Transformation Temperature TM Research Articles

Tuning effect of Cu doping on martensitic transformation and magnetic and mechanical properties of Heusler alloy Mn50Ni40In10−xCux (x = 0–10) ribbons

Single-phase Mn50Ni40In10−xCux (x = 0, 1, 2, 4, 6, 8, and 10) Heusler alloys were synthesized by melt-spinning method. Martensitic transformation is retained within the whole composition range studied. The substitution of Cu for In leads to a drastic increase in the martensitic transformation temperature TM from below room temperature to above 750 K, and enhances the antiferromagnetic character of the martensite. The increase in TM is related to the increasing electron concentration e/a and decreasing cell volume V with Cu doping. The energy difference between martensite and austenite also increases with Cu doping, which tends to elevate the TM. Cu doping can enhance the metallic bonding character in NiMn-based magnetic shape memory alloys (MSMAs) and is a grain refining strategy of their ribbons. The oriented columnar grains in In-rich samples change to fine equiaxial grains in Cu-rich samples. All this has a positive effect on the strength and ductility. The tensile strength of the Mn50Ni40Cu10 ribbon is 3.6 times higher than that of Mn50Ni40In6Cu4. So, Mn–Ni–Cu can be a promising all-d-metal Heusler alloy platform for developing MSMAs with interesting properties.

Effects of Bi-doping on electronic structure and martensitic transformation in Ni2MnGa and Mn2NiGa MSMAs: A theoretical approach

The effects of Bi-doping on the electronic structure, martensitic transformation and magnetic properties of Heusler alloys Ni2MnGa1-xBix and Mn2NiGa1-xBix (x = 0, 0.25, 0.5, 0.75, 1) were investigated theoretically. The substitution of Bi for Ga has a negative effect on the martensitic transformation. The total energy difference ΔEM between the martensite and austenite decreases gradually with the doping of Bi. When x = 0.5, 0.75 and 1.0 in Ni2MnGa1-xBix and x = 1.0 in Mn2NiGa1-xBix, the martensite phase becomes higher in energy comparing with the austenite. Thus the doping of Bi can lower the martensitic transformation temperature TM and finally suppresses it. The possible reasons for this trend are related to the increasing valence electron concentration e/a and cell volume V with Bi-doping. The competition between them may be ascribed further by the electron density, which combines the effect of valence electron concentration and size effect. The electron densities of Ni2MnGa1-xBix and Mn2NiGa1-xBix decrease monotonously with increasing Bi content. This agrees well with the variation of ΔEM or TM. The doping of Bi does not change the magnetic structure of Ni2MnGa1-xBix and Mn2NiGa1-xBix. The former is ferromagnetic and the latter is ferrimagnetic in both martensite and austenite state. These results can help to introduce new component like Bi to Heusler alloys and discover new functional materials in them.

Electronic structure, magnetic properties and martensitic transformation of Ni50Mn36.5Sb13.5-xSix alloys

A series of Ni50Mn36.5Sb13.5-xSix (x = 0 - 2.5) Heusler alloys was prepared. Effect of Si-doping on the formation, martensitic transformation and magnetic properties of Ni-Mn-Sb alloys was investigated experimentally and theoretically. The increasing Si content leads to the increase of the Curie temperature TC and decrease of the saturation magnetization Ms at 5K. The martensitic transformation temperature Tm also shows a decreasing tendency with the substitution of Si for Sb. First-principles calculations indicate that the martensite has a lower total energy comparing with the austenite, which supports the phase transition observed experimentally. The energy difference between the martensite and austenite ΔEM suggests that the martensitic transformation temperature TM will decrease with a small amount of Si-doping, but increase when Si content is high. This explains the decrease of TM observed in the experimental samples, for in these samples, the Si content is in the ΔEM-decreasing region. The calculated total spin moment of the martensite shows a decreasing tendency with increasing Si content, which agrees with the variation of saturation magnetization Ms at 5K.

