Relaxation in Ni–Mn–Ga ferromagnetic shape memory alloys

Evidence of relaxation has been observed in ferromagnetic Ni–Mn–Ga single crystals. The relaxation may be explained by a change in symmetry-conforming short-range ordering according to Ren and Otsuka in this off-stoichimetric ordered alloy. Martensite stabilization has also been found after martensite ageing.     

Magnetic and mechanical properties of Ni–Mn–Ga/Fe–Ga ferromagnetic shape memory composite

A ferromagnetic shape memory composite of Ni–Mn–Ga and Fe–Ga was fabricated by using spark plasma sintering method. The magnetic and mechanical properties of the composite were investigated. Compared to the Ni–Mn–Ga alloy,the threshold field for magnetic-field-induced strain in the composite is clearly reduced owing to the assistance of internal stress generated from Fe–Ga. Meanwhile, the ductility has been significantly improved in the composite. A fracture strain of 26% and a compressive strength of 1600 MPa were achieved.     

Annealing effect on phase transformation in nano structured Ni–Mn–Ga ferromagnetic shape memory alloy

Nano structured Ni_52.6Mn_23.7Ga_24.3 alloy was prepared using the ball milling technique. High martensitic transition temperatures are observed in the range between 336 and 367 K. The X-ray diffraction profile revealed that annealed Ni–Mn–Ga powder at 1073 K displays mixture phases of austenite and martensite. Annealing at 1173 K induces phase transformation from mixture phase to Heusler L2₁ structure, which confirms the high-temperature shape memory effect. On the contrary, the milled sample shows no evidence of shape memory effect. Furthermore, annealing at higher temperature (1273 K) shows the accumulation of oxidation, which leads to the loss of shape memory effect. The grain size increases with increasing annealing temperature and causes deterioration in the soft magnetic properties.     

Ab initio calculations of the ferromagnetic shape memory alloy Ni–Mn–Al

Results of the first-principles calculations of the magnetic shape-memory alloy NiMnAl are presented. The properties of a series of alloys in the composition range Ni₅₀Mn _x Al_{50? x} with 0 ≤ x ≤ 50 were deduced from the ab initio simulations. One essential result is that the alloy is ferromagnetic in the range from 14 to 31?at.% Mn. Furthermore, the martensitic phases 2?M, 10?M, and 14?M with long-periodic structure were calculated. They are metastable in the stoichiometric Ni₂MnAl alloy due to additional bonding between specific atomic sites. Their properties are discussed in terms of the density of states and their charge distribution.     

New aspects of martensite stabilization in Ni–Mn–Ga high-temperature shape memory alloy

We report on new aspects of martensite stabilization in high-temperature shape memory alloys. We show that, due to the difference in activation energies among various structural defects, an incomplete stabilization of martensite can be realized. In material aged at high temperatures, this gives rise to a variety of unusual features which are found to occur in the martensitic transformation. Specifically, it is shown that both forward and reverse martensitic transformations in a Ni–Mn–Ga high-temperature shape memory alloy can occur in two steps. The observed abnormal behaviour is evidence that, in certain circumstances, thermoelastic martensitic transformation can be induced by diffusion.     

Thickness dependent phase transformation of magnetron-sputtered Ni–Mn–Sn ferromagnetic shape memory alloy thin films

