Martensite Stabilization Research Articles

Dependence of martensitic stabilization on Ms temperature in Cu-Al-Ni shape memory alloys

The occurrence of martensitic stabilization shows a strong positive dependence on Ms temperature in Cu-Zn-Al alloys. Compared with Cu-Zn-Al alloys, Cu-Al-Ni alloys have a more excellent resistance to martensitic stabilization. However, whether the occurrence of martensitic stabilization is dependent on Ms temperature in Cu-Al-Ni alloys is still unknown. To clarify this question, we investigated the relationship between SME and Ms temperature in directly-quenched Cu-Al-Ni alloys. Results showed that the positive dependence of martensitic stabilization on Ms temperature indeed existed in directly-quenched Cu-Al-Ni alloys. The Cu-Al-Ni alloys with Ms≤488 K possess a good resistance to martensitic stabilization. For the directly-quenched Cu-Al-Ni alloys with remarkably higher than 488 K, they cannot obtain SME due to the occurrence of martensitic stabilization after recovery heating at <25 K/s. The original number of vacancies at martensite boundaries (Nvmb) rising with the increment in Ms temperature was responsible for the remarkable lowering in resistance to martensitic stabilization during the process of slow heating. To adjust the original Nvmb in alloys with remarkably high Ms temperature, we changed the Ms temperature through precipitating α phase. The precipitation of α phase can enrich the matrix with Al and Ni elements and thus decrease the Ms temperature. The reduction in Ms temperature to below ∼500 K significantly decreased the original Nvmb and improved the resistance to martensitic stabilization during the slow heating process.

Modeling the effects of initial grain size, martensitic transformation induced dynamic grain refinement, phases, and texture on strength of a high entropy alloy using crystal plasticity

Microstructurally flexible high entropy alloys (HEAs) can be tailored for strength via dual-phase strengthening mechanisms, which originate from the strain-induced phase transformation microstructural phenomena. One such alloy is a recently synthesized Fe42Mn28Co10Cr15Si5 (at%) HEA consisting of metastable gamma austenite (γ), stable epsilon martensite (ε), and stable sigma (σ) phases. In this work, a crystal plasticity homogenization incorporating a physically based phase transformation and hardening models is used to interpret and better understand the effects of strain induced phase transformation phenomena on the overall hardening response of the alloy. The transformation model is conceived based on the grain-scale stress sensitive motion of partial dislocations forming shear bands, while the hardening model is an extended Kocks-Mecking-type dislocation density-based formulation sensitive to grain size and shape. The deformation of constituent grains per phase in the alloy is modeled as a combination of anisotropic elasticity, crystallographic glide, and γ→ε phase transformation. Parameters of the hardening and transformation models within the homogenization are calibrated and validated on a suite of data including flow stress curves and phase fractions measured under compression, while the corresponding texture evolution data is used solely for verification. Good predictions of the model elucidate that the transformation induced dynamic Hall-Petch-type barrier effect is the primary origin of strain hardening along with the increase in the ε-phase fraction and dislocation density, while the individual strength of the phases gives rise to the overall strength. The modeling framework described in the present work is expected to be applicable to other HEA systems.

An investigation on stabilisation and phase transformation characteristics of Cu-Al-Be shape-memory alloys quenched in different media

ABSTRACT The aim of the present investigation is to fabricate the ternary Cu87.59, Al11.99 and Be0.42 (wt.%) shape memory alloy (SMA), and to study the effect of quenching medium on the phases, phase transformation, stabilisation, and shape recovery ratio. SMA was fabricated, betatized at high temperature (β) and then quenched into different media and quench types. The quenched SMAs are examined for phases, microstructure, phase transformation temperatures, martensite stabilisation and shape recovery ratio. XRD results reveal that all the quenched alloys are of complete martensite of 18 R and 2 H variants. Meta quench oil quenched SMAs possess pure martensite, smaller grain size, lower transformation temperatures, and rapid recovery, whereas silicone oil exhibits larger grain size and delay in recovery and the mechanisms are discussed in detail in the discussion part. MQ oil quenched SMAs are suitable and recommended for rapid response/recovery actuator applications with enhanced properties.

