Increase Of Martensitic Transformation Temperatures Research Articles

Influence of hot rolling on microstructure, mechanical properties, and martensitic transformation of TiNiCuNb shape memory alloy

TiNiCuNb shape memory alloys are a promising new class of materials with the potential to be applied as elastocaloric components. However, the mechanical processing of these alloys remains a challenge and new insights on this topic must enlighten the knowledge about this system. In this work, the effects of hot rolling on Ti46Ni38Cu10Nb6 alloy were investigated. The results showed that hot rolling leads to the increase of martensitic transformation temperatures and the formation of a matrix containing both B2 and B19 phases. The coarsening of the lamellar eutectic constituent was observed, and part of the β-Nb phase precipitated into the matrix after being dissolved due to hot work. Microstructural aspects and ultra-microhardness measurements suggest that dynamic recrystallization occurred during hot rolling.

Comparisons of performances in Ti-V-Al composites with single and multiple in-situ reinforcements

In the present study, various kinds of in-situ reinforcements were introduced into Ti-V-Al shape memory alloys to enhance strength. The results revealed that the addition of TiB2 and B4C ceramic particles can form TiB whiskers and a mixture of TiB whiskers and TiC particles, respectively. Meanwhile, both TiB2 and B4C ceramic particles can refine grain size of Ti-V-Al shape memory alloys. The addition of TiB2 and B4C ceramic particles caused an increase in martensitic transformation temperature. The grain refinement and introduction of in-situ TiB whisker and TiC particles resulted in an obvious improvement in strength without sacrificing the strain recovery characteristics. For comparison, the addition of B4C ceramic particles can obtain high-performance Ti-V-Al shape memory alloys.

Full Variation of Site Substitution in Ni-Mn-Ga by Ferromagnetic Transition Metals

Systematic doping by transition elements Fe, Co and Ni on each site of Ni2MnGa alloy reveal that in bulk material the increase in martensitic transformation temperature is usually accompanied by the decrease in ferromagnetic Curie temperature, and vice versa. The highest martensitic transformation temperature (571 K) was found for Ni50.0Mn25.4(Ga20.3Ni4.3) with the result of a reduction in Curie temperature by 55 K. The highest Curie point (444 K) was found in alloy (Ni44.9Co5.1)Mn25.1Ga24.9; however, the transition temperature was reduced to 77 K. The dependence of transition temperature is better scaled with the Ne/a parameter (number of non-bonding electrons per atom) compared to usual e/a (valence electrons per atom). Ne/a dependence predicts a disappearance of martensitic transformation in (Ni45.3Fe5.3)Mn23.8Ga25.6, in agreement with our experiment. Although Curie temperature usually slightly decreases while the martensitic transition increases, there is no significant correlation of Curie temperature with e/a or Ne/a parameters. The doping effect of the same element is different for each compositional site. The cascade substitution is discussed and related to the experimental data.

Microstructural and Mechanical Response of NiTi Lattice 3D Structure Produced by Selective Laser Melting

Nowadays, additive manufacturing (AM) permits to realize complex metallic structural parts, and the use of NiTi alloy, known as Nitinol, allows the integration of specific functions to the AM products. One of the most promising designs for AM is concerning the use of lattice structures that show lightweight, higher than bulk material deformability, improved damping properties, high exchange surface. Moreover, lattice structures can be realized with struts, having dimensions below 1 mm—this is very attractive for the realization of Nitinol components for biomedical devices. In this light, the present work regarded the experimental characterization of lattice structures, produced by selective laser melting (SLM), by using Ni-rich NiTi alloy. Differential scanning calorimetry (DSC), electron backscatter diffraction (EBSD), and compression testing were carried out for analyzing microstructure, martensitic transformation (MT) evolution, and superelasticity response of the SLMed lattice samples. The lattice microstructures were compared with those of the SLMed bulk material for highlighting differences. Localized martensite was detected in the nodes zones, where the rapid solidification tends to accumulate solidification stresses. An increase of martensitic transformation temperatures was also observed in lattice NiTi.

Martensitic transformation, twin boundary and phase interface mobility of directionally solidified Ni-Mn-Ga alloys during compression by EBSD tracing

Directionally solidified Ni-Mn-Ga alloy with orderly arrayed austenite and non-modulated martensite (NM) was formed due to microsegregation between dendrite arm and interdendritic region, leading to preferred orientation of <001>A and <110>M coexistence at ambient temperature. The stress-induced martensitic transformation took place due to the increase of martensitic transformation temperature under step-wise uniaxial compression. The detwinning process accompanied with dislocation mobility was easy to carry out on the twin planes 45° inclined to the compression axis, compared with the twin planes parallel to the compression axis in the dominant twinned groups. The martensitic transformation and reorientation of variants were investigated by electron backscatter diffraction.

