Stress-induced Martensite Variants Research Articles

Microstructure evolution and shape memory behaviors of Ni47Ti44Nb9 alloy subjected to multistep thermomechanical loading with different prestrain levels

Ni47Ti44Nb9 shape memory alloy (SMA) is a promising material in the aerospace field due to its wide transformation hysteresis. The application of shape memory effect depends on multistep thermomechanical loading, viz., low-temperature deformation and subsequent heating to recovery. Low-temperature deformation prestrain plays a pivotal role in shape memory properties tailoring of SMA components. However, microstructure evolution and deformation mechanisms of Ni47Ti44Nb9 SMA subjected to various prestrain levels are still unclear. To this end, microstructure evolution and shape memory behaviors of Ni47Ti44Nb9 alloy subjected to multistep thermomechanical loading with prestrain levels of 8% - 16% at -28 °C (Ms + 30 °C) were investigated. The results demonstrate that the stress-strain curve of the specimen exhibits four distinct stages at a maximal prestrain of 16%. Whereas stage II and stage III end at prestrains of ∼8% and ∼12%, respectively. In stage II, the stress-induced martensitic transformation is accompanied by the dislocation slip of the NiTi matrix and β-Nb inclusions. In stage III, in addition to the higher density of dislocations and further growth of stress-induced martensite variants (SIMVs), (0 0 1) compound twins are introduced as a result of the (0 0 1) deformation twinning in stress-induced martensite. More {2 0 -1} martensite twins are gradually introduced in stage IV. Correspondingly, after subsequent unloading and heating, a higher density of {1 1 4} austenite twins form in the specimen with a larger prestrain of 16%. With increasing prestrain from 8% to 16%, the recoverable strain εreT upon heating increases first and then decreases. The εreT obtains a maximum of 7.03% at 10% prestrain and decreases to 6.17% at 16% prestrain. The increase of εreT can be attributed to the formation of new SIMVs, the further growth of existing SIMVs, and the recoverable (0 0 1) compound twins. While the decrease of εreT is mainly associated with the irrecoverable strain by {2 0 −1} martensite twins. The effect of β-Nb inclusions on the evolution of SIMVs is also found herein that deformed β-Nb inclusions can significantly hinder the growth and recoverability of adjacent stress-induced martensite.

Comparative study on mechanical and shape memory properties of hot forged NiTiNb in axial and radial direction

The NiTiNb shape memory alloys (SMAs) used in practice usually exist in a polycrystalline form with a strong texture. However, the performance differences in forged or rolled rods in the axial and radial directions caused by microstructures such as texture are often ignored. This study compared the mechanical and shape memory properties of the hot-forged NiTiNb alloy in the axial and radial directions. This investigation shows an increase in elongation of tensile specimens in the axial direction when compared with that in the radial direction (the maximum difference is 22.6%). However, the shape memory properties of hot-forged NiTiNb rods in the radial direction are more excellent than those in the axial direction. The types of stress-induced martensite variants are different in the axial and radial directions, but the recoverable transformation strains are the same. The primary reason is that the contribution of {2 0 1‾} compound twins to transformation strain in the radial direction is equivalent to the plastic deformation of the β-Nb phase caused by the increase in flow stress. The maximum recovery stress of the radial sample with pre-strain 10% is 20.7% higher than that of the axial sample.

Atomistic study of the effect of crystallographic orientation on the twinning and detwinning behavior of NiTi shape memory alloys

Understanding the effect of crystallographic orientation on the twinnin/detwinning mechanisms in NiTi shape memory alloys at an atomistic scale can help to control and tune the mechanical properties and failure behavior of such materials. In this work, we employed classical molecular dynamics (MD) and density functional theory (DFT) computational methods to better understand how twinning and detwinning occurs through a combination of slip, twin, and shuffle on 〈010〉, 〈110〉, and 〈111〉 crystallographic orientations under uniaxial tensile test. Elastic constants including Young's Modulus (E), Bulk modulus (B), Poisson’s ratio (ν), and Shear Modulus (G) are obtained and computed for resultant stress-induced martensite variants as a function of crystallographic orientation using DFT calculations. In addition, computational nanoindentation tests are carried out using MD simulations to evaluate the effect of crystallographic orientation on the twinning and detwinning characteristics in martensite in NiTi alloys under sphere indenter, both qualitatively and quantitatively. Based on a careful polyhedral template matching (PTM) and dislocation analysis (DXA) by taking into account the textures, it is determined that the microscopic stress-strain and load-displacement responses strongly depend on the crystallographic orientation. Our findings reveal that the size of twinned and detwinned zones in martensite increases in the order of 〈111〉 < 〈010〉 < 〈110〉. Based on DFT results, against 〈111〉 direction, abrupt changes in the free energy-strain curves occurs at 4% strain in 〈001〉, and 8% strain in 〈110〉 directions. The twinning and detwinning mechanisms are controlled by monoclinic martensite (B19′) → orthorhombic martensite (B19) phase transformation in 〈110〉 orientation and by body-centered orthorhombic martensite (BCO) → an intermediate structure (B19″) → monoclinic martensite (B19′) phase transformation in 〈001〉 orientation. Finally, the predicted orientation-dependent critical energy release rate is analyzed to examine the effect of the twinning and detwinning process on the fracture toughness of the material. Our results show that reducing the density of twins results in increasing the critical energy release rate. Therefore, the fracture stress intensity increases in the order of 〈001〉 < 〈110〉 < 〈111〉.

