B19 Martensitic Transformation Research Articles

Actuating Bimorph Microstructures with Magnetron-Sputtered Ti-Ni-Cu Shape Memory Alloy Films.

The generation of microactuation using narrow thermal hysteresis Ti-Ni-Cu shape-memory alloy films deposited on non-metallic substrates as the active element is studied based on a model previously developed for Ni-Ti/Si bimorphs. To this end, the compositional range in which the B2 (monoclinic) → B19 (orthorhombic) martensitic phase transformation occurs was considered, and films were deposited by magnetron sputtering on heated Si and Kapton substrates. Ultra-fine grains were observed for the 550 °C deposition temperature. The selected composition was close to Ti50Ni35Cu15, so the narrowing of the thermal hysteresis is not associated with a significant reduction in shape recovery capability. The microstructure and composition of the target materials and as-deposited films used in our experiments were characterized by X-ray diffraction and scanning electron microscopy, whereas the temperature dependence of the volume fraction of the martensite phase was derived using differential scanning calorimetry records for the target materials and from the temperature dependence of the electrical resistance data for the films. An original model was used to predict the actuation of cantilever-type bimorphs with Kapton and Si substrates.

Structure and Multistage Martensite Transformation in Nanocrystalline Ti-50.9Ni Alloy

An electron microscopic study of the evolution of the size, morphology, and spatial distribution of coherent Ti3Ni4 particles with a change in the aging temperature in a nanocrystalline (NC) Ti-50.9 at % Ni alloy with an inhomogeneous grain–subgrain B2-austenitic nanostructure has been carried out. It was found that with an increase in the aging temperature, along with a change in the size and shape of Ti3Ni4 nanoparticles, their spatial distribution changes from location at dislocations to precipitates at subboundaries. Research has shown that the presence of different types of internal interfaces in the nanostructure contributes to the heterogeneous distribution of coherent Ti3Ni4 nanoparticles in the volume of the B2 matrix, which is associated with the precipitation of particles in the region of low-angle subboundaries and the suppression of the Ti3Ni4 precipitation in nanograins with high-angle boundaries. The difference in the structural-phase state of nanograins and subgrains regions is the main reason for the implementation of the anomalous R-phase transformation effect in the sequence of multistage martensitic transformations B2↔R↔B19′.

Structure and functional properties of rapidly quenched TiNiCu alloys with high copper contents

Thin materials exhibiting the shape memory effect, with a narrow temperature hysteresis, are required to create miniature and high-speed devices. Quasi-binary intermetallic TiNi-TiCu alloys with high copper contents (more than 10 at %) demonstrate the reversible martensitic transformation with a small (4-6 K) hysteresis. Alloys of the TiNi–TiCu system with a copper content of 30–40 at.% were fabricated in an amorphous state by the planar flow casting technique at a melt cooling rate of 106 K/s in the form of ribbons 30–50 μm thick. The alloy samples were subjected to dynamic crystallization using a single electric current pulse with duration of 5 ms. X-ray diffraction studies revealed almost fully martensitic state of the alloys with B19 structure at room temperature. TEM examination showed their structure to contain typical B19-martensite plates with a mean size of 20–80 nm. At the same time, the alloys exhibit a one-stage phase martensitic transformation B2↔B19 in the temperature range of (55÷75)°C, as well as pronounced shape memory effect, whose properties are largely determined by the structural parameters of the alloys.

Effect of Solution-Treated Temperature on Microstructure in TiNixCu12 (x = 39, 40 at %) Alloys

Ti–xNi–12Cu (at%) (x = 39, 40) alloys were prepared and then their transformation behavior and microstructures were investigated after solution treatment at various temperatures by means of scanning electron microscopy, energy dispersive X-ray spectroscopy, and differential scanning calorimetry. Matrix (B2), Ti2 (Ni, Cu) and Ti(Ni, Cu)2 phases were observed in all specimens except the TiNi39Cu12 alloy solution treated at 1373 K in which matrix (B2) and Ti2 (Ni, Cu) were observed. Volume fraction of Ti(Ni, Cu)2 phase decreased with increasing solution treatment temperature. All specimens showed the B2–B19 martensitic transformation behavior. Transformation temperatures and hysteresis decreased with increasing solution treatment temperature, which was ascribed to the decrease in the volume fraction of Ti(Ni, Cu)2 phase resulting in the increase in Ni content of matrix.