Insights into effects of Fe doping on phase stability, martensitic transformation, and magnetic properties in Ni-Mn-Ti-Fe all-d-metal Heusler alloys

In this work, the effects of Fe doping on phase stability, martensitic transformation, and magnetic properties of Ni50Mn37.5-xTi12.5Fex (x = 3.125, 6.25, 9.375) all-d-metal Heusler alloys were systematically investigated by first-principles calculations. The results indicate a tendency for doped Fe atoms to aggregate within the Ni50Mn37.5-xTi12.5Fex alloys. As Fe concentration increases, a gradual reduction in both lattice constants and phase stability of austenite and martensite is observed. In the absence or presence of minimal Fe doping, both austenite and martensite exhibit antiferromagnetic behavior. However, at x = 9.375, a transition to a ferromagnetic state is observed in the austenite phase. This transition is attributed to the activation of the ferromagnetic coupling effect in the austenite phase induced by Fe doping in the Ni-Mn-Ti alloy. In contrast, the martensite phase maintains its antiferromagnetic characteristics throughout the doping range. A comprehensive analysis of the electronic structure elucidates the underlying physical mechanisms responsible for the martensitic transformation and magnetic property changes.

Effect of laser energy density on the phase transformation behavior and functional properties of NiTi shape memory alloy by selective laser melting

NiTi shape memory alloy (SMA) was prepared via selective laser melting (SLM). Orthogonal tests were conducted with varying laser power and scanning speeds to explore the influence of laser energy density on microstructure, phase transformation behavior, mechanical properties and functional properties (superelasticity and shape memory effect). As energy density increases from 24.3 to 106.3 J/mm³, microstructure transforms from fine to columnar grains. Ni consumption intensifies, boosting transformation temperature and B2 content at room temperature. Post cyclic stretching, B2 content in alloy escalates further at room temperature. Tensile strength peaked at 53.6 J/mm³, while the elongation rate significantly increases with increasing energy density. During cyclic stretching, low or high energy densities in SLM NiTi induce high-density dislocations and stable residual martensite, respectively, significantly inhibiting martensitic transformation and resulting in extremely poor functional properties. The alloy produced at 53.6 J/mm³ boasts exceptional superelasticity due to stress-induced martensitic transformation and phase inversion post-unloading. Following heating past the critical transition, it nearly fully recovers (97.78 % recovery rate). Despite multiple shape memory fatigue tests, it maintains a recovery rate above 90 %. This study provides crucial theoretical insights for optimizing NiTi SMA phase transformation behavior and enhancing the performance.

Shape memory effect enhancement via aging treatment of the Cu-Al-Mn-Si alloy manufactured using laser powder bed fusion

This study investigated the effects of aging treatments (200–500℃, 1 h) on the Cu-11.1Al-8.15Mn-0.37Si shape memory alloy manufactured using laser powder bed fusion (LPBF). As the aging temperature increased, the residual stress of the alloy decreased, and the microstructure transitioned from an austenite-martensite dual-phase structure to an austenite single-phase structure. A phenomenon of martensite stabilization induced by low-temperature aging at 200–300℃ was observed, leading to increased phase transformation temperatures and decreased mechanical properties. After aging at 400℃, the plasticity was improved and the shape memory effect (SME) recovery strain experienced a significant 48 % enhancement and reached a maximum value of 3.10 %. This enhancement was due to the improved ordering and texture homogenization of the austenite phase, confirmed by EBSD results. Meanwhile, EPMA analyses revealed the pinning effect of Mn5Si3 precipitations migrated from the grain interior to the grain boundaries which no longer obstructed the movement of martensitic habit planes during the recovery process. Thus, the SME improved. Additionally, after aging at 500℃, a large amount of α phase precipitated along grain boundaries leading to a decrease in mechanical properties. These findings underscore the importance of tailored aging treatments for optimizing shape memory properties in LPBF-manufactured Cu-based shape memory alloys.