In this study, the influence of film thickness on the first-order martensite–austenite phase transformation of Ni–Mn–Sn ferromagnetic shape memory alloy thin films has been systematically investigated. Different thicknesses of the Ni–Mn–Sn films (from ~100 to 2,500 nm) were deposited by DC magnetron sputtering on Si (100) substrates at 550 °C. X-ray analysis reveals that all the films exhibit austenitic phase with the L2₁ cubic crystal structure at room temperature. The grain size and crystallization extent increase with the increase in film thickness, but the films with thickness above ~1,400 nm show structural deterioration due to the formation of MnSn₂ and Ni₃Sn₄ precipitates. The improvement in the crystallinity of the film with thickness is attributed to the decrease in film–substrate interfacial strain resulting in preferred oriented growth of the films. Temperature-dependent magnetization measurements as well as electrical measurements demonstrate the complete absence of phase transformation for the film of thickness of ~120 nm. For thickness greater than 400 nm, film exhibits the structural transformation, and it occurs at higher temperature with better hysteresis as film thickness is increased up to ~1,400 nm, after which degradation of phase transformation phenomenon is observed. This degradation is attributed to the disorders present in the films at higher thicknesses. Film with thickness ~1,400 nm possesses the highest magnetization with the smallest thermal hysteresis among all the films and therefore best suited for the actuators based on first-order structural phase transformation. Nanoindentation measurements reveal that the higher values of hardness and elastic modulus of about 5.5 and 215.0 GPa obtained in film of 1,014 nm thickness can considerably improve the ductility of ferromagnetic shape memory alloys (FSMA) and their applicability for MEMS applications. The exchange bias phenomenon is also found to be present in the films of thickness 1014, 1412, and 2022 nm exhibiting prominent martensitic transformation. Film of thickness 2,022 nm exhibits maximum exchange bias of ~50 Oe and higher exchange bias blocking temperature of 70 K as compared to other films.     

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 (T_C) also decreased with increasing Al concentration. Ni₅₀Mn_36.3Sb_10.4Al_3.3 thin film showed significant improvement in exchange bias field as compared to pure Ni_50.3Mn_36.9Sb_12.8 thin film. This enhancement in the exchange bias field H_EB = 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.     

Phase transformation in Ni–Mn–Sn ferromagnetic shape memory alloy thin films induced by dense ionization

The effects of 200 MeV Au ions irradiation on the structural and magnetic properties of Ni–Mn–Sn ferromagnetic shape memory alloy (FSMA) thin films have been systematically investigated. In order to understand the role of initial microstructure and phase of the film with respect to high energy irradiation, the two types of Ni–Mn–Sn FSMA films having different phases at room temperature were irradiated, one in martensite phase (Ni_58.9Mn_28.0Sn_13.1) and other in austenite phase (Ni₅₀Mn_35.6Sn_14.4). Transmission electron microscope (TEM) and scanning electron microscope (SEM) images along with the diffraction patterns of X-rays and electrons confirm that martensite phase transforms to austenite phase at a fluence of 6×10¹² ions/cm² and a complete amorphization occurs at a fluence of 3×10¹³ ions/cm², whereas ion irradiation has a minimal effect on the austenitic structure (Ni₅₀Mn_35.6Sn_14.4). Thermo-magnetic measurements also support the above mentioned behaviour of Ni–Mn–Sn FSMA films with increasing fluence of 200 MeV Au ions. The results are explained on the basis of thermal spike model considering the core and halo regions of ion tracks in FSMA materials.     

Ab initio study of the composite phase diagram of Ni–Mn–Ga shape memory alloys

The magnetic and structural properties of a series of nonstoichiometric Ni–Mn–Ga Heusler alloys are theoretically investigated in terms of the density functional theory. Nonstoichiometry is formed in the coherent potential approximation. Concentration dependences of the equilibrium lattice parameter, the bulk modulus, and the total magnetic moment are obtained and projected onto the ternary phase diagram of the alloys. The stable crystalline structures and the magnetic configurations of the austenitic phase are determined.     

Stress–strain–temperature behaviour of [001] single crystals of Co49Ni21Ga30 ferromagnetic shape memory alloy under compression

The conventional shape memory effect (SME) and pseudoelasticity (PE) in as-grown [100] single crystals of Co₄₉Ni₂₁Ga₃₀ alloy under compression are reported. The parent single crystals exhibit about 5% transformation strain at compressive stress levels as low as 4?MPa, and a pseudoelastic strain of 4.5%. Complete PE was observed in the temperature range from 35 to 285°C, along with increasing stress hysteresis with temperature. The latter is attributed to increasing number of variants and the corresponding variant–variant interactions. We demonstrate that the current material can be utilized in applications that demand high strength at elevated temperatures. Moreover, the current results also indicate the potential of this material to exhibit magnetic shape memory effect, which could broaden the scope of utility of this material upon further research.     