Influence of Cr-doping on the formation, martensitic transformation and magnetic properties of Mn2NiGa1-xCrx (x = 0–0.5) ribbon samples

Mn2NiGa1-xCrx (x = 0–0.5) alloys were synthesized successfully to explore possible Cr-based all-d-metal Heusler alloys with magnetic shape memory effects. When x = 0–0.25, Mn2NiGa1-xCrx are single BCC Heusler phase, but when x = 0.375 some FCC phase co-exists with the main Heusler phase, finally a pure FCC phase is formed in Mn2NiGa0.5Cr0.5. In BCC Mn2NiGa1-xCrx, Cr-doping leads to the decrease of martensitic transformation temperature TM and increase of the saturation magnetization Ms at 5 K. While the FCC phase does not show the trace of phase transition and has a very low magnetization till 5 K. First-principles calculations suggest that, in BCC Mn2NiGa1-xCrx the partial spin moment of Cr is large and parallel to that of Mn (B), which leads to the increase of Ms when x = 0–0.25. A higher stability of the martensite than the austenite is found, which supports the phase transition observed experimentally. It may be noticed that the TM of Mn2NiGa1-xCrx decreases with both increasing e/a and decreasing cell volume, which is opposite to the empirical rule in traditional Heusler alloys. Calculation suggests that Cr–Ga disorder may be a possible reason for this decrease.

Martensite phase structure of Mg-Sc lightweight shape memory alloy and the effect of rare earth elements doping

With the continual increase of application requirement of lightweight shape memory alloys (SMAs), MgSc alloys have drawn great attention as a new SMA. However, the ambiguous martensitic structure, unclear transformation path, and low martensitic transformation temperature (Tm) of Mg-Sc alloys are still issues. Herein, the martensitic structure and transformation path of Mg-Sc alloys were investigated systematically. The martensite phase was identified finally to be an orthorhombic structure by formation energy. Moreover, to improve the Tm, the effect of rare earth elements doping on Tm was studied, and Tm was increased by 28.2 K with 1.25 at.% Gd doping. The total density of states indicated that the phase stability determined the increase of Tm. Furthermore, the partial density of states indicated that the d orbital electrons coupling between Sc and Gd atom led to the changing of Tm. Our results provide a firm base for designing the new Mg-Sc-based lightweight SMAs.

Magnetic-field-driven reverse martensitic transformation with multiple magneto-responsive effects by manipulating magnetic ordering in Fe-doped Co-V-Ga Heusler alloys

Nowadays, searching for the materials with multiple magneto-functional properties and good mechanical properties is vital in various fields, such as solid-state refrigeration, magnetic actuators, magnetic sensors and intelligent/smart devices. In this work, the magnetic-field-induced metamagnetic reverse martensitic transformation (MFIRMT) from paramagnetic martensite to ferromagnetic austenite with multiple magneto-responsive effects is realized in Fe-doped Co-V-Ga Heusler alloys by manipulating the magnetic ordering. The martensitic transformation temperature Tm reduces quasi-linearly with increasing Fe-content. In strikingly contrast with the Fe-free alloys, the magnetization difference (ΔM') across martensitic transformation increases by three orders of magnitude for Fe-doped alloys. The increased ΔM' should be ascribed to the reduction of Tm, almost unchanged Curie temperature of austenite and the increased magnetic moment in the samples with higher Fe-content. The large ΔM' provides strong driving force to realize the MFIRMT and accordingly multiple magneto-responsive effects, such as magnetocaloric, magnetoresistance and magnetostriction effects. Meanwhile, giant Vickers hardness of 518 HV and compressive strength of 1423 MPa are achieved. Multiple magneto-responsive effects with exceptional mechanical properties make these alloys great potential candidates for applications in many fields.

