Decrease Of Martensitic Transformation Temperature Research Articles

Microstructure and martensitic transformation of Ni–Ti–Pr alloys

The effect of Pr addition on the microstructure and martensitic transformation behavior of Ni50Ti50−x Pr x (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9) alloys were investigated experimentally. Results show that the microstructures of Ni–Ti–Pr alloys consist of the NiTi matrix and the NiPr precipitate with the Ti solute. The martensitic transformation start temperature decreases gradually with the increase in Pr fraction. The stress around NiPr precipitates is responsible for the decrease in martensitic transformation temperature with the increase in Pr fraction in Ni–Ti–Pr alloys.

Magnetostructural transformation and magnetocaloric effect in Mn-Ni-Sn melt-spun ribbons

The martensitic transformation and magnetic properties of Mn50Ni50-x Sn x $(7\le x\le 10)$ melt-spun ribbons were investigated. It is shown that the increase of Sn content results in a nearly linear decrease of martensitic transformation temperatures in the ribbons, with an average rate of 70 K per at % of Sn. In Mn50Ni40Sn10 ribbons, the field-induced reverse martensitic transformation from a weak magnetic martensite to a ferromagnetic austenite was realized due to the strong magnetostructural coupling. Under a field change of 5T, the large magnetic entropy change of 13.7 J kg-1K-1 and the effective refrigerant capacity of 72.9 Jkg-1 were obtained in Mn50Ni40Sn10 ribbons.

Phase transition of Ni55−xCoxMn20Ga25 (8.5 ≤ x ≤ 11.0) alloys with different compositions and magnetic fields

Studies on Co-doped Ni–Mn–Ga ferromagnetic shape memory alloys (FSMAs) have been quite active topics in recent years. Unlike previous reports where the amount of Co doping was generally less than 8 at%, in this work Ni55−xCoxMn20Ga25 (8.5 ≤ x ≤ 11.0) alloys were studied with high Co doping. Unusual effects of both composition and magnetic field on phase transitions were observed. With the increase in substitution of Co for Ni, the magnetic transition temperatures increase gradually but the martensitic transformation temperature decreases quite sharply. In particular, the average decrease in the martensitic transformation temperatures is up to 100 K which is nearly twice that in the case of Co content of less than 8 at%. Further, under an applied magnetic field ranging from 0.03 to 0.60 T, abnormal stabilization of a martensite phase with lower magnetization was monitored. Magnetic entropy change of 1.6 J·kg−1·K−1 was induced at the martensitic transformation of Ni46.5Co8.5Mn20Ga25 alloy by an applied field of 1 T. The magnetic contributions, including the magnetocrystalline anisotropy and the Zeeman energy, to the thermodynamics of the martensitic transformation are considered to understand the observed unusual phenomena. This work results in new insights into the understanding of Co-doped Ni–Mn-based ferromagnetic shape memory alloys.

Influence of Cu Doping on Martensitic and Magnetic Transitions in Ni-Mn-Sn Alloys

In this work, the fourth element Cu was introduced to substitute Ni in polycrystalline Ni-Mn-Sn alloys. It was shown that Cu doping did not change the crystal structure of the martensite in Ni50-xCuxMn39Sn11 (x=0, 1, ......,7) alloys, but resulted in the decrease of martensitic transformation temperatures. Due to the higher atomic radius of Cu with respect to that of Ni, the lattice volume of martensite unit cell increases with the gradual substitution of Ni by Cu. For the alloys with the Cu content of 0-4%, the martensitic transformation is from weak magnetic (paramagnetic) austenite to weak magnetic (paramagnetic) martensite. When the Cu content is higher than 4%, the paramagnetic to ferromagnetic transition of austenite was introduced. The temperature interval between magnetic transition and structural transformation was enlarged with the increase of Cu content. Due to the relatively smaller magnetization difference between austenite and martensite, the field induced inverse martensitic transformation behavior is not significant in the present Cu-doped alloys.

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.

Enhanced exchange bias in magnetron-sputtered Ni–Mn–Sb–Al ferromagnetic shape memory alloy thin films

In the present study, the influence of aluminium (Al) addition on the martensite-austenite phase transformation and exchange bias of Ni–Mn–Sb films have been investigated. Ni–Mn–Sb–Al films with different Al concentration (∼0–5.6%) were deposited by co-sputtering of Ni–Mn–Sb and Al targets. Experimental results revealed the decrease in martensitic transformation temperature with increasing Al content upto a certain extent (3.3%) beyond which martensitic transformation was suppressed. Paramagnetic to ferromagnetic transition temperature (TC) also decreased with increasing Al concentration. Ni50Mn36.3Sb10.4Al3.3 thin film showed significant improvement in exchange bias field as compared to pure Ni50.3Mn36.9Sb12.8 thin film. This enhancement in the exchange bias field HEB = 611 Oe at 10 K is attributed to the increase of AFM-FM interactions that result from the decrease of Mn–Mn distance due to the incorporation of Al atoms. This behaviour is an additional property of the FSMA thin films apart from various other multifunctional properties and therefore, is of technological importance for their applications in magnetic storage devices.