Role of the chemical ordering on the magnetic properties of Fe–Ni cluster alloys

The spin and orbital moments of fcc Fe-Ni cluster alloys are determined within the framework of a d-band Hamiltonian including the spin-orbit coupling non perturbatively. Different sizes (up to 321 atoms), compositions, and chemical configurations (random alloys as well as core-shell arrays of iron and nickel atoms) are considered in order to reveal the crucial role played by local order and stoichiometry on the magnetic moments of the clusters. Interestingly, we have found considerably reduced average magnetizations for Fe-Ni clusters with Fe cores compared to that of the bulk alloy with the same composition. Indeed, in these configurations not only antiparallel arrangements between the local moments of some Fe atoms within the iron core are found, but also the total magnetization of the surface Ni atoms is significantly quenched. On the opposite, the disordered and Ni-core cluster alloys are characterized by high magnetizations resulting from saturated-like contributions from both Ni and Fe atoms, in agreement with recent ab-initio calculations. In general, the local orbital magnetic moments are strongly enhanced with respect to their bulk values. Finally, the variation of the orbital-to-spin moment ratio with the chemical order is discussed.     

Effect of irradiation on microstructure and hardening of Cr–Fe–Ni–Mn high-entropy alloy and its strengthened version

ABSTRACT

A single-phase fcc high-entropy alloy (HEA) of 20%Cr–40%Fe–20%Mn–20%Ni composition and its strength with yttrium and zirconium oxides version was irradiated with 1.4?MeV Ar ions at room temperature and mid-range doses from 0.1 to 10 displacements per atom (dpa). Transmission electron microscopy (TEM), scanning transmission electron microscopy with energy dispersive X-ray spectrometry (STEM/EDS) and X-ray diffraction (XRD) were used to characterise the radiation defects and microstructural changes. Nanoindentation was used to measure the ion irradiation effect on hardening. In order to understand the irradiation effects in HEAs and to demonstrate their potential advantages, a comparison was performed with hardening behaviour of 316 austenitic stainless steel irradiated under an identical condition. It was shown that hardness increases with irradiation dose for all the materials studied, but this increase is lower in high-entropy alloys than in stainless steel.     

Effect of upsetting deformation temperature on the formation of the fine-grained cast alloy structure of the Ni–Mn–Ga system

The plastic behavior during deformation by upsetting and its effect on the microstructure in the polycrystalline Ni_2.19Fe_0.04Mn_0.77Ga alloy are studied. The temperatures of martensitic and magnetic phase transformations were determined by the method for analyzing the temperature dependence of the specific magnetization as M _F = 320 K, A _S = 360 K, and T _C = 380 K. Using differential scanning calorimetry, it is shown that the phase transition from the ordered phase L2₁ to the disordered phase B2 is observed in the alloy during sample heating in the temperature range of 930–1070 K. The melting temperature is 1426 K. An analysis of the load curves constructed for sample deposition at temperatures of 773, 873, and 973 K shows that the behavior of the stress–strain curve at a temperature of 773 K is inherent to cold deformation. The behavior of the dependences for 873 and 973 K is typical of hot deformation. After deforming the alloy, its microstructure is studied using backscattered scanning electron microscopy. Plastic deformation of the alloy at study temperatures results in grain structure fragmentation in the localized deformation region. At all temperatures, a recrystallized grain structure is observed. It is found that the structure is heterogeneously recrystallized after upsetting at 973 K due to the process intensity at such a high temperature. The alloy microstructure after plastic deformation at a temperature of 873 K is most homogeneous in terms of the average grain size.     