First-principles study of Zn-doping effects on phase stability and magnetic anisotropy of Ni-Mn-Ga alloys

The effect of Zn doping on Ni-Mn-Ga magnetic shape memory alloy was studied by the first-principles calculations using exact muffin-tin orbital method in combination with the coherent-potential approximation and projector augmented-wave method. Trends in martensitic transformation temperature TM and Curie temperature TC were predicted from calculated energy differences between austenite and nonmodulated martensite, ΔEA−NM, and energy differences between paramagnetic and ferromagnetic state, ΔEPM−FM. Doping upon the Ga-sublattice results in stabilization of martensitic phase which indicates the increase in TM. TC is affected only weakly or slightly decreases, because ΔEPM−FM of martensite does not change significantly with doping. The substitution of Mn atoms by Zn causes the decrease in both TM and TC. Comparing to Cu-doped Ni-Mn-Ga alloys, we predict that doping with Zn results in smaller decrease in TC but also in smaller increase in TM. Moreover, Cu doping upon the Ga-sublattice strongly decreases the magnetic anisotropy energy of martensite, whereas such strong effect was not observed for Zn doping. Based on the calculations of Zn-doped Ni-Mn-Ga alloys we suggest that simultaneous doping with Zn and an element increasing TC can result in significant increase in both transformation temperatures without strong decrease of magnetic anisotropy.

Magnetic and martensitic transformation properties under ambient and hydrostatic pressures of Ni50Mn25.2Fe2.8Ga22 ferromagnetic shape memory alloy

The magnetic and martensitic transformation properties of Ni50Mn25.2Fe2.8Ga22 ferromagnetic shape memory alloy were systematically investigated. The Curie temperature TC and the martensitic transformation temperature TM were determined. The temperature variations of the lattice parameters were measured by using an in situ X-ray powder diffractometer. This alloy crystallizes in cubic L21-type structure for the parent phase and the martensitic transformation was observed to start at 294 K with a small volume change ΔV. The effect of hydrostatic pressure on TC and TM was investigated by measuring the temperature dependence of the initial permeability at various pressures up to 9.4 kbar. It was found that TM increases with increasing pressure. By use of the Clausius-Clapeyron equation, the values of dTM/dp and ΔV determined in this study were found to be consistent with the result of the entropy change ΔS, which was estimated by thermoanalysis. On the other hand, TC was found to slightly decrease with increasing pressure, which is a unique behavior compared to other Ni2MnZ (Z = Ga, In, Sn, Sb) alloys. On the basis of the experimental results, the exchange mechanism of the Ni50Mn25.2Fe2.8Ga22 alloy is discussed.

Role of Reversed Austenite Behavior in Determining Microstructure and Toughness of Advanced Medium Mn Steel by Welding Thermal Cycle.

Reversed austenite transformation behavior plays a significant role in determining the microstructure and mechanical properties of heat affected zones of steels, involving the nucleation and growth of reversed austenite. Confocal Laser Scanning Microscope (CLSM) was used in the present work to in situ observe the reversed austenite transformation by simulating welding thermal cycles for advance 5Mn steels. No thermal inertia was found on cooling process after temperature reached the peak temperature of 1320 °C. Therefore, too large grain was not generated in coarse-grained heat-affected zone (CGHAZ). The pre-existing film retained austenite in base metal and acted as additional favorable nucleation sites for reversed austenite during the thermal cycle. A much great nucleation number led to the finer grain in the fine-grained heat-affected zone (FGHAZ). The continuous cooling transformation for CGHAZ and FGHAZ revealed that the martensite was the main transformed product. Martensite transformation temperature (Tm) was higher in FGHAZ than in CGHAZ. Martensite transformation rate was higher in FGHAZ than in CGHAZ, which is due to the different grain size and assumed atom (Mn and C) segregation. Consequently, the softer martensite was measured in CGHAZ than in FGHAZ. Although 10~11% austenite retained in FGHAZ, the possible Transformation Induced Plasticity (TRIP) effect at −60 °C test temperature may lower the impact toughness to some degree. Therefore, the mean absorbed energy of 31, 39 and 42 J in CGHAZ and 56, 45 and 36 J in FGHAZ were exhibited at the same welding heat input. The more stable retained austenite was speculated to improve impact toughness in heat-affected zone (HAZ). For these 5Mn steels, reversed austenite plays a significant role in affecting impact toughness of heat-affected zones more than grain size.