Composition-dependent properties and phase stability of Fe-Pd ferromagnetic shape memory alloys: A first-principles study

The composition-dependent properties and their correlation with the phase stability of Fe75+xPd25−x (−10.0≤x≤10.0) alloys are systematically investigated by using first-principles exact muffin-tin orbitals (EMTO)-coherent potential approximation (CPA) calculations. It is shown that the martensitic transformation (MT) from L12 to body-centered-tetragonal (bct) occurs in the ordered alloys with about −5.0≤x≤10.0. In both the L12 and bct phases, the evaluated a and c/a agree well with the available experimental data; the average magnetic moment per atom increases whereas the local magnetic moments of Fe atoms, dependent on both their positions and the structure of the alloy, decrease with increasing x. The tetragonal shear elastic constant of the L12 phase (C′) decreases whereas that of the bct phase (Cs) increases with x. The tetragonality of the martensite (|1−c/a|) increases whereas its energy relative to the austenite with a negative value decreases with Fe addition. All these effects account for the increase of MT temperature (TM) with x. The MT from L12 to bct is finally confirmed originating from the splitting of Fe 3d Eg and T2g bands upon tetragonal distortion due to the Jahn-Teller effect.

Martensitic transformation and detwinning in directionally solidified two-phase Ni-Mn-Ga alloys under uniaxial compression

A kind of Ni-Mn-Ga alloy consisting of orderly arrayed austenite and non-modulated (NM) martensite was designed and directionally solidified. The influences of step-wise uniaxial compression on martensitic transformation and detwinning processes of the above-mentioned sample were investigated by the electron backscatter diffraction (EBSD) tracing. Experimental results show that the stress has induced martensitic transformation at ambient temperature, which is completed after the three cycles (2%, 4% and 6% strain, respectively). This should be attributed to the increase of martensitic transformation temperature under compression. Moreover, the stress has caused detwinning of the twins with high Schmid factor, resulting in martensite only composed of favorable variants with 〈110〉M orientation parallel to the compression axis. The kernel average misorientation (KAM) maps reveal that the dislocation density decreases during stress-induced processes.

Effect of Cr addition on the structural, magnetic and mechanical properties of magnetron sputtered Ni–Mn–In ferromagnetic shape memory alloy thin films

The effect of Cr substitution for In on the structural, martensitic phase transformation and mechanical properties of Ni–Mn–In ferromagnetic shape memory alloy (FSMA) thin films was systematically investigated. X-ray diffraction results revealed that the Ni–Mn–In–Cr thin films possessed purely austenitic cubic L21 structure at lower content of Cr, whereas higher Cr content, the Ni–Mn–In–Cr thin films exhibited martensitic structure at room temperature. The temperature-dependent magnetization (M-T) and resistance (R-T) results confirmed that the monotonous increase in martensitic transformation temperatures (TM) with the addition of Cr content. Further, the room temperature nanoindentation studies revealed the mechanical properties such as hardness (H), elastic modulus (E), plasticity index (H/E) and resistance to plastic deformation (H3/E2) of all the samples. The addition of Cr content significantly enhanced the hardness (28.2 ± 2.4 GPa) and resistance to plastic deformation H3/E2 (0.261) of Ni50.4Mn34.96In13.56Cr1.08 film as compared with pure Ni–Mn–In film. As a result, the appropriate addition of Cr significantly improved the mechanical properties with a decrease in grain size, which could be further attributed to the grain boundary strengthening mechanism. These findings indicate that the Cr-doped Ni–Mn–In FSMA thin films are potential candidates for microelectromechanical systems applications.

Magneto-structural transformations in Ni50Mn37.5Sn12.5−xInx Heusler powders

The effect of ball milling and subsequently annealing of melt spun ribbons on magneto-structural transformations in Ni50Mn37.5Sn12.5−xInx (x=0, 2, 4, 6) ribbons is presented. Short time vibration milling allows to obtain chemically homogenous powders of angular particle shapes and size within 10–50μm. Milling does not change the characteristic temperatures of martensitic transformation in comparison to the melt spun ribbons. The effect of In substitution for Sn on martensitic transformation has a complex mechanism, associated with electron density change. Substitution of Sn by In in both milled and annealed powders leads to decrease of Curie temperature of austenite and increase of martensitic transformation temperature, stabilizing martensitic phase. The coexistence of magnetic transformation of austenite and martensitic transformation at low magnetic field was observed. The intermartensitic transformation of 4O martensite to L10 martensite was observed during cooling at low magnetic field and this was confirmed by TEM microstructure observations. The annealing process of as-milled powders leads to the change of their martensitic structure due to relaxation of internal stresses associated with anisotropic columnar grain microstructure formed during melt spinning process. The level of stresses introduced during milling of ribbons has no significant influence on martensitic transformation. The annealing process of as milled powders leads to enhancement of their magnetic properties, decrease of Curie temperature of austenite, and marginal change of temperature of martenisitic transformation.