Specific Features of the Atomic Structure of Ti50Ni25Cu25 Alloy Rapidly Quenched from Melt

A complex study of the structure of Ti50Ni25Cu25 alloy obtained in the initial amorphous state by rapid quenching (melt spinning) have been performed for the first time. The investigations are carried out by methods of neutron diffraction, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and electron microdiffraction. It is found that the topological and composition short-range atomic order, corresponding to three superstructure types (B2, L21, and L12), is formed within localized nanodomains (up to 1 nm) in amorphous Ti50Ni25Cu25 alloy upon solidification. It is also demonstrated for the first time that the alloy has an amorphous–crystal structure, which contains (along with individual microspherulites that underwent thermoelastic martensitic transformation B2 ↔ B19) nanocrystalline ensembles with the B2, L21, or L12 structure up to 10 nm in size, which are localized in the amorphous matrix.

Мартенситные превращения сплавов на основе никелида титана с разными легирующими добавками

Investigations of the martensitic transformations of TiNi-based alloys grade TN-10, doped with 1 at.% V, Nb, Ag, were presented. It was shown that alloying elements (Ag, Nb, V) cause an increase in internal elastic stresses inside the crystalline structure of the B2 phase, which leads to an increase in the beginning temperature of direct martensitic transformation. Of the selected alloying elements, vanadium has a significant effect on this temperature, shifting it to the region of high values ​​by ~ 45 ° C. In alloys with vanadium and silver, a complex sequence of martensitic transformations B2-> R-> B19 'is realized, and in alloys with niobium, a simple B2-> B19' is realized.

Dynamic Scenarios of the Formation of Martensite with the {110} Habits in the Ni50Mn50 Alloy

Martensitic transformation B2–L10 in the ordered alloy Ni50Mn50, which occurs at comparatively high temperatures (980–920 K), is discussed with the use of dynamic concepts of the wave control of the threshold deformation. The proximity of the observed orientations of martensite-crystal habits (and of twin boundaries) to the planes of the {110} family makes it possible to use the longitudinal waves along the axes 〈001〉 (in the basis of the initial phase) as the driving factors. It is shown that at temperatures of the onset of the transformation there is a satisfactory correspondence between the calculated and experimental data on the tetragonality of martensite and on the volume effect. The opportunity of different dynamic scenarios of the formation of the final phase is noted, namely, of separate crystals; layered structures, in which the crystals of martensite with the identical orientation relationships alternate with the untransformed regions of austenite; and packets of pairwise-twinned crystals. Examples are given of morpho-types corresponding to these scenarios.

A hidden single-stage martensitic transformation from B2 parent phase to B19′ martensite phase in an aged Ni51Ti49 alloy

The aged Ni-rich NiTi shape memory alloys (SMAs) exhibit the multi-stage martensitic transformation (MMT), which has important influences on functional properties and practical applications of the NiTi SMAs. A hidden single-stage martensitic transformation from B2 parent phase to B19′ martensite phase is found in an aged Ni51Ti49 alloy, which happens concurrently with a commonly observed two-stage martensitic transformation B2–R–B19′ (R: martensite phase) and actually composes one stage of a multi-stage martensitic transformation (MMT) together with the two-stage one. B2–B19′ martensitic transformation occurs in the NiTi matrix containing Ni4Ti3 precipitates with relatively large inter-particle space, while B2–R–B19′ transformation takes place in the NiTi matrix with Ni4Ti3 precipitates having relatively small inter-particle space.

Computational study of binary titanium-based potential shape memory alloys

First-principles density functional theory approach was employed to investigate the martensitic temperature of the equiatomic TiPt, TiZr and TiTa shape memory alloys. Firstly, their structural, thermodynamic, elastic and phonon dispersion have been deduced to mimic the relative stabilities. These alloys have potential to operate as high-temperature shape memory alloys since they possess a reversible B2 to B19 martensitic transformation. It was found that the lattice parameters are in good agreement with the experimental results to within 5%. Furthermore, the TiPt structure is thermodynamically stable displaying lowest heats of formation, while TiZr and TiTa are not. Our results show that the B2 phases of these binary alloys are vibrational unstable due to the presence of soft modes along high symmetry direction on the phonon dispersion curves. TiTa alloy is mechanically stable (C′ > 0), which may be used to develop future high-temperature alloys.

Structure and Thermoelastic Martensitic Transformations in Ternary Ni–Ti–Zr Alloys with High-Temperature Shape Memory Effects

X-ray diffraction analysis and transmission electron and scanning electron microscopies were used to study the effect of alloying with zirconium (0–20 at %) on the phase composition and structural and morphological features of thermoelastic martensitic transformations in ternary Ni–Ti–Zr alloys. The electrical resistivity of alloys has been measured in a wide range of temperatures, the critical temperatures have been determined, and a complete diagram of the high-temperature thermoelastic forward and reverse martensitic transformations B2 ↔ B19' occurring with a hysteresis in the range of 32–50 K has been constructed based on XRD data. The intercritical range of the temperatures of transformations increases as the zirconium content increases within the indicated limits. The lattice parameters of the monoclinic crystal lattice of the B19' martensite have been measured at room temperature. The twinning types most frequently observed in B19' martensite have been found as follows: I-(011), $$\left( {11\;\bar 1} \right)$$ and (001), II-(011).