The Effect of Different Atomic Substitution at Mn Site on Magnetocaloric Effect in Ni50Mn35Co2Sn13 Alloy

The effect of different atomic substitutions at Mn sites on the magnetic and magnetocaloric properties in Ni50Mn35Co2Sn13 alloy has been studied in detail. The substitution of Ni or Co for Mn atoms might lower the Mn content at Sn sites, which would reduce the d-d hybridization between Ni 3d eg states and the 3d states of excess Mn atoms at Sn sites, thus leading to the decrease of martensitic transformation temperature TM in Ni51Mn34Co2Sn13 and Ni50Mn34Co3Sn13 alloys. On the other hand, the substitution of Sn for Mn atoms in Ni50Mn34Co2Sn14 would enhance the p-d covalent hybridization between the main group element (Sn) and the transition metal element (Mn or Ni) due to the increase of Sn content, thus also reducing the TM by stabilizing the parent phase. Due to the reduction of TM, a magnetostructural martensitic transition from FM austenite to weak-magnetic martensite is realized in Ni51Mn34Co2Sn13 and Ni50Mn34Co2Sn14, resulting in a large magnetocaloric effect around room temperature. For a low field change of 3 T, the maximum ∆SM reaches as high as 30.9 J/kg K for Ni50Mn34Co2Sn14. A linear dependence of ΔSM upon μ0H has been found in Ni50Mn34Co2Sn14, and the origin of this linear relationship has been discussed by numerical analysis of Maxwell’s relation.

Magnetic and Structural Transitions Tuned through Valence Electron Concentration in Magnetocaloric Mn(Co1–xNix)Ge

The structural and magnetic properties of magnetocaloric Mn(Co1–xNix)Ge compounds have been studied. Two responses to the increase of valence electron concentration on substitution of Ni (3d84s2) for Co (3d74s2) in the orthorhombic phase (Pnma) are proposed: expansion of unit-cell volume and redistribution of valence electrons. We present experimental evidence for electronic redistribution associated with the competition between magnetism and bonding. This competition in turn leads to complex dependences of the reverse martensitic transformation temperature TM (orthorhombic to hexagonal (P63/mmc)) and the magnetic structures on the Ni concentration. Magnetic transitions from ferromagnetic structures below x = 0.50 to noncollinear spiral antiferromagnetic structures above x = 0.55 at low temperature (e.g., 5 K) are induced by modification of the density of states at the Fermi surface due to the redistribution of valence electrons. TM is found to decrease initially with increasing Ni content and then increa...

Orthorhombic intermediate phase originating from {110} nanotwinning in Ni50.0Mn28.7Ga21.3 modulated martensite

The temperature evolution of the structure of Ni50.0Mn28.7Ga21.3 single crystals exhibiting magnetic shape memory effect was investigated near the martensitic transformation by X-ray diffraction. Upon heating, five-layered modulated monoclinic martensite (10M) with a=0.5974 nm, b=0.5949 nm, c=0.5589 nm, and γ=90.34∘ at room temperature (297 K) changed to an intermediate (10M′) phase with a=b=0.5945 nm, c=0.5616 nm, and γ=90.25∘ at a few tenths of kelvin below the transformation to austenite at TA=327 K. This previously unreported 10M′ phase between 10M martensite and austenite can be described as orthorhombic using axes diagonal to the original a and b axes derived from the cubic L21 cell. Upon cooling, the phase remained stable in a broader temperature interval down to temperature T=318 K at which it transformed back to monoclinic 10M structure with a≠b. The transformation temperature coincided with the martensitic transformation temperature TM=318 K. The SEM observations and theoretical X-ray diffraction calculations indicate that the intermediate phase is actually the nanotwinned original 10M phase. This {110} nanotwinning with ≈ 20 nm mean twin width originates from the complex hierarchical branching on the austenite nucleus–martensite interface.