Unraveling factors affecting the reversibility of martensitic phase transformation in FeNiCoAlTi shape memory alloys: Insights from HR-EBSD and acoustic emission analysis

The reversibility of the martensitic phase transformation is crucial for the functionality of shape memory alloys. The easier the martensite is pinned, the faster occurs the degradation of the shape memory effect during cyclic loading. This study investigates various factors influencing the stabilization of martensite during deformation of single-crystalline states of the FeNiCoAlTi system. In order to investigate early pinning mechanisms in more detail a 〈011〉 single crystal with expected lower reversibility of the martensitic phase transformation and, consequently, lower superelasticity was chosen. Complementary in situ methods, such as digital image correlation and acoustic emission, were used to characterize the evolution of the martensitic phase transformation and the role of dislocations during deformation. Thereby, experimental evidence was elaborated pointing at mechanisms that so far have been underrated in terms of functional degradation. Beside interaction of martensitic variants and dislocation activity, detwinning, associated with high acoustic energy, was identified as an additional significant factor contributing to reduced reversibility and superelasticity. In addition, high-resolution electron backscatter diffraction measurements were applied for the first time to identify residual stresses in the austenitic matrix, which significantly contribute to the reverse transformation. Micro-mechanical experiments using pillar compression tests were carried out to study the influence of residual back stresses of the austenitic matrix on the reversibility of the martensitic phase transformation. A reduced reversibility was found in the case of the absence of back stresses. The findings from this study foster the understanding of pinning mechanisms during loading of the FeNiCoAlTi shape memory alloy eventually enabling targeted optimization for enhanced superelastic material behavior.

Complementary In Situ Characterization of Superelasticity in Fe–Mn–Al–Ni–Ti Shape Memory Alloys by Acoustic Emission, Digital Image Correlation and Infrared Thermography

AbstractCoupled in situ investigations were conducted on a Fe–Mn–Al–Ni–Ti single crystal deformed in compression and two Fe–Mn–Al–Ni–Ti oligo-crystals deformed in tension. Acoustic emission measurements were employed to monitor the degradation of superelasticity and the stabilization of martensite due to dislocation processes. These observations were corroborated by the application of digital image correlation and infrared thermography measurements. A poor strain reversibility and a premature plastification of the parent phase were observed in case of the single crystal due to an unfavourable crystal orientation. A contradictory transformation behaviour of the two oligo-crystals was observed, with one specimen showing a promising strain reversibility and characterisitic signs of degradation, and the other specimen exhibiting a limited strain reversibility due to an unusual confinement of the martensitic phase transformation to an unfavourably oriented grain. In the former case, an increase in the dislocation density within five cycles was detected through a shift of the acoustic signals’ median frequencies. In the latter case, a strong coupling between martensite nucleation and dislocation generation led to a pronounced martensite stabilization after one loading cycle. For all specimens, temporal sequence effects related to the coupling of martensite nucleation and dislocation generation were detected by means of acoustic emission.

Investigation of the magnetic and structural properties in the non‐stoichiometric Heusler alloy Ni50Mn25+xSn25‐x; x = 13, 14

AbstractMemory shape magnetic alloys, especially Heusler alloys, are important materials in replacing conventional cooling with magnetic systems. In the present study off stoichiometric Heusler alloys with nominal composition Χ50Υ25+xΖ25‐x (X = Ni; Y = Mn; Z = Sn; x = 13, 14) were prepared by arc melting followed by thermal treatment. Structural properties were analyzed with X‐ray diffraction at room temperature (RT) and at elevated temperatures, above the martensite—austenite transition area, to determine the relevant crystallographic parameters and observe the transition. Martensite stabilization at RT appears to be a challenge, coexistence of martensite—austenite phases were observed and calculated for both 38–12 and 39–11 (16% and 12% austenite, respectively). Magnetization measurements versus temperature and field were recorded in the areas of interest where 1st order transitions were expected (355 K for x = 13 and 408 K for x = 14), and the magnetic entropy's changes (ΔSm) were determined [0.4 (J/kgK) for x = 13 and 0.3 J/(kgK) for x = 14; Hmax = 1 T]. The complex character of the magnetic properties and their dependence on Mn‐Sn ratio and on the distance between Mn atoms is discussed. The structure and the lattice parameters were determined using an anisotropic strain broadening model; stress and strain were detected in the structure due to crystal phase coexistence.