Stress induced martensite variants revealed by in situ high resolution electron backscatter diffraction (HR-EBSD)

In situ HR-EBSD during FeMnSi-based micro-pillar compression is used to correlate phase transformations and its underlying stresses. A simple experimental configuration containing two grains, one grain boundary and a uniaxial compression causes complex stress fields. Two out of three differently orientated stress induced martensite variants are expected to form due to high Schmid factors. To reveal the origin of growth of the third variant with a low Schmid factor, we calculate global stress maps and local resolved shear stress maps based on HR-EBSD measurements. Localized shear stresses at the grain boundary are identified to cause the growth of the not expected near grain boundary variant.

Influence of Dynamic Compression on Phase Transformation of Martensitic NiTi Shape Memory Alloys

Shape memory alloys (SMAs) exhibit high damping capacity in both austenitic and martensitic phases, due to either a stress-induced martensite phase transformation or a stress-induced martensite variant reorientation, making them ideal candidates for vibration suppression devices to protect structural components from damage due to external forces. In this study, both quasi-static and dynamic compression was conducted on a martensitic NiTi SMA using a mechanical loading frame and on a Kolsky compression bar, respectively, to examine the relationship between microstructure and phase transformation characteristics of martensitic NiTi SMAs. Both endothermic and exothermic peaks disappear completely after experiencing deformation at a strain rate of 103 s−1 and to a strain of about 10 pct. The phase transformation peaks reappear after the deformed specimens were annealed at 873 K (600 °C) for 30 minutes. As compared to samples from quasi-static loading, where a large amount of twinning is observed with a small amount of grain distortion and fracture, samples from dynamic loading show much less twinning with a larger amount of grain distortion and fracture.

Stress-induced martensite variant reorientation in magnetic shape memory Ni–Mn–Gasingle crystal studied by neutron diffraction

Stress-induced martensite variant reorientation in magnetic shape memory Ni–Mn–Gasingle crystal was studied in situ by the neutron diffraction technique. Principles ofdetermination of individual tetragonal martensitic variants in shape memory alloys areexplained. Using neutron diffraction we show that the macroscopic strain originates solelyfrom the martensite structure reorientation or variant redistribution. Neutron diffractionalso reveals that the reorientation of martensite is not fully completed even at a stressvalue of 25 MPa, which is about 20 times larger than the mean stress needed forreorientation. Only one twinning system is active during the reorientation process.

Effect of thermomechanical training temperature on the two-way shape memory effect of TiNi and TiNiCu shape memory alloys springs

In this article, the effect of thermomechanical training temperature on the two-way shape memory effect (TWSME) of TiNiCu and TiNi alloys springs were investigated. The results showed that when the springs were thermomechanical-trained at pure martensite, there is an increase of the recovery rate to a saturated value, the maximum recovery rate was about 55% and 45% for TiNiCu and TiNi alloys, respectively. As the springs were thermomechanical-trained at pure austensite and martensite+austensite, there is an increase of the recovery rate to a maximum value and decreased with ongoing training after having passed the maximum value and the maximum TWSME recovery rate is less than that of the shape memory alloys spring-trained at pure martensite. Dislocations generated by martensite reorientation were effective in developing two-way memory effect. Since the amount of the stress-induced martensite variants is less than that of thermal-induced martensite variants, thus, the recovery rate showed a different rule with increasing thermomechanical training cycles at different training temperature.

Stress-induced martensitic transformation behavior of a Ti–Ni–Hf high temperature shape memory alloy

The stress-induced martensitic transformation behavior and the microstructure of stress-induced martensite (SIM) in a Ti 36Ni 49Hf 15 high temperature shape memory alloy (SMA) have been investigated using tensile tests and transition electronic microscopy (TEM) observations. It shows that, compared with the TiNi SMAs, there is no stress plateau in the stress-strain curve as the TiNiHf alloy deformed in an austenite. The martensitic transformation enthalpy is calculated to be −106.84 cal/mol. The martensite variants mainly show preferential oriented morphologies for the TiNiHf alloy deformed to 8% at 523 K. The substructure of SIM and deformed SIM in the present alloy are (001) compound twin. Martensite variants are (011) type I twin related. Further, increasing the deformation temperature and deformation strain, the preferential oriented SIM variants gradually develop to martensite variants with variant-crack/variant-intersect morphologies. The inexistence of stress plateau in the stress-strain curve of TiNiHf may mainly result from the dislocation slip during the stress-induced martensitic transformation.