Martensitic Transformation and Crystal Structure Characterization of Ni-Fe-Ga-Co Ferromagnetic Shape Memory Alloys

In this paper, the martensitic transformation temperature, the microstructure and the crystal structure of the complicated martensitic phases of Ni56-xFe19Ga25Cox (x =0, 1.5, 3, 4.5, 6) alloys were investigated by DSC, XRD, SEM and TEM techniques. DSC results show that the martensitic transformation temperature Tm, which is above the room temperature, decreases with the increasing Co content. The microstructure of the Ni56-xFe19Ga25Cox (x =0, 1.5, 3, 4.5, 6) alloys is composed by the martensitic lath and randomly distributed γ phase. The 6M+14M mixed modulated martensite and the γ second phase were detected in the Ni53Fe19Ga25Co3 alloy by XRD and TEM tests.

Thermal and magnetic field-induced martensitic transformation in Ni50Mn25−xGa25Cux (0 ⩽ x ⩽ 7) melt-spun ribbons

We have studied the phase transformation behavior of melt-spun ribbons with x = 0, 1, 2, …, 7 (at%). It is shown that Cu substitution simultaneously increases and decreases the martensitic transformation temperature TM and the magnetic transition temperature of austenite , respectively. In Ni50Mn18Ga25Cu7 ribbons, the magnetic and structural transformations are coupled. The field-induced martensitic transformation from a paramagnetic austenite with lower magnetization to a ferromagnetic martensite with higher magnetization has been studied. The critical field μoHcr above which the magnetic field can induce the martensitic transition is well defined and decreases linearly with the decrease of the temperature at a reduction rate of 1 T K−1. Under a magnetic field of 5 T, the starting temperature of martensitic transformation (Ms) increases by ~9 K. Such a magnetic field-induced transformation is irreversible and temperature dependent, giving rise to the maximum magnetic entropy change of 17.8 J kg−1 K−1 and hysteresis losses of 43.6 J kg−1 under the magnetic field change of 5 T, respectively.

Wheel speed-dependent martensitic transformation and magnetocaloric effect in Ni–Co–Mn–Sn ferromagnetic shape memory alloy ribbons

This study investigated systematically the effects of different wheel speeds (10–30m/s) on the structural, microstructural, magnetic, and magnetocaloric properties of as-spun and annealed Ni–Co–Mn–Sn ribbons. The ribbons comprised a mixture of austenite with L21 cubic structure and non-modulated martensite with L10 tetragonal structure. Both as-spun and annealed ribbons showed an increase in the reverse martensitic transformation temperature (Tm) and a decrease in the thermal hysteresis (Thys) when the wheel speed was increased from 10m/s to 25m/s. Unexpectedly, the Tm decreased and the Thys increased in the ribbons prepared with a wheel speed of 30m/s. This finding may be attributed to the appearance of the γ-precipitated phase, which was lacking in the other samples. The martensitic transformation with a large magnetocaloric effect can be tailored in a wide temperature range, i.e., 30K for the as-spun ribbons and 33K for the annealed ribbons. The as-spun ribbons prepared at 15m/s and annealed ribbons prepared at 25m/s achieved large effective refrigerating capacities of 48.8 and 47.8J/kg, respectively, in the field change of 0–1T. These results provided significant insights into the effects of wheel speed on the magnetic and functional properties, as well as into the underlying mechanisms, of Ni–Mn-based multifunctional ribbon alloys.

