•Ni40Co10Mn40Sn10 powders were studied with the phase transition and magnetocaloric effect.•Effect of annealing temperatures on the magnetostructural transition behaviors id clarified.•The magnetocaloric effect of powders is very close to that of the bulk alloy.The phase structure, the magnetostructural transition behavior and the associated magnetocaloric effect have been investigated in Ni40Co10Mn40Sn10 bulk and powders, mainly focusing on the effects of annealing temperatures ranging from 573 K to 773 K. A non-modulated martensitic structure was detected in the bulk and as-ground powders, but a seven-layered orthorhombic structure was observed after annealing. The magnetostructural transition in the bulk alloy from weak magnetic martensite to ferromagnetic austenite was also confirmed in the as-ground and annealed powders. Compared with the bulk alloy, the transition temperatures of the as-ground powders decreased markedly; the transition temperature hysteresis broadened, and the magnetization change across the transition was severely reduced. With increasing annealing temperatures, the parameters of the magnetostructural transition gradually improved. When the powders were annealed at 773 K, the magnetostructural transition behaved the most similar to the bulk alloy. Consequently, the large magnetic entropy change of 27 J/(Kg K) and the effective cooling capacity of 171.3 J/Kg that were obtained under the magnetic field of 5 T were very close to the bulk alloy. The internal stress and surface effects were considered to understand the observed phenomena.

The influence of heat treatment with different cooling rates on phase transition behaviors and magnetocaloric effect is systematically studied. Difference in atomic order is induced by changing cooling rates, where ordered phase is obtained in the furnace cooled (FC) sample while disordered phase is reserved in the water quenched (WQ) sample. The coupled magneto-structural transition is detected in both samples but the characteristic temperature significantly shifts to lower temperatures with increasing atomic order. Giant magnetic entropy change (ΔSmag) derived from magnetic field induced martensitic transformation is confirmed for both samples, and can be remarkably enhanced by the atomic ordering. The largest ΔSmag of 20. 9 J/(kg · K) is obtained at 307. 5 K under 5 T in the FC sample.

The elastocaloric effect (eCE) is studied in a Ni57Mn18Ga21In4 alloy undergoing the magnetostructural transition from paramagnetic austenite (PMA) to ferromagnetic martensite (FMM). Superelasticity associated with the