Effect of heat treatment on magnetostructural transformations and exchange bias in Heusler Ni48Mn39.5Sn9.5Al3 ribbons

The influence of low temperature annealing and high temperature quenching on martensitic transformation, microstructure, magnetic and exchange bias properties of Ni48Mn39.5Sn9.5Al3 metamagnetic shape memory alloy ribbons have been investigated. It is shown that with increasing thermal treatment temperature the martensitic transformation temperature increases and the exchange bias is reduced. In the case of low temperature annealing this effect is mediated by stress and structural relaxations, whereas in the case of high temperature annealing followed by water quenching this effect is ascribed to the combined influence of grain size enlargement and composition change resulting from metastability of the Ni–Mn–Sn–Al phase at elevated temperatures. Furthermore low temperature thermal treatment leads to the change in the martensite structure from 4O to 10M as is demonstrated by in situ TEM studies. It is demonstrated that low temperature annealing is a preferable method for fine tuning magnetostructural properties of Ni–Mn–Sn–Al ribbons.

Thermal stability of B2 CuZr phase, microstructural evolution and martensitic transformation in Cu–Zr–Ti alloys

The effects of minor Ti addition on the thermal stability of B2 CuZr phase, the microstructure and the martensitic transformation (MT) in Cu50Zr50−xTix (x = 0, 2.5, 7.5 and 10 at%) alloys were investigated. It was found that the crystallization products, i.e. Cu10Zr7 and Cu(Ti, Zr)2, of Cu–Zr–Ti amorphous alloys transform to B2 CuZr phase at high temperatures. The corresponding eutectoid transformation temperature gradually increases with increasing Ti content, implying the decrease of the thermodynamic stability of the B2 CuZr phase. The microstructures of Cu–Zr–Ti martensitic alloys were proven to contain B2 CuZr, CuZr martensite, Cu10Zr7, Cu(Ti, Zr)2, and ZrTiCu2 crystals. Dilatometric measurements reveal that the MT temperature reduces with increasing Ti content, which would be of electronic nature. With increasing thermal cycles, the MT temperature gradually decreases while the reverse MT temperature increases, which results from the enhancement of the dislocation density, the partial decomposition of the equiatomic CuZr crystals and the partially reversible MT.

Stress-Induced Thermoelastic Martensitic Transformations and Functional Properties in [011]-oriented NiTiHfPd Single Crystals

The stress-induced martensitic transformation in the [011]-oriented Ni45.3Ti29.7Hf20Pd5 (at. %) single crystals in as-grown, homogenized and aged states were investigated in compression. It is experimentally shown that heat treatments of single crystals result in increase in martensitic transformation temperatures, two-fold decrease in reversible strain and increase in strain-hardening coefficient. As-grown single crystals demonstrate large temperature range of superelasticity (up to 140 K), large reversible strain (up to 4.3%) and large work output in comparison with homogenized and aged crystals.

Martensitic phase transformations and magnetocaloric effect in Al co-sputtered Ni–Mn–Sb alloy thin films

Abstract We systematically investigated the influence of aluminium (Al) content on the martensitic transformations and magnetocaloric effect (MCE) in Ni–Mn–Sb ferromagnetic shape memory alloy (FSMA) thin films. The temperature-dependent magnetization (M–T) and resistance (R–T) results displayed a monotonic increase in martensitic transformation temperature (TM) with increasing Al content. From the isothermal magnetization (M–H) curves, a large magnetic entropy change (ΔSM) of 23 mJ/cm3 K was observed in N49.8Mn32.97Al4.43Sb12.8. A remarkable enhancement of MCE could be attributed to the significant change in the magnetization of Ni–Mn–Sb films with increasing Al content. Furthermore, a high refrigerant capacity (RC) was observed in Ni–Mn–Sb–Al thin films as compared to pure Ni–Mn–Sb. The substitution of Al for Mn in Ni–Mn–Sb thin films with field induced MCE are potential candidates for micro length scale magnetic refrigeration applications where low magnetic fields are desirable.