Phase transition and magnetocaloric effect of Ni50Mn29Ga21−xTbx (0 ⩽ x ⩽ 1) alloys

Ni50Mn29Ga21−xTbx (0⩽x⩽1) alloys were studied with the microstructure, phase transition, and magnetocaloric effect. Dual-phase microstructure containing the martensite matrix and Tb-rich precipitations were formed. The martensitic transformation was observed over the whole composition range, with the transformation temperature TM significantly increased by the addition of terbium. The magnetic transition temperatures of the austenite and martensite, i.e. TCA and TCM, were monitored for 0⩽x⩽0.16 and 0.27⩽x⩽1, respectively. Both TCA and TCM were slightly decreased by the addition of terbium. For 0.16⩽x⩽0.27 the martensitic transformation was coincided with the magnetic transition in case of TM=TC, giving rise to the coupled magneto-structural transition from ferromagnetic martensite to paramagnetic austenite. Sizable magnetic entropy change was induced by magnetic field in the vicinity of the coupled magneto-structural transition.

Effect of magnetic field on the first order Martensite phase transition in Ni55Fe20Al25 shape memory alloy

The temperature and magnetic field effect on the transport and magnetic properties of a ferromagnetic shape memory alloy Ni55Fe20Al25 has been studied. The resistivity and thermo magnetization results provide the evidence of martensite transformation and the presence of coexisting austenite and martensite phases by first order magnetic phase transition (FOMT) at different fields. Martensite transformation temperature (Tm) shifts towards higher temperature with the application of magnetic field which reveals the sensitivity of this alloy. Ni-Fe-Al alloys are potential new candidates for ferromagnetic shape memory alloys.

First-Principles Investigations on Magnetic Properties in Off-Stoichiometric Ni<sub>2</sub>MnIn Magnetic Shape Memory Alloys

Ni-Mn-In is a new type of magnetic shape memory alloy, but in the stoichiometric Ni2MnIn, the martensitic transformation cannot perform. Thus, in order to obtain the appropriate Curie temperature TC and the martensitic transformation temperature TM, the composition adjustment must be carried out around the stoichiometric Ni2MnIn. The process of composition adjustment would induce various point defects. This paper scientifically studies the magnetic properties of off-stoichiometric Ni2MnIn alloys by first-principles calculations.

Magneto-Structural Properties of Ni2MnGa Ferromagnetic Shape Memory Alloy in Magnetic Fields

The purpose of this review was to investigate the correlation between magnetism and crystallographic structures as it relates to the martensite transformation of Ni2MnGa type alloys, which undergo martensite transformation below the Curie temperature. In particular, this paper focused on the physical properties in magnetic fields. Recent researches show that the martensite starting temperature (martensite transformation temperature) TM and the martensite to austenite transformation temperature (reverse martensite temperature) TR of Fe, Cu, or Co-doped Ni–Mn–Ga ferromagnetic shape memory alloys increase when compared to Ni2MnGa. These alloys show large field dependence of the martensite transformation temperature. The field dependence of the martensite transformation temperature, dTM/dB, is −4.2 K/T in Ni41Co9Mn32Ga18. The results of linear thermal strain and magnetization indicate that a magneto-structural transition occurred at TM and magnetic field influences the magnetism and also the crystal structures. Magnetocrystalline anisotropy was also determined and compared with other components of Ni2MnGa type shape memory alloys. In the last section, magnetic field-induced strain and magnetostriction was determined with some novel alloys.

Effect of Co substitution on the martensitic transformation and magnetocaloric properties of Ni50Mn35−xCoxSn15

Martensitic transformation and magnetic entropy change in Co substituted Ni50Mn35−xCoxSn15 (x = 0, 1.0, 1.5, 2.0, and 3.0) Heusler alloys have been investigated by X-ray powder diffraction analysis, differential scanning calorimetry, and magnetic measurements. X-ray diffraction analysis reveals that the Ni50Mn35−xCoxSn15 alloys have L21 Heusler structure at room temperature. The phase decomposition of the sample with x = 3.0, after annealing 48 h at 1173 K, is confirmed by both X-ray powder diffraction analysis and energy-dispersive x-ray spectroscopy in scanning electron microscopy. With the increase of the Co content from 0 to 2.0, the martensitic transformation temperature TM increases from 185 to 245 K, which is in good agreement with the rule of valence electron concentration e/a-dependence of TM. The magnetic entropy change ∆SM is investigated in the vicinity of the martensitic transformation.