Reverse martensitic phase transformation induced in Nb–Co multilayers by ion irradiation

Employing ab initio calculations, the relative stabilities of the possible crystalline structures at compositions of CoTa 3 , Co 3 Ta and CoTa were predicted and based on thermodynamic calculations, the possible structural phase transitions were predicted over a broad composition range for the metastable crystalline Co–Ta alloys. To confirm the relevance of the calculations, ion-beam-mixing experiments were conducted using Co–Ta multilayered films with various overall compositions. It turned out that the experimentally observed formation of metastable crystalline Co–Ta phases agreed reasonably well with the theoretical predictions and that the glass-forming range of the Co–Ta system was determined to be within 25–70 at. % of Ta, matching fairly well with that estimated from thermodynamic calculations. Moreover, a reverse martensitic phase transformation was observed in the CoTa 2 multilayers upon ion irradiation. In addition, steady-sate thermal annealing of the Co–Ta multilayered films also provided some...

Nano-twinned Fe-Mn alloy prepared by reverse martensitic phase transformation

Reversible martensitic phase transformations in a partially disordered Ni–Al alloy within the composition range from 60to65at.% of Ni are investigated using molecular dynamics simulation. During a complete temperature cycle a wide hysteresis in enthalpy, volume, and shape of the simulated crystals is observed. The temperature T0 of the phase transformation is found from the calculated free energy evolution. To investigate the atomic-scale development during the phase transformation a local order parameter is defined which is based on a combined method of Voronoy tessellation [J. Reine Angew. Math. 134, 198 (1908)] with common-neighbor analysis. This local order parameter allows us to get a detailed localized picture of nucleation and growth of the new phases. Both homogeneous formation of the new phase and heterogeneous nucleation are observed. The velocity of new phase growth front is estimated.

When uranium-niobium alloys containing between 13.9 and 17.9 at. pct Nb are quenched to room temperature from the BCC (γ) phase at elevated temperatures, diffusion-controlled precipitation of the equilibrium phases is prevented and martensitic transformations to transition phases occur instead. Dilatometry was used to detect transformation temperatures and with the help of X-ray diffraction analysis, a metastable phase diagram was established. At room temperature after quenching, alloys containing < 15.2 at. pct Nb were monoclinic (α″) and those with < 16.6 at. pct Nb were tetragonal (γ°). The deformation behavior and shape memory effects (SME) accompanying the reverse martensitic phase transformations in polycristalline specimens were surveyed and characterized phenomelogically. From uniaxial tensile tests at room temperature, macroscopic stress-strain parameters, associated with the reversible deformation modes in the α″ and γ° martensites, were defined and their composition and structural state dependencies delineated. A diffuse maximum manifested in the stress-strain diagrams was identified with the reversible strain limit, which varied inversely and continuously with composition. A concentration-independent value of 693 MPa was found for the plastic yield strength of the alloys. All the alloys exhibited heat-activated shape recovery but the degree depended on structural state and composition. The α″ alloys showed a much larger effect than γ° alloys. Shape recovery occurred in two stages in all alloys. The first stage of recovery accompanied martensite reversion but final reversion to the equilibrium y phase was not accomplished until much higher temperatures were reached. Rapid, low temperature aging reactions were thought to affect the finish of shape recovery and delay it to higher temperatures.

We report the discovery of a rigorous nucleation mechanism for {101¯2} twins in hexagonal close-packed (hcp) magnesium through reversible hcp-tetragonal-hcp martensitic phase transformations with a metastable tetragonal phase as the intermediate state. Specifically, the parent hcp phase first transforms to a metastable tetragonal phase, which subsequently transforms to a twinned hcp phase. The evanescent nature of the tetragonal phase severely hinders its direct observation, while our carefully designed molecular dynamics simulations rigorously reveal the critical role of this metastable phase in the nucleation of {101¯2} twins in magnesium. Moreover, we prove that the reversible hcp-tetragonal-hcp phase transformations involved in the twinning process follow strict orientation relations between the parent hcp, intermediate tetragonal, and twin hcp phases. This phase transformation-mediated twinning mechanism is naturally compatible with the ultrafast twin growth speed. This work will be important for a better understanding of the twinning mechanism and thus the development of novel strategies for enhancing the ductility of magnesium alloys.