Origins of functional fatigue and reversible transformation of precipitates in NiTi shape memory alloy

The work clarifies several key questions in shape memory research that have eluded previous studies. The findings show that dislocation slip emanates at austenite-martensite interfaces during unloading and aligns with the internal twin boundary interface of martensite. It was observed that the type II internal twins of the martensite become parallel dislocations in the austenite. During reloading, these dislocations act as nucleation sites for the martensitic twins, reducing the nucleation barrier and the transformation stress. The precipitates facilitate martensite nucleation but also act as an obstacle to martensite front motion, restrict detwinning, and pin the interfacial dislocations during unloading, thereby contributing to residual strains and martensite stabilization. Martensite nucleation is not suppressed by the size of the thin film, which is of the order of 85 to 105 nanometers thick, and repeated transformation occurred cycle after cycle. Single crystals deformed in the <101>LD exhibited the best recoverability of up to 5.5 % and tensile stresses of up to 1.4 GPa. It was demonstrated for the first time that, when favorably oriented, Ni4Ti3 precipitates undergo a reversible phase transformation to R-phase and can accommodate up to 4 % reversible strains.

Reverse ε→γ transformations of temperature-induced and stress-induced martensites in Fe–Mn–Si alloys with shape memory effect

In this work we argue that the reverse martensitic transformation (MT) of the stress-induced martensite (SIM) of Fe–Mn–Si-based alloys for moderate prestrains occurs in essentially thermoelastic-like way, in contrast to non-thermoelastic MT of temperature-induced martensite (TIM). We compare, using a number of complementary experimental techniques, the temperatures and the kinetics of the reverse ε→γ martensitic transformation of SIM and TIM for commercial and model Fe–Mn–Si alloys after various thermomechanical treatments. We claim that two studied phenomena are generic in Fe–Mn–Si-based alloys of different compositions and microstructure: first, opposite to the conventional effect of stabilization of martensite by prestrain, the reverse MT during heating of alloys prestrained in bending and tension starts well below (up to 100 K) the start temperature of the reverse MT for the TIM of the same alloy. The difference between engineering temperatures of the start of the reverse transformation, As, for SIM and TIM reaches ca. 50 K, pointing to a need to consider different As for TIM and SIM, AsTIM and AsSIM. Second, the reverse transformations of TIM and SIM show very different kinetics. The reverse MT of SIM is diffuse over the temperature range exceeding 100 K. The reverse MT of TIM, on the contrary, is well localized within the temperature range of ca. 20 K. The experimental observations can be interpreted assuming that, due to the differences in the formation mechanisms of SIM and TIM, the latter shows non-thermoelastic behaviour, whereas the SIM shows similarities with the thermoelastic martensites stabilized by prestrain.

Outstanding shape memory effect and thermal stability of Cu–Al–Ni-based high temperature shape memory alloys achieved by Gd and Mn doping

In this paper, the effects of Mn element and rare earth element Gd on the microstructure, thermal stability, mechanical properties and shape memory effect of polycrystalline Cu-12.0Al-4.0Ni and Cu-12.0Al-4.0Ni-1.5Mn-xGd high-temperature shape memory alloy were investigated. The results show that the Cu-12.0Al-4.0Ni alloy exhibits millimeter-sized grain, dual-phase structure with coexistence of 18R and 2H martensite and poor thermal stability. When the Mn and Gd element was added to Cu-12.0Al-4.0Ni alloys, the grain size can be refined by an order of magnitude, and the 2H martensite disappeared completely, accompanied by the formation of the AlCu4Gd phase, meanwhile, thermal stability is significantly improved. The grain refinement improves the mechanical properties and shape memory effect of Cu-12.0Al-4.0Ni-1.5Mn-xGd alloys. When x = 0.3, the compressive fracture stress, compressive fracture strain and recoverable strain can reach 969 MPa, 15 % and 8 %, respectively.

