bulk Ti–Ni–Hf. This is attributed to the very small grain size (18±5nm) of the samples. Alloys with superior thermal cycling stability are identified and the ability to control the transformation temperature through multiple thermal cycling is demonstrated.

Influence of α morphology and volume fraction on the stress-induced martensitic transformation in Ti–10V–2Fe–3Al

This paper reports an investigation into the influence of α morphology and volume fraction on the critical stress levels for the stress-induced martensitic transformation in Ti–10V–2Fe–3Al. Control of the stress-induced martensite transformation offers a new route to the optimization of the load bearing capabilities of titanium alloys. Various heat treatments in α + β and β + (α + β) conditions were imposed to study their effect on both microstructure and mechanical properties. The results show that in samples with globular α, the β matrix stability is the main factor controlling the stress-induced martensitic transformation. For samples with acicular α, the “grain size” (i.e. the size of the retained β domain), which governs the mean length and the width of martensitic plates, also plays an important role in such transformations. The linear combination of both effects provides a good prediction of the stress-induced martensite formation stress as a function of the volume fraction and morphology of the α phase.

Interfacial microstructure and strength of the dissimilar joint Ti 3Al/TC4 welded by the electron beam process

Dissimilar weld joints in Ti 3Al/TC4 were welded using the electron beam (EB) process. The microstructure evolution characterizations of the joints was investigated by means of OM, SEM, XRD, TEM and the tensile strengths of the joints were tested. The microstructure of the weld metal of every joint was identical. The structures were characterized by martensite, appearing coarse equiaxed grains. There existed grain coarsening and martensitic transformation in Ti 3Al/HAZ and TC4/HAZ. With the increase of heat input, the grain size was significantly raised, yet the composition of the weld metal was independent of heat input. The highest tensile strength of the joints can reach 831 MPa, equaled almost to 92% of that of Ti 3Al-based alloy.