Effect of Al incorporation for Co on the gamma-beta phase boundary of rapidly solidified CoNiAl ferromagnetic shape memory alloys

The alloys Co64−xNi36Alx(at%, X=23,24,26 and 28) prepared through melt spinning technique revealed β- and γ-phase structure at room temperature. A monotonous increase in β-phase and simultaneous decrease in γ-phase has been observed with increasing Al content. The synergistic competition between β- and γ-phase influence β(B2)→L10 martensitic transformation. Therefore transformation broadening has been observed with Al content. Consequently, martensitic transformation temperature decreases and austenite transformation temperature increases. The γ-rich alloys showed magnetically soft phase, with high initial susceptibility and enhanced saturation magnetization. These alloys also revealed large magneto-strain values which are also supported by micro-structural morphology.

Tuning martensitic transformation and large magnetoresistance in Ni50−xCuxMn38Sn12 Heusler alloys

In this paper, polycrystalline Ni50−xCuxMn38Sn12 alloys (x = 0, 2, 4, 6) were prepared. The influence of Cu doping on the martensitic transformation and magnetic properties were investigated in these alloys. Experimental results indicate that the martensitic transformation temperature decreases and the Curie temperature increases with the increasing of substitution of Cu for Ni. Therefore, the magnetic properties in both austenitic and martensitic phases could be tuned by Cu content in these alloys. In addition, magnetoresistance were also performed and discussed in detail. A large magnetoresistance (up to 39%) was obtained by the magnetic field induced reverse martensitic transformation.

Microstructure, thermomechanical and shape memory properties of two-phase Ni–Mn–Fe–Ga high-temperature shape memory alloys with B addition

Microstructure, martensitic transformation, thermomechanical stability and shape memory effect of two-phase Ni–Mn–Fe–Ga high-temperature shape memory alloys with small B addition, were investigated by thermomechanical cycling. It is found that the high degree of martensite stabilization is observed in all studied alloys due to the compressive pre-strain. The results show that B addition and thermomechanical cycling facilitate the precipitation of the ordered γ' phase. The precipitation alters the chemical compositions of martensite and results in the decrease of martensitic transformation temperature of the alloys. As a result, the formation of the ordered γ' precipitate has negative effect on the thermomechanical stability of the alloys. Results further demonstrate that the martensite reorientation proceed becomes easier with small B addition due to the decrease of the yield strength, which can effectively improve shape memory effect of the alloys. However, with increasing B concentration and number of cycles, the shape memory effect gradually decreases due to the combined effects of the roles of γ phase and γ' precipitate, and thermomechanical cycling process.

Magnetic properties and structural transformations in Ni–Co–Mn–Sn multifunctional alloys

A systematic study of the magnetic properties and both the temperature- and magnetic-field-induced structural transformations in the Ni50−xCoxMn39Sn11 (0⩽x⩽10) multifunctional alloys over a large temperature range from 500K down to 10K was performed. It is revealed that, with increasing x, the martensitic transformation temperatures first decrease slowly when 0⩽x⩽4 and then decrease rapidly when 5⩽x⩽8; no martensitic transformation was observed in the alloys with 9⩽x⩽10. The magnetic properties of these alloys are very sensitive to their chemical composition. The austenite in the alloys with 0⩽x⩽4 shows paramagnetic behavior and the martensite exhibits paramagnetic, superparamagnetic (SPM), and superspin glass (SSG) behaviors in different temperature ranges during cooling. In contrast, the austenite in the alloys with 5⩽x⩽8 is paramagnetic above its Curie temperature TC and ferromagnetic below TC, and the martensite shows SPM and SSG behaviors in different temperature ranges. The austenite in the alloys with 9⩽x⩽10, which remains stable down to 10K, shows paramagnetic and ferromagnetic behaviors above and below TC, respectively. The complete phase diagram for the Ni50−xCoxMn39Sn11 (0⩽x⩽10) alloy system, from high temperature down to 10K, is established. A significant magnetic-field-induced decrease of martensitic transformation temperatures and almost fully reversible magnetic-field-induced structural transformation are achieved across a broad temperature range in the alloys with 5⩽x⩽8. These results are important for understanding the composition and temperature-dependent functional properties, as well as their underlying mechanisms, in the Ni–(Co)–Mn–X (X=In, Sn, Sb) multifunctional alloys.