Revealing the evolution behavior of multiple carbides precipitation and mechanical response in M54 secondary hardening steel

As a new type of secondary hardening ultra-high strength steel, the evolution behavior of multiple carbides and its effect on the strength–toughness of M54 steel have been studied by subjecting to tempering duration (2– 10 h) at 520 °C. Within tempering of 2– 4 h, there is incompletely dissolved cementite with intra-granular distribution; however, it does not deteriorate the impact toughness. Without the observation of cementite, it shows the largest precipitation density of M2C carbides after 8 h-tempering, with the maximum density of 650 μm−2 and minimum inter-particle distance of 12.3 nm. Prolonged tempering of 10 h does not lead to coarsening due to the low diffusivity of constituent W element and coherent relationship of M2C/matrix interface. However, there is obvious segregation of MC carbides along the long-axis of M2C carbides in growth. The favorable precipitation of M2C carbides mostly contributes to the highest yield strength of 1747 MPa and ultimate tensile strength of 2037 MPa in case of 8 h-tempering. The stable ultrafined martensite blocks (∼0.30 μm) and high ratio of high-angle grain boundaries (∼50%) guarantee the desirable V-notched impact absorbed energy of ∼30 J for the 2– 8 h-tempering. Unfortunately, due to the insufficient ability of arresting propagating cracks, the possible preferential MC carbides-distributed sub-grain boundaries leads to an obvious degradation of impact toughness (∼18 J) in the 10 h-tempered samples.

Phase Transformations and Martensite Stabilization in Ni&lt;sub&gt;2.36&lt;/sub&gt;Mn&lt;sub&gt;0&lt;/sub&gt;&lt;sub&gt;.64&lt;/sub&gt;Ga High-Temperature Shape Memory Alloy

We report on experimental investigations of a Ni2.36Mn0.64Ga Heusler alloy, which transforms to tetragonal martensite at cooling below Ms ≈ 271°С. The evolution of lattice constants was tracked by in situ neutron diffraction measurements. It was found that the martensite tetragonality c/a gradually decreases during heating from room temperature to austenite transition start temperature As ≈ 272°С. The phenomenon of martensite stabilization was investigated by differential scanning calorimetry utilizing three different protocols of the martensite aging. It was found that the martensite aging at a constant temperature T = 255°С merely shifts the reverse transformation to higher temperatures, while the reverse transformation temperature interval (Af – As) remains the same (≈ 30°C) independently of aging time. On the other hand, a multistep aging at different temperatures starting from T = 255°С not only shifts the reverse transformation temperature, but makes the transformation temperature interval narrower down to As – Af ≈ 10°C.

Thermal stability of Ag-doped Ni-Mn-Ga high-temperature shape memory alloy

In the present study, the microstructure features and martensitic transformation behaviour of a Ni-Mn-Ga alloy were tailored by adjusting the Ag content. In addition, the effect of Ag content on the thermal stability was also explored. The results revealed that the non-modulated martensite phase and Ag particles coexisted in the Ni-Mn-Ga-Ag alloys. Different martensite variants were in the {112} type I twinning relationship. With increasing Ag content, not only all transformation temperatures but also the thermal hysteresis decreased continuously in a quasilinear manner. This was attributed to the introduction of small and soft Ag particles, which were applied to release the elastic energy during the martensitic transformation. The decreasing trend of elastic energy was highly nonlinear. The alloy with 4 at% Ag addition was the percolation threshold. In addition, the relationship of energy dissipation against Ag is not monotonic. While the energy dissipation of Ni48Mn25Ga19Ag8 is almost zero, the others were much higher. Furthermore, unlike others, Ni48Mn25Ga19Ag8 featured good martensitic transformation stability upon thermal cycling.