Fine structure and mechanical properties of the shape-memory Ni50Ti32Hf18 alloy rapidly quenched by spinning

We have reported the results of investigations of the structure and chemical and phase compositions of the amorphous Ni50Ti32Hf18 alloy prepared by rapid quenching from melt by spinning and subjected to heat treatments. The specific features of the fine polycrystalline alloy structure formation depending on the heat-treatment mode have been studied by transmission and scanning electron microscopy, chemical microanalysis, electron diffraction, and X-ray diffraction analysis. According to the data on the temperature behavior of electrical resistivity, critical temperatures of devitrification and subsequent thermoelastic martensitic transformation B2 → B19′ have been determined. The mechanical properties in different heat-treatment modes have been investigated.

Phase transformations of severely plastically deformed Ti–Ni–Pd high-temperature shape memory alloys

Severe plastic deformation of a Ti[Formula: see text]Ni[Formula: see text]Pd[Formula: see text] high-temperature shape memory alloy yields a nanoscale mixture of an amorphous phase and retained lamellae of the parent B19 martensite. The thermal stability of the deformed martensite is significantly enhanced. Upon heating, B2 austenite occurs within the martensite and by crystallization of the amorphous phase, concomitant with recovery and the formation of an ultrafine grained structure. In the small grains, the B2 austenite to B19 martensitic transformation is shifted to lower temperatures and incomplete.

Dependence of the enthalpy of the direct martensitic transformation in titanium nickelide on the stress

An original technique of differential thermal analysis for studying thermal properties of samples loaded with a tangential stress has been created. In a series of experiments studying the direct martensitic transformation B2 → B19′ in titanium nickelide during cooling under constant stress, it has been found that the enthalpy of transformation linearly decreases with an increase in stress and, at a stress of 100 MPa, it is 30% less than that of the sample in a free state.

Relationship between crystallographic compatibility and thermal hysteresis in Ni-rich NiTiHf and NiTiZr high temperature shape memory alloys

The relationship between the crystallographic compatibility of austenite and martensite phases and the transformation thermal hysteresis (ΔT) of Ni-rich Ni50.3Ti29.7Hf20 and Ni50.3Ti29.7Zr20 alloys undergoing B2–B19′ martensitic transformation was studied as a function of microstructure, via differential scanning calorimetry, transmission electron microscopy, and X-ray diffraction. An experimental linear relationship of ΔT vs λ2 (the second eigenvalue of the transformation stretch matrix) was observed for these NiTi(Hf/Zr) alloys, but with a shallower slope as compared to the universal behavior followed by alloys showing B2–B19 martensitic phase transformation. Several ternary NiTiCu and binary NiTi alloys undergoing the B2–B19′ transformation were also found to deviate from the universal behavior attributed to alloys that undergo a B2–B19 transformation, and instead, follow the trend revealed for the present alloy systems. Aged NiTi(Hf/Zr) samples, which consist of very fine nano-precipitates, followed the newly established ΔT vs λ2 linear relationship, due to only minor changes in the microstructure. In contrast, samples with large precipitates, exhibited a large deviation from this relationship due to much more drastic changes in microstructure. Finally, despite the poor crystallographic compatibility of the austenite and martensite lattices observed in the present alloys, rationalized by large deviation of λ2 values from 1, relatively low ΔT values were measured. This behavior is actually consistent with the newly established relationship for ΔT vs λ2 for B2–B19′ transforming alloys, which is much less sensitive to compatibility (shallower slope). It is concluded that such a difference in the ΔT vs λ2 slope must be a consequence of the crystallography of monoclinic martensite formation in NiTi-based alloys as long as other factors such as plasticity or major constraints to the martensitic transformation do not intervene.

Structure and Mechanical Properties of Shape-Memory Alloys of the Ti – Ni – Cu System

The effect of copper additives in an amount of up to 35 at.% on the structure and phase transformations in ternary Ti – Ni – Cu alloys in conventional microcrystalline, submicrocrystalline and nanocrystalline states is studied. Complete diagrams of thermoelastic martensitic transformations B2↔B19′, B2↔B19↔B19′ and B2 ↔ B19 are plotted as a function of the copper content. Special features of variation of the microstructure are determined. Tensile mechanical properties including the parameters of the shape memory effect are measured.