Magnetostructural Transformation and Magnetoresponsive Properties of ${\rm MnNiGe}_{1-x}{\rm Sn}_{x}$ Alloys

The martensitic and magnetic phase transformations in MnNiGe1-xSnx (0 \leq x \leq 0.200) alloys were investigated using X-ray diffraction (XRD), differential thermal analysis (DTA) and magnetization measurements. Results indicate that the increasing Sn substitution in MnNiGe1-xSnx results in (i) decrease of martensitic transformation temperature from 460 to 100 K and (ii) conversion of AFM spiral to antiparallel AFM strcuture in martensite. Based on these, the remarkable magnetic-field-induced PM/spiral-AFM and FM/AFM magnetostructural transformations and, large positive and negative magnetocaloric effects are obtained. The magnetoresponsive effects of MnNiGe1-xSnx alloys are enhanced by Sn substitution. A structural and magnetic phase diagram of MnNiGe1-xSnx alloys has been proposed.

Martensitic transformation and magnetic properties in ferromagnetic shape memory alloy Ni43Mn46Sn11−xSix

This study investigated the effects of Si addition and heat treatment on the martensitic transformation and magnetic properties of Ni43Mn46Sn11−xSix (x = 1, 2, 3) alloys. The martensitic transformation temperatures were found to increase with increasing Si content in the alloy. A magnetic field-induced martensite-to-austenite transformation was found in Ni43Mn46Sn10Si1. Ageing of the Ni43Mn46Sn10Si1 alloy at 673 K resulted in the formation of a (Mn,Ni)–SiSn precipitate. The precipitate contains very low Sn content, causing the increase of the Sn content in the matrix phase and a decrease of martensitic transformation temperature. Ageing at 573 K is found to increase the Curie temperature and the saturation magnetization. This is attributed to the increase of atomic ordering of the matrix. Solution treatment of the aged samples at 1073 K was effective to restore the original transformation behaviour and magnetic properties.

Martensitic and magnetic transformations in Ni-Mn-In alloys

Dilatometry, magnetometry, and optical metallography were used to study structural and magnetic transformations in Ni-Mn-In alloys of different compositions. Temperatures of structural and magnetic transformations of the Ni47 − x Mn42 + x In11 alloys (with x = 0–2) were determined. It is shown that, as the valence electron concentration decreases, the martensitic transformation temperature decreases. Some of the Ni-Mn-In alloys exhibit a ferromagnetic transformation in both the initial austenite and in the arising martensite.

Microstructure and martensitic transformation of Ni–Fe–Ga–Si ferromagnetic shape memory alloys

The effects of the fourth element Si on the martensitic transformation and magnetic properties of Ni–Fe–Ga magnetic shape memory alloys were investigated. A complete thermoelastic martensitic transformation in Ni–Fe–Ga–Si alloys was observed in the temperature range of 218–285 K. The martensitic transformation temperatures of Ni–Fe–Ga alloys are obviously decreased by the substitution of Si for Ga element, that is, the substitution of 1 at.% Si for Ga leads to a decrease of martensitic transformation temperature of about 39.6 K. Moreover, the substitution of Si for Ga leads to a decrease of the saturation magnetic field and the magnetic anisotropy constant K 1 obviously.

Influence of Sb on the magnetic and magnetocaloric properties of ferromagnetic shape memory alloy NiMnIn

We investigate the magnetic properties and the magnetic entropy change around the martensitic transformations in Sb-doped Ni50Mn35In15 ferromagnetic shape memory alloys. We find that the martensitic transformation temperature decreases, and the thermal hysteresis associated with the transition increases when Sb is added. The alloy with 5.5 at. % Sb enhances the ferromagnetism of austenitic phase and improves the field shift martensitic transition ability, resulting in a great increase in magnetic entropy change up to 8 J kg−1 K−1 at 2 T and 21 J kg−1 K−1 at 5 T. This is attributed to magnetic interaction that is sensitive to the variation of p-d hybridization and conduction electron concentration arising from the substitution of sp element.