Effect of aging and ball milling on the phase transformation of Ni50Mn25Ga17Cu8−xZrx alloys

This study investigated phase transformation of Ni50Mn25Ga17Cu8-xZrx (x = 0, 4, 8) alloys after aging and ball milling. For the Cu4Zr4 and Zr8 dual phase alloys, aging has enhanced magnetic susceptibility and magnetic exchange of the matrix, resulting in an increase of the Curie temperature of austenitic matrix. The decrease of martensitic transformation temperatures for the Cu4Zr4 alloy and the increase of martensitic transformation temperatures for the Zr8 alloy after aging should be related to the dissolution and precipitation of the second phase in the matrix, respectively. Ball milling is effective to smash the Cu4Zr4 and Zr8 alloys to fine particles, but cannot fracture the Cu8 alloy to particles, indicating an inherent high ductility and strength of the Cu8 alloy. Therefore, the macroscopic brittleness of the polycrystalline Cu8 alloy was mainly caused by the weak grain boundaries. For the Cu4Zr4 and Zr8 particles, the martensitic transformation and Curie transition of austenitic matrix disappeared and the Curie transition of second phase remained after ball milling. After post-annealing at 800 °C, the Curie transition of austenite was recovered due to the restoration of atomic order, but the martensitic transformation cannot be retrieved which might be caused by the grain refinement of the austenitic matrix after ball milling.

Effects of Cu on the martensitic transformation and magnetic properties of Mn50Ni40In10 alloy

The structures, the martensitic transformations, and the magnetic properties are studied systematically in Mn50Ni40−xCuxIn10, Mn50−xCuxNi40In10, and Mn50Ni40In10−xCux alloys. The partial substitution of Ni by Cu reduces the martensitic transformation temperature, but has little influence on the Curie temperature of austenite. Comparatively, the martensitic transformation temperature increases and the Curie temperature of austenite decreases with the partial replacement of Mn or In by Cu. The magnetization difference between the austenite phase and the martensite phase reaches 70 emu/g in Mn50Ni39Cu1In10; a field-induced martensite-to-austenite transition is observed in this alloy.

Influence of Ti additions on martensitic transformation and magnetic properties of cast Ni51Fe22−xGa27Tix shape memory alloys

The effect of Ti addition on the microstructure, martensitic transformation, magnetic and mechanical properties of polycrystalline Ni51Fe22−x Ga27Ti x (x=0, 2 and 4) ferromagnetic shape memory alloy was investigated by scanning electron microscope, differential scanning calorimetry and X-ray diffraction. The results showed that the martensitic transformation temperature increases monotonously with the increase of fraction of Ti substitution for Fe. The increase in the martensite transformation temperatures should be related to the change of the electron concentration after the addition of Ti to Ni51Fe22−x Ga27Ti x alloys. According to the results of X-ray diffraction and magnetic properties, Ti has significant effect the structure of Ni51Fe22-x Ga27Ti x . Adding of 4 at% Ti altered the structure of the matrix from five-layered tetragonal martensite of Ni51Fe22Ga27 and Ni51Fe20Ga27Ti2 alloys to non-modulated tetragonal martensite. Magnetic properties proved that the alloy transits from ferromagnetic, five-layered tetragonal martensite, to paramagnetic, non-modulated martensite structure, with increasing Ti content to 4 at.%. Saturation magnetization, remnant magnetization and coercivity of the alloy were significantly influenced by Ti additions. Hardness values of Ni51Fe22Ga27 increased by the addition of Ti.

Effect of Ti addition on the structural, mechanical and damping properties of magnetron sputtered Ni–Mn–Sn ferromagnetic shape memory alloy thin films

Titanium (Ti) co-sputtered Ni50.4Mn34.7Sn14.9 films deposited by magnetron sputtering onto Si(1 0 0) substrates at 823 K were investigated. X-ray diffraction profiles revealed the formation of highly (2 2 0)-oriented Ni–Mn–Sn–Ti austenite phase with significant decrease in grain size with increasing Ti power. Hardness (H), elastic modulus (Er), damping (tan δ), figure of merit (FOM) and coefficient of restitution (e) of the films were evaluated using nanoindentation tests. A significant improvement in the hardness (10.5 GPa) and toughness (0.040) was observed in the Ni51.0Mn28.2Sn11.0Ti9.7 nanocomposite film as compared with pure Ni50.4Mn34.7Sn14.9films. An impact model, which incorporates material behaviour, is presented that predicts the experimentally observed material quantities, including energy dissipation metrics such as the coefficient of restitution e with high accuracy. The highest damping factor (tan δ = 0.061), high FOM (0.79) with low coefficient of restitution (e = 0.28) quantifies excellent energy dissipation capacity in the Ni51.0Mn28.2Sn11.0Ti9.7 nanocomposite. Temperature dependence of magnetization (M–T) curves showed an increase in martensitic transformation temperatures with increasing Ti content. The Ni–Mn–Sn–Ti composite films exhibit ferromagnetic behaviour at room temperature.