Ag implantation-induced modification of Ni–Ti shape memory alloy thin films

Nanocrystalline thin films of Ni–Ti shape memory alloy are deposited on an Si substrate by the DC-magnetron co-sputtering technique and 120?keV Ag ions are implanted at different fluences. The thickness and composition of the pristine films are determined by Rutherford Backscattering Spectrometry (RBS). X-Ray diffraction (XRD), atomic force microscopy (AFM) and four-point probe resistivity methods have been used to study the structural, morphological and electrical transport properties. XRD analysis has revealed the existence of martensitic and austenite phases in the pristine film and also evidenced the structural changes in Ag-implanted Ni–Ti films at different fluences. AFM studies have revealed that surface roughness and grain size of Ni–Ti films have decreased with an increase in ion fluence. The modifications in the mechanical behaviour of implanted Ni–Ti films w.r.t pristine film is determined by using a Nano-indentation tester at room temperature. Higher hardness and the ratio of higher hardness (H) to elastic modulus (E_r) are observed for the film implanted at an optimized fluence of 9?×?10¹⁵ ions/cm². This improvement in mechanical behaviour could be understood in terms of grain refinement and dislocation induced by the Ag ion implantation in the Ni–Ti thin films.     

Temperature dependence of twinning stress – Analogy between Cu–Ni–Al and Ni–Mn–Ga shape memory single crystals

Abstract

To obtain a direct non-magnetic analogy to Ni–Mn–Ga 10M martensite with highly mobile twin boundaries, we present the recalculation of twinning systems in Cu–Ni–Al martensite. In this approach, the twinning planes denoted as Type I, Type II and compound have similar orientations for both alloys (Ni–Mn–Ga and Cu–Ni–Al). In Cu–Ni–Al, compound twinning exhibits the twinning stress of 1 to 2 MPa comparable to twining stress of Ni–Mn–Ga. In contrast Type II twinning stress of Cu–Ni–Al is approximately 20 MPa, i.e. much higher than twinning stress for Type II in Ni–Mn–Ga (0.1 to 0.3 MPa). Similarly to Ni–Mn–Ga, the twinning stress of Type II in Cu–Ni–Al is temperature independent. Moreover, no temperature dependence was found also for compound twinning in Cu–Ni–Al.     

Pressure effects on magnetic properties and martensitic transformation of Ni–Mn–Sn magnetic shape memory alloys

The mechanism for the effects of pressure on the magnetic properties and the martensitic transformation of Ni-Mn- Sn shape memory alloys is revealed by first-principles calculations. It is found that the total energy difference between paramagnetic and ferromagnetic austenite states plays an important role in the magnetic transition of Ni-Mn-Sn under pressure. The pressure increases the relative stability of the martensite with respect to the anstenite, leading to an increase of the martensitic transformation temperature. Moreover, the effects of pressure on the magnetic properties and the martensitic transformation are discussed based on the electronic structure.     

Effect of thermo-mechanical process on structure and high temperature shape memory properties of Ti–15Ta–15Zr alloy

The effect of thermo-mechanical treatment on microstructure evolution, martensite transformation, and shape memory behavior of Ti–15Ta–15Zr high temperature shape memory alloy were investigated. Different martensite morphologies were found with different annealing temperatures. The Ti–15Ta–15Zr alloy exhibits almost perfect shape memory recovery strain of 6% after annealing at 973 K for 0.5 h.     

Shape recovery mechanism observed in single crystals of Cu–Al–Ni shape memory alloy

Micromechanism of the shape recovery process is optically observed in single crystals of the Cu–Al–Ni shape memory alloy. Formation of X and λ-interfaces (interfacial microstructures with two intersecting habit planes) is documented, both in a thermal gradient and during a homogeneous heating. The observed growth mechanisms (i.e. mechanisms of nucleation and growth of the twinned structures) are described and analysed. Weakly non-classical boundaries between austenite and two crossing twinning systems are also documented.     

Effect of cobalt doping on thermoelastic martensitic transformations and physical properties of magnetic shape memory alloys Ni50 − x Co x Mn29Ga21

The magnetic and thermoelastic martensitic transformations and physical properties (magnetization, electrical resistivity, thermoelectric power, relative elongation, and thermal expansion coefficient) of multicomponent magnetic shape memory alloys Ni_{50 ? x} Co _x Mn₂₉Ga₂₁ (x = 0, 1, 2, 3, 10 at %) have been investigated. The critical temperatures of thermoelastic martensitic transformation and magnetic transitions have been determined. It has been found that the alloy with 10 at % Co undergoes a martensitic transformation in the temperature range of 6–10 K.