Effect of thermocycling on the temperatures of phase transformations, structure, and properties of the equiatomic alloy Ti50.0Ni50.0

This article is devoted to studying the influence of thermocycling in the range of temperatures of the thermoelastic martensitic transformation B2–B19' on the microstructure, the temperatures of the martensitic transformations, and the mechanical properties of the equiatomic alloy Ti50Ni50 in the coarse-grained (CG) and ultrafine-grained (UFG) states, the latter obtained by equal-channel angular pressing (ECAP). One hundred cycles of thermocycling and the related increase in the dislocation density in the CG alloy led to a decrease in the temperatures of martensitic transformations. In the UFG alloy, the temperatures of the forward transformation (Ms, Mf) decrease by 2–3 K, and the temperatures of the reverse transformation (As, Af) increase by 6 K. The ultimate strength remains almost unaltered upon the thermocycling, but the yield stress increases substantially from 430 to 550 MPa and from 935 to 1120 MPa for the CG and UFG states, respectively.

Morphological features in B2→B19 martensitic transformation and their description based on the concept of controlling wave process

The paper analyzes the possibility of describing morphological features in the B2→B19 transformation in the framework of a dynamic theory based on the concepts of heterogeneous nucleation and wave growth of martensite crystals. It was shown, using Ti-Ni-Cu alloys as an example, that the predicted and observed orientations of habit planes agree well. The issue that strain ratios remain valid in going from threshold to final strains is considered, and calculation results for orientation relations and their fulfilment conditions are discussed.

Ambient-temperature high damping capacity in TiPd-based martensitic alloys

Shape memory alloys (SMAs) have attracted considerable attention for their high damping capacities. Here we investigate the damping behavior of Ti50(Pd50−xDx) SMAs (D=Fe, Co, Mn, V) by dynamic mechanical analysis. We find that these alloys show remarkably similar damping behavior. There exists a sharp damping peak associated with the B2–B19 martensitic transformation and a high damping plateau (Q−1~0.02–0.05) over a wide ambient-temperature range (220–420K) due to the hysteretic twin boundary motion. After doping hydrogen into the above alloys, a new relaxation-type damping peak appears in the martensite phase over 270–360K. Such a peak is considered to originate from the interaction of hydrogen atoms with twin boundaries and the corresponding damping capacity (Q−1~0.05–0.09) is enhanced by roughly twice that of the damping plateau for each alloy. Moreover, the relaxation peaks are at higher temperatures for the TiPd-based alloys (270–370K) than for the TiNi-based alloys (190–260K). We discuss the influence of hydrogen diffusion, mobility of twin boundaries and hydrogen–twin boundary interaction on the temperature range of the relaxation peak. Our results suggest that a martensite, with appropriate values for twinning shear and hydrogen doping level, provides a route towards developing high damping SMAs for applications in desired temperature ranges.

Влияние тёплой деформации методом abc-прессования на механические свойства никелида титана

The effect of temperature on the ultimate tensile strength (σuts) and fracture elongation (δ) under tension of titanium nickelide (50.2 at.% Ni) with initial coarse-grained structure and structures after abc-pressing at 873K (alloy 1, true deformation е = 2.2) and after “step-by-step” pressing at 873K→673K (alloy 2, е = 4.2) are studied. The grain dimensions in the samples of alloy 1 (2-40 μm) are smaller than ones in initial samples (20-70 μm). The structure of alloy 2 is submicrocrystalline (grain/subgrain dimensions are 100-700 nm). It was found that the martensitic transformations (MT) B2←→B19ʹ had been observed under cooling and heating of initial samples. The sequences of В2→R→В19ʹ→В2 MT were observed in alloys 1 and 2 (R – the rhombohedral martensitic phase). The finish of B19ʹ→B2 MT take place in the temperature range 340-355 K. The tension tests were carried out at temperatures 293K, 343K and 473K (or 523 K), i.e. in the B19ʹ phase, pretransition temperature range and B2 phase, accordingly. The “stress-strain” dependences obtained at these temperatures are quantitavely the same for all samples. The martensitic plateau connected with the reorientation and/or generation of the B19ʹ martensitic crystallites are observed under tension of samples at 293K and 343K. No the generation of stress-induced B19ʹ martensite was observed under tension at 473K (or 523K). The increase of σuts was found after abc-pressing. The maximum values of σuts are observed under tension of samples of alloys 1 and 2 at 293K and 343K (in B19ʹ phase). The alloy 1 demonstrates the highest plasticity (δ about 80%) at temperature range 293-343 K. The σuts and δ decrease essentially (about 2 times) with the temperature increase to 473K (or 523K).

Dynamic Model of the Formation of an Intermediate Mesorcopic State During B2 → B19 Martensitic Transformation

Morphological characteristics of martensite after B2–B19 transformation are considered within the limits of the concept of the control wave process. It is demonstrated that there is a possibility for the fast formation of an intermediate mesoscopic state. The instability of this state for the subsequent transition to final strains is supposed.