Effect of Aging on Microstructure and Shape Memory Properties of a Ni-48Ti-25Pd (At. Pct) Alloy

The microstructure and properties of a precipitation-hardenable Ni-48Ti-25Pd (at. pct) shape memory alloy have been investigated as a function of various aging conditions. Both the hardness and martensitic transformation temperatures increased with increasing aging time up to 100 hours at 673 K (400 °C), while no discernable differences were observed after heat treatment at 823 K (550 °C), except for a slight decrease in hardness. For aging at 673 K (400 °C), these effects were attributed to the formation of nano-scale precipitates, while precipitation was absent in the 823 K (550 °C) heat-treated specimens. The precipitation-strengthened alloy exhibited stable pseudoelastic behavior and load-biased-shape memory response with little or no residual strains. The precipitates had a monoclinic base-centered structure, which is the same structure as the P-phase recently reported in Ni(Pt)-rich NiTiPt alloys. 3D atom probe analysis revealed that the precipitates were slightly enriched in Ni and deficient in Pd and Ti as compared with the bulk alloy. The increase in martensitic transformation temperatures and the superior dimensional stability during shape memory and pseudoelastic testing are attributed to the fine precipitate phase and its effect on matrix chemistry, local stress state because of the coherent interface, and the ability to effectively strengthen the alloy against slip.

Ion irradiation induced modifications of nanostructured Ni–Mn–Sn ferromagnetic shape memory alloy thin films

Thin films of Ni–Mn–Sn ferromagnetic shape memory alloy, grown on Si substrate by DC magnetron sputtering were bombarded by 450 keV Ar + 4 ions at different fluences ranging from 1 × 10 14 to 3 × 10 16 ions/cm 2 in order to investigate the effect of ion irradiation on characteristic transformation temperatures and thus on shape memory behavior. Temperature dependent resistivity measurements reveal the increase in martensitic transformation temperature (~ 100 K) upto a fluence of 1 × 10 15 ions/cm 2, above which shape memory behavior degrades and completely loses its behavior at 3 × 10 16 ions/cm 2, which is ascribed to the amorphization of Ni–Mn–Sn structure at a fluence of 3 × 10 16 ions/cm 2 as evidenced from X-ray diffraction pattern. The diffuse rings in the electron diffraction pattern also confirmed the amorphization of the film at highest fluence. The temperature dependent magnetization measurements also show the increase in martensitic transformation temperature upto a fluence of 1 × 10 15 ions/cm 2 in support of resistivity data. This work gives a possibility to acquire a better control on the properties of FSMA thin films using ion irradiation.

Structural and magnetic properties of magnetron sputtered Ni–Mn–Sn ferromagnetic shape memory alloy thin films

In the present study, structural and magnetic properties of Mn-rich, off-stoichiometric, nanocrystalline Ni–Mn–Sn ferromagnetic shape memory alloy thin films, grown on Si (100) substrates at 550 °C by dc magnetron sputtering have been systematically investigated. The crystallization, surface morphology, and structural features were studied using x-ray diffraction, atomic force microscopy, and field emission scanning electron microscopy. The structural transition from austenite to martensite was observed with an increase of Mn content. Austenitic phase with mixed L21/A2+B2 structure has been observed at room temperature in Ni52.6Mn23.7Sn23.6 (S1) and Ni51.5Mn26.1Sn22.2 (S2) films, while those with composition of Ni58.9Mn28.0Sn13.0 (S3) and Ni58.3Mn29.0Sn12.6 (S4) show martensitic phase with 14M modulated monoclinic structures. Field induced martensite-austenite transformation has been observed in magnetization studies using superconducting quantum interference device magnetometer. Temperature dependent magnetization measurements demonstrate the influence of magnetic field on the structural phase transition temperature. The investigations reveal an increase of martensitic transformation temperature (TM) with corresponding increase in substitution of Mn. The films exhibit ferromagnetic behavior at low temperatures below Curie temperature (TC). The decrease in saturation moment with increasing Mn content, indicates the existence of antiferromagnetic correlations within ferromagnetic matrix.