Evolution of microstructure and strength of a high entropy alloy undergoing the strain-induced martensitic transformation

In a recent work, we have reported outstanding strength and work hardening exhibited by a metastable high entropy alloy (HEA), Fe42Mn28Co10Cr15Si5 (in at. %), undergoing the strain-induced martensitic transformation from metastable gamma austenite (γ) to stable epsilon martensite (ε). However, the alloy exhibited poor ductility, which was attributed to the presence of the brittle sigma (σ) phase in its microstructure. The present work reports the evolution of microstructure, strength, and ductility of a similar HEA, Fe38.5Mn20Co20Cr15Si5Cu1.5 (in at. %), designed to suppress the formation of σ phase. A cast and then rolled plate of the alloy was processed into four conditions by annealing for 10 and 30 min at 1100 °C and by friction stir processing (FSP) at tool rotation rates of 150 and 400 revolutions per minute (RPM) to facilitate detailed examinations of variable initial grain structures. Neutron diffraction and electron microscopy were employed to characterize the microstructure and texture evolution. The initial materials had variable grain size but nearly 100% γ structure. Diffusionless strain induced γ→ε phase transformation took place under compression with higher rate initially and slower rate at the later stages of deformation, independent on the initial grain size. The transformation facilitated part of plastic strain accommodation and rapid strain hardening owing to a transformation-induced dynamic Hall-Petch-type barrier effect, increase in dislocation density, and texture. The peak strength of nearly 2 GPa was achieved under compression using the structure created by double pass FSP (150 RPM followed by 150 RPM). Remarkably, the tensile elongation exhibited by the alloy was nearly 20% with fracture surfaces featuring a combination of ductile dimples and cleavage.

Precipitation behavior of the G-phase strengthened 7Ni maraging steels

In this study, G-phase strengthened 7Ni maraging alloys were studied using a combination of thermodynamic prediction based on the TCFE-7 database and advanced experimental techniques, including micro-hardness testing, electron backscatter diffraction (EBSD), in-situ X-ray diffraction (XRD), transmission electron microscopy (TEM), and atom probe tomography (APT). The martensite reversion and phase stability of overcooled austenite were precisely determined for a series of Fe–7Ni–2Si-based alloys, validating the effectiveness of thermodynamic predictions in martensite transformation.Based on these theoretical prediction, aging hardness measurements and microstructural observations further revealed that Ni16X6Si7-G (X = Ti, Nb, Ta) precipitates are effective strengthening phases in 7Ni maraging steel, with the exception of Ni16X6Si7 (X = Mn, Zr) due to their significantly different thermal stabilities. Experimental results showed that the Ni16X6Si7-G (X = Ti, Nb, Ta) precipitates remained stable and densely distributed within the martensitic matrix after aging at 500 °C, resulting in high aging hardness values ranging from 350 to 550 HV. Among the studied alloys, the 1Ti alloy strengthened by the Ni16Ti6Si7-G phase exhibited the finest particle radius (estimated at 1.4 nm) and the highest number density (estimated at 1.9 × 1024/m3). Additionally, it is worth noting that the Ni16Zr6Si7-G phase was believed to form through eutectic reaction with α-Fe during solidification and the Ni16Mn6Si7-G phase was only stable at temperatures below 460 °C and was not detected experimentally.These findings enhance our comprehension of G-phase precipitation and strengthening in 7Ni maraging steel and underscore the potential for utilizing thermodynamic calculations and advanced experimental techniques to guide the design and optimization of high-strength alloys.

Influence of Manganese Content on Martensitic Transformation of Cu-Al-Mn-Ag Alloy.