Microstructural development inside the stress induced martensite variant in a Ti–Ni–Nb shape memory alloy

The microstructural development inside the stress induced martensite (SIM) variants in Ti–Ni–Nb alloy with various degrees of deformation have been revealed by electron microscopic observations. The orientation relationship between the SIM and the parent phase has been found: [1 1 ̄ 0] M //[11 1 ̄ ] B2 , (001) M 5° away from (101) B2. The lattice invariant shear of the SIM variants at the slightly deformed stage is dominantly (11 1 ̄ ) Type I twin. Besides the ordinary slip, the adjustment and development of the internal secondary twinning from (11 1 ̄ ) Type I twin to 〈011〉 Type II/or (011) Type I twin, (001) compound twin and (111) Type I twin happen concurrently or in combination inside the SIM variants with the further deformation. The corresponding deformation mechanisms include stress induced reorientation of SIM substructural bands by the most favorably oriented twin system, stress induced migration of the SIM substructural boundary through internal twinning and stress induced injection of foreign SIM variant to the preexisting substructural bands.

Study of Ti-Ni-Nb Shape Memory Alloys with a Wide Hysteresis

It has been shown that TiNiNb shape memory alloys exhibit a wide transformation hysteresis compared to TiNi binary alloys (about 145°C versus 30°C). This wide hysteresis has important engineering utility. The present paper was undertaken to review of the microstructure. mechanical behavior, deformation-induced martensitic transformation and the effect of deformation on the stability of martensite and its reverse transformation in TiNiNb alloys. Simultaneously, the strain recovery characteristic and interface structure of martensite were explained, and the mechanism for increasing martensite stability by deformation was discussed. In addition, the microstructure and interface structure inside the stress-induced martensite (SIM) variants were evaluated. It is found that the morphology and substructure of SIM depend on deformation condition. The tensile deformation at parent phase condition has great influence on the substructure and interface structure of SIM variants. The most distinctive substructural feature of the stress-induced martensite is the presence of the various secondary twins. Based on the analysis of the stability and shape memory effect of various kinds of martensite, the concept of stability of deformed stress-induced martensite was proposed and explained by the mechanism for the formation of several secondary deformation twins.

High resolution electron microscopy studies on the interface structure of deformed stress induced martensite variants in a Ti–Ni–Nb shape memory alloy

Abstract The interfacial structure of deformed stress induced martensite variants in the Ti-44.7 at.% Ni-9 at.% Nb alloy have been investigated by high resolution electron microscopy. Bands conversions were revealed to happen inside the martensite variants to accommodate the macroscopic strain. The evolution of twinning modes from ( 11 1 ) Type I to (001) compound Type, Type II and (111) Type I were found. These newly formed (001) deformation twins nucleate and grow inside the substructural bands to perform the coalescence of substructural bands. It also causes the rearrangement of the unfavorably oriented bands. The appearance of the Type II twinning plate seems to eliminate the stress concentration during the coalescence of martensite bands. The (111) Type I twinning plate is also introduced to accommodate the shape deformation in different directions.

Stabilization and the two way shape memory effect (TWME) in Cu–Zn–Al polycrystals

Abstract The influence of diffusion on the two way shape memory effect (TWME) in Cu–Zn–Al polycrystals has been studied. The excess vacancy concentration which controls the diffusion velocity has been monitored by different heat treatments: (a) a slow air cooling and aging in the austenite prior to the transformation, leading to a low vacancy concentration; (b) a quench from 800°C; and (c) a quench from 300°C with an increase in the excess vacancies. The TWME has also been studied at low temperatures where diffusion is absent. All these results show that in Cu–Zn–Al polycrystals, diffusion has little or no influence on the TWME, and that instead a spectrum of martensite plate configurations is created by the stress, which then serve as nuclei for the subsequent transformation without an applied stress. The TWME obtained in the polycrystals is in all cases lower than that in single crystals after the stabilization of the stress induced martensite variant.