Microstructure changes in two phase β+ γ Co-Ni-Al ferromagnetic shape memory alloys in relation to Al/Co ratio

The microstructure of Co 35+x - Ni 40-x - Al 25 (x = 0, 2.5, 5.0) ferromagnetic shape memory alloys annealed at 1200°C and water quenched has been investigated by optical (OM) and analytical transmission electron microscopy (TEM). The microstructure consists of elongated grains of martensite and 7 phase. Annealing twins were found frequently in the 7 phase. The volume fraction of γ phase has been determined to be about 40%. TEM observations show twinned L1 0 martensite with {111}-type twinning plane and tetragonality c/a = 0.85 in all investigated alloys. Determination of the composition of β phase, allows to calculate Al/Co and e/a ratio in the range 7.68-8.40 in correlation with its Ms temperatures between 66 and 167 °C, respectively. An increase in the Al/Co ratio together with a decrease of Ni content in β phase causes a decrease of the e/a ratio, which promotes a decrease of martensitic transformation temperatures.

Thermal characteristics of Ni–Fe–Ga–Mn and Ni–Fe–Ga–Co ferromagnetic shape memory alloys

The martensitic transformation (MT) characteristics of three quaternary polycrystalline alloys with compositions Ni53.5Fe16.5Ga27Co3, Ni54Fe16Ga27Mn3 and Ni52Fe18Ga27Co3 have been studied in order to investigate the effects of additions of Mn or Co to Ni–Fe–Ga, in substitution of Ni or Fe. The shifts in the MT temperatures, compared to the corresponding ternary alloys, are in agreement with the e/ a ratio changes, although an atomic size factor seems to be also at play when Co substitutes Fe. The increase in the degree of L2 1 order brings about a decrease of the MT temperatures, the behaviour being equivalent to that of ternary alloys. The martensitic phases observed in the quaternary alloys differ slightly from those of the ternary; in particular, an unusual four-layered phase has been found in the alloy containing Mn. Different martensite stabilization degrees, depending on the quenching rate, have been observed in the Ni53.5Fe16.5Ga27Co3 alloy transforming above room temperature. For intermediate quenching rates, the effect is concomitant with the quenched-in disorder, which is gradually recovered when heating above the end of the MT. Annealing treatments at 1270 K promote the formation of ordered γ′ (L1 2 structure) precipitates in the Ni52Fe18Ga27Co3 alloy. The particles are richer in Fe and poorer in Ga than the matrix, leading to a slight reduction in the e/ a ratio of the matrix, which results in a decrease of MT temperatures, as confirmed by DSC runs.

New Sequences of Phase Transition in Ni-Mn-Ga Ferromagnetic Shape Memory Nanoparticles

In-situ high-energy X-ray diffraction was used for tracing the structural transitions in Ni2MnGa and Ni47Mn25Ga22Co6 nanoparticles that were prepared by a ball-milling and subsequent annealing process. A decrease in martensitic transformation temperature was evidenced in all annealed nanoparticles. It was found that a three-layered modulated structure, usually observed as a metastable premartensitic phase in coarse-grained alloys, becomes a stable phase in the annealed Ni2MnGa nanoparticles up to 4 K.

Martensitic and magnetic transformation in Ni–Mn–Ga–Co ferromagnetic shape memory alloys

The effect of Co addition on crystal structure, martensitic transformation, Curie temperature and compressive properties of Ni 53− x Mn 25Ga 22Co x alloys with the Co content up to 14 at.% was investigated. An abrupt decrease of martensitic transformation temperature was observed when the Co content exceeded 6 at.%, which can be attributed to the atomic disorder resulting from the Co addition. Substitution of Co for Ni proved efficient in increasing the Curie temperature. Compression experiments showed that the substitution of 4 at.% Co for Ni did not change the fracture strain, but lead to the increase in the compressive strength and the decrease in the yield stress. This study may offer experimental data for developing high performance ferromagnetic shape memory alloys.

Martensitic Transformation and Superelasticity of Ti-Nb-Pt Alloys

Shape memory properties and martensitic transformation of Ti-Nb-Pt alloys were investigated in order to develop Ni-free biomedical superelastic alloys. The effect of Pt addition on the transformation strain was investigated by measuring the lattice constants of the parent and martensite phases of Ti-Nb-Pt alloys. The Ti-Nb-Pt alloys were fabricated by arc melting. The ingots were cold-rolled up to 99% reduction in thickness. The cold-rolled specimens were heat treated at 873 K for 0.6 ks. The shape memory effect and/or superelastic behavior were observed in the Ti-Nb-Pt alloys. The martensitic transformation temperature decreases by about 160 K with a 1 at% increase of Pt in the Ti-Nb-Pt alloys. The addition of Pt as a substitute of Nb was effective to increase the transformation strain when compared with compositions which reveal similar transformation temperatures. An aging treatment at 573 K was effective for increasing the stress for inducing the martensitic transformation and the critical stress for plastic deformation, resulting in good superelasticity. [doi:10.2320/matertrans.48.400]