In recent years, ferromagnetic shape memory alloys (FSMAs) have been widely investigated in many countries [1–4]. Of the developed FSMAs, NiMnGa alloys have been found to exhibit nearly 10% magnetic-fieldinduced strain which is the maximum obtained MFIS in these FSMAs [5]. Based on NiMnGa alloys, researchers have developed new ferromagnetic shape memory alloys, i.e., NiMnFeGa alloys [6, 7]. The structure, shape memory effect, and magnetic properties of Fe-doped NiMnGa alloys have been studied. It is assumed that the addition of Fe element enhances the alloy plasticity without sacrificing its magnetic and thermoelastic properties [6]. However, little work has been devoted to studying transformation behavior and the mechanical properties of polycrystalline Fe-doped NiMnGa alloys. The aim of the present letter is to study the effect of additions of Fe on transformation behavior and the hardness of NiMnGa alloys. A range of alloys with the nominal composition of Ni48.7Mn30.1−x Fex Ga21.2 (at%) (x = 0, 2, 5, 8, 11), each weighing about 80 g, were prepared. The alloys were fabricated by arc melting under an argon atmosphere using appropriate quantities of nickel (99.99% purity), manganese (99.9% purity), iron (99.9% purity) and gallium (99.99% purity). The ingots obtained were sealed in a quartz tube under a vacuum, followed by annealing at 1123 K for 24 hrs and quenched into ice water for homogeneity. Rectangular samples 4×4×6 mm were cut from the post-treated ingots for hardness measurements. Some 10 mg samples were taken from the ingots for DSC measurements. The transformation temperatures were determined by differential scanning calorimetry using a TA-2920. The heating and cooling rates were 10 K/min. The crystal structure at room temperature was confirmed by powder X-ray diffraction using D/MAX-RA X-ray diffractometer with Cu Kα radiation. Micro Vickers hardness (HV) of eight points was measured using a Akashi HM-102 Vickers Hardness Tester. The applied load and time were 200 g and 15 s, respectively. The HV values were averaged excluding the highest and lowest values [8]. Table I summarizes the transformation temperatures measured by DSC analysis in Ni48.7Mn30.1−x Fex Ga21.2 alloys (x = 0, 2, 5, 8, 11). It can be seen, at room temperature, that the parent phase and martensite coexist in the Ni48.7Mn30.1Ga21.2 alloy and the Ni48.7Mn28.1Fe2Ga21.2 alloys, whereas in the other three alloys only the parent phase exists. The results are in agreement with X-ray diffraction results in Fig. 1. X-ray patterns depict that the cubic parent phase and tetragonal martensite coexist when x = 0, 2, while only the cubic parent phase exists when x = 5, 8, 11. As shown in Table I, the thermal hysteresis of these five alloys are −1.38, −3.30, 0.46, 2.60 and 2.20 ◦C, respectively. For the Ni48.7Mn30.1Ga21.2 alloy and the Ni48.7Mn28.1Fe2Ga21.2 alloys, the thermal hysteresis is negative. Hence Fe-doped NiMnGa alloys exhibit very narrow thermal hysteresis which means that the chemical energy of the phase transformation is very small. The narrow thermal hysteresis also shows that the driving force for inducing the martensitic transformation in Fe-doped NiMnGa alloy is very small. Wayman [9] divided thermal transformations into two types: the first is Af > As > Ms > Mf while the second is Af > Ms > As > Mf. From Table I it can be seen that the first two alloys belong to the second type of thermal transformation, while the other three alloys belong to the first type of thermal transformation. In addition, only single forward and reverse transformation are determined by DSC curves. Fig. 2 depicts the phase transformation temperatures vs. Fe content for Ni48.7Mn30.1−x Fex Ga21.2 alloys (x = 0, 2, 5, 8, 11). It can be seen that with increase of Fe content all the transformation temperatures, Ms (martensitic transformation start temperature), Mf (martensitic transformation finish temperature), As (reverse martensitic transformation start temperature), and A f (reverse martensitic transformation finish temperature), decrease. The relationship between Ms temperature and Fe content is nearly linear, varing from 26.40 ◦C for Ni48.7Mn30.1Ga21.2 without Fe to −55.00 ◦C for Ni48.7Mn19.1Fe11Ga21.2 which has the maximum Fe content. Fig. 3 exhibits the relationship between transformation temperatures and valence electron numbers per atom of Ni48.7Mn30.1−x Fex Ga21.2 alloys (x = 0, 2, 5, 8, 11) and the shape of the curves is similar to Fig. 2. The tendency is clear that all transformation temperatures decrease with increase of electronic concentration in these five alloys, and the increasing rate is also basically consistent with the change of the electronic concentration. Here, it is assumed that the number of valence electrons per atom for Ni, Mn, Fe, and Ga atoms is 10(3d84s2),7(3d54s2),8(3d64s2) and 3(3s24p1), respectively. It is believed that two factors, the conduction electronic concentrations [10] and the size factor [11],