The influence of manganese content on the formation of martensite structure and the final properties of a quaternary Cu-Al-Mn-Ag shape memory alloy (SMA) was investigated. Two alloys with designed compositions, Cu- 9%wt. Al- 16%wt. Mn- 2%wt. Ag and Cu- 9%wt. Al- 7%wt. Mn- 2%wt. Ag, were prepared in an electric arc furnace by melting of high-purity metals. As-cast and quenched microstructures were determined by optical microscopy and scanning electron microscopy equipped with EDS. Phases were confirmed by high-energy synchrotron radiation and electron backscatter diffractions. Austenite and martensite transformations were followed by differential scanning calorimetry and hardness was determined using the Vickers hardness test. It was found that the addition of silver contributes to the formation of the martensite structure in the Cu-Al-Mn-SMA. In the alloy with 7%wt. of manganese, stable martensite is formed even in the as-cast state without additional heat treatment, while the alloy with 16%wt. of manganese martensite transforms only after thermal stabilization and quenching. Two types of martensite, β1' and γ1', are confirmed in the Cu-9Al-7Mn-2Ag specimen. The as-cast SMA with 7%wt. Mn showed significantly lower martensite transformation temperatures, Ms and Mf, in relation to the quenched alloy. With increasing manganese content, the Ms and Mf temperatures are shifted to higher values and the microhardness is lower.

Manipulation of magnetocaloric and elastocaloric effects in Ni–Mn–In alloys by lattice volume and magnetic variation: Effect of Co and Fe co-doping

The effects of Co and Fe co-doping Ni–Mn–In alloy on the phase stability, lattice parameters, magnetic properties, and electronic structures are systematically investigated by using the first-principles calculations. Results indicate that Fe atoms replace the excess Mn2 atoms by direct and indirect coexistence (Fe→Mn2 and Fe→In→Mn2); Co substitutes the Ni atoms by direct substitution (Co→Ni) for the Ni–Mn–In alloy. The austenites all exhibit the ferromagnetic (FM) state for the studied compositions. The NM martensites are in the ferrimagnetic (FIM-1) state for the Ni2Mn1.5In0.5, Ni2Mn1.25In0.5Fe0.25, Ni1.75Mn1.5In0.5Co0.25, and Ni1.75Mn1.25In0.5Co0.25Fe0.25 alloys, while the other compositions are in the FM state. The phase stability of austenite and martensite decreases with increasing Co and Fe co-doping. A magnetic-structural coupling transition occurs at x < 0.25 and y < 0.25. The Ni1.91Mn1.5In0.5Co0.08 and Ni1.91Mn1.42In0.5Co0.08Fe0.08 alloys exhibit an A→6M→NM transformation, accompanied by a magnetic transition. When Co and Fe are co-doped, the hybridization strength between Co and Fe is greater than that between Co/Fe and Mn. The enhancement of magnetocaloric and elastocaloric effects is favored by larger magnetization difference (∆M) and lattice volume change (∆V/V0). Based on the calculated phase stability, magneto-structure coupling, ∆V/V0 and c/a ratio, one can predict that the Ni2-xMn1.5-yIn0.5CoxFey alloy with Co content 0 ≤ x ≤ 0.25 and Fe content 0 ≤ y ≤ 0.05 is predicted to have good magneto-controlled functional behavior.

Effect of Ni alloying on vacancy behavior and damping capacity in martensitic ductile Cu–Al–Mn alloys

Ductile Cu–Al–Mn shape memory alloys obtain the damping capacity through the movement of martensite boundaries. To reduce the number of vacancies at martensite boundaries can improve their damping capacity. Our results showed that Ni alloying could remarkably suppress the vacancy movement during the process of cooling from high temperature and heating as well as ageing. The Cu-18.0Al-8.4Mn-2.5Ni alloy showed remarkably higher damping capacity than the Cu-18.0Al-9.4Mn alloy did due to the reduced vacancies at martensite boundaries by the Ni alloying, regardless of the direct quenching, up-quenching, and step-quenching. The occurrence of martensitic stabilization could rationalize the lower improvement of damping capacity in the up-quenched Cu-18.0Al-9.4Mn alloy. The present results also further confirmed that the original number of vacancies at the martensite boundaries determined the occurrence of martensitic stabilization.