On the mechanism of two way shape memory effect obtained by stabilised stress induced martensite

Two way shape memory effect (TWME) can be obtained by suitable thermomechanical processing which involve repetitive training routines. J. Perkins found that TWME take place as a result of a macroscopic non-uniform residual stress field, concluding that plastic deformation was necessary to get TWME. K. Enami et al found that complex dislocations arrays are generated by thermomechanical cycling during the training procedure. TWME results obtained by the above training methods are not as good as would be expected, because during the thermal cycles, above and below A[sub f] and M[sub f] respectively, new dislocations are generated which interact with the dislocations that control TWME so giving a loss of TWME. A different training method has been developed by J.M. Guilemany et al based on the stabilization of stress induced martensite variants (SSIM). This method has been derived from the observation made by J. Perkins and R.O. Sponholz who found that retained (not stabilized) martensite acts as a nucleation site of thermal martensite. Thus, during cooling the stabilized martensite would grow or influence the nucleation and growth of the thermal martensite giving TWME. The effect of training temperature, time and stress on TWME obtained by SSIM has been studied.

Stress-induced Martensitic Transformation of Small Fe-Co Particles in a Cu Matrix.

The f.c.c. to b.c.c. martensitic transformation in small coherent Fe-Co particles has been induced by applying external stress to single crystals of a Cu-Fe-Co alloy without activating crystal dislocations. Orientation of the stress-induced martensite variants is sensitively dependent on the direction ([416]f or [001]f) and sense (tension or compression) of the applied stress. The experimental results can be well explained by postulating that the applied stress helps the lattice shear deformation involved in the f.c.c. to b.c.c. lattice change.

TEMPERATURE AND STRESS DEPENDENCE OF THE SHAPE MEMORY EFFECT PRODUCED BY ALLOYING ADDITIONS IN Cu-Al-Ni ALLOYS

-The effect produced on microstructure and properties of CuAlNi alloys by additions of elements such as manganese and boron have been studied. Ductilities between 4-6% at room temperature and 12-16% at high temperature (250-300°C) have been achieved.These high strains were produced in all cases by the formation of stress-induced martensite variants which are favourable to produce maximum elongation at each temperature.

Pseudoelastic effects in CuZn single crystals

Mechanical properties associated with pseudoelastic phenomena in CuZn single crystals have been studied and characterized. As a result it is possible to clarify superelastic properties and to report on a new rubber-like behavior. Two-surface analysis at a range of temperatures above M s demonstrated that the habit plane and shape strain direction of the stress-induced martensitic variant were those associated with the lowest Schmid factor of the 24 possible variants. The magnitudes of the measured strains were identical to the resolved theoretical values calculated from the phenomenological theory. The influence of temperature, strain rate, and the amount of superelastic cycling above M s was observed. The onset of ‘advanced deformation’ (by either the nucleation of reversible α s martensite or plastic flow) caused modification in the pseudoelastic loops of the stress-strain curves. ‘Training’ of specimens above M s caused the nucleation of the same variant when cooling to near M s . However, for certain orientations of the stress-induced martensite (SIM) single crystals further cooling caused a subsequent transformation of the SIM into a (2, 0, 10) 9R twin as the temperature approached M ƒ . The twinned single crystal of martensite was observed to behave in a rubber-like fashion on load cycling below M ƒ . The rubber-like behavior is due to twin boundary movement of the trained SIM variant and its (2, 0, 10) 9R twin. Magnitudes of the rubber-like strain have been measured and compared to the predicted theoretical values.

A stress-induced martensitic transformation of spherical iron particles in a CuFe alloy

γ→α martensitic transformation in spherical iron precipitates under the uniaxial applied stress was examined by using a Cu-1 wt.% Fe alloy single crystal. Distribution of the stress-induced martensite variants both in tension and in compression indicates the strong effect of the applied stress on the {111} ƒ〈211〉 ƒshear systems in the initial stage of the transformation. Together with the previous studies of FeNiCr alloys, it is concluded that not the total shape deformation but the lattice shear deformation in the γ→α martensitie transformation is preferentially aided by the applied stress. The number of bands in the α-iron particle which have been identified as alternative α-twin layers is strongly size dependent. This has been discussed in terms of the elastic energy and the twin-interface energy of a particle. Twin-interface energy of an α-iron can be estimated to be ~1.4J/m 2,

Orientation of martensite formed in Fe-23Ni-5Cr crystals under uniaxial stress along [001

Single crystals of an Fe-23Ni-5Cr alloy have been stressed along [001] f axis under tension and compression to induce martensite. Compressive loading induces more martensites than tensile loading. Orientation of the stress-induced martensite variants is sensitively dependent on senses of applied stresses. From these observations, it is concluded that applied stresses play an important role in the initial stage of lattice deformation accompanying the transformation. In contrast, the discussion based on the interaction energy between the total shape deformation and the applied stress has given less rational explanation for the experimental results. Together with the previous results [ Acta Met. 22, 313(1974) and 24, 853(1976)], it is concluded that the orientation relationship as well as the distribution of martensites transformed under an external stress is determined in the earliest stage of the transformation.