Phase field-elasticity analysis of austenite–martensite phase transformation at the nanoscale: Finite element modeling

Direct measurement and imaging of magnetocaloric effect inhomogeneities at the microscale in Ni44Co6Mn30Ga20 with infrared thermography

The properties and structure of the martensitic phase of alloys with a near-stoichiometric equiatomic Ni50Mn50 composition, as well as martensitic transformations in them, are investigated in a wide temperature range by measuring the resistivity and thermal expansion coefficient and applying transmission electron microscopy, scanning electron microscopy, electron diffraction, and X-ray diffraction. It is found that Ni50Mn50 and Ni49Mn51 alloys experience the B2 → L10 highly reversible thermoelastic martensitic transformation and its related high-temperature deformation of the transformation and shape memory effect. Critical temperatures, volume (ΔV/V = ∓1.7%) and linear size effects attributed to the direct and reverse martensitic transformations, and the high-temperature dependences of the martensitic and austenite lattice parameters are determined. It is found that the morphology of tetragonal L10 martensitic represents a hierarchy of thin coherent sheets of submicrocrystallites and nanocrystallites with plane near-{111}L10 habit boundaries, the crystallites being pairwise twinned according to the {111}〈11 $\bar 2$ 〉 L10 ∥ {011}〈-1 $\bar 1$ 〉 B2 twinning shear scheme.

The study covers the effect of recrystallization annealing temperature and time on the characteristic temperatures of martensitic transformations and mechanical properties of the Ti–50.7at.%Ni shape memory alloy in the form of wire after cold drawing at room temperature. Six modes of post-deformation annealing with different temperatures and holding times were studied for the alloy to obtain structures with different sizes of recrystallized grains. The recrystallized grain size was determined by electron backscatter diffraction (EBSD). It was shown that the size of recrystallized grains increases from 2.5 to 9 μm, with both an increase in the annealing temperature (600– 700 °С) and an increase in the holding time (0.5–5.0 h). The characteristic temperatures of direct and reverse martensitic transformations were determined using differential scanning calorimetry. It was shown that the threefold growth of the recrystallized grain size reduces the starting temperature of the direct martensitic transformation, and extends the temperature range of the reverse martensitic transformation. The results of mechanical tests (stretching tests) at room temperature showed that an increase in the grain size leads to a decrease in the dislocation yield strength and an increase in the phase yield strength. It was established that the dislocation yield strength obeys the Hall–Petch law, and the phase yield strength is determined by the test temperature position relative to the starting (or peak) temperature of the direct martensitic transformation. Heat treatment modes for specific products should be recommended taking into account these two competing factors, as well as reverse martensitic transformation temperatures determining the alloy strain recovery temperatures.

Chemical complexity, microstructure and martensitic transformation in high entropy shape memory alloys

We report on a novel phenomenon, that is a high-strain-induced reverse martensitic transformation in an ultrafine-grained Ti–36Nb–2Ta–3Zr (wt.%) alloy processed by equal channel angular pressing (ECAP) at room temperature. Our results show that a martensitic transformation from body-centred cubic β matrix to orthorhombic α″ martensite occurs under low-strain ECAP conditions and that a large portion (~34%) of martensite transforms into a matrix phase (i.e. reverse martensitic transformation) with increasing ECAP strain to a high value of 4 (i.e. 6 passes) with a corresponding reduction in the α″-lath thickness and a refinement of grain size in the matrix phase.

Alloys with the shape memory effect have recently found application in engineering. Specifically, parts made of such alloys can be actuating members in structures of thermoregulators and thermorelays. It is known that the shape memory effect is realized in alloys with a thermoelastic martensite transformation. When heated, specimens strained in a martensite state recover their initial shape partially or completely as a result of a reverse martensite transformation. If an external load is applied in thermocycling, the shape will be changed both in heating and in cooling, i.e., recovered in the reverse martensite transformation and deformed under the effect of the external load in the direct martensite transformation. This change in shape is commonly said to be reversible. The aim of the present work consists in analyzing the effect of deformation in the martensite state prior to thermocycling and of an external load during the thermocycling on the reversible change of shape in Mn−Cu-base alloys.

On the plastic deformation accompanying cyclic martensitic transformation in thermomechanically loaded NiTi

The transformation behavior and the shape memory effect due to cyclic stress/strain for Ti–49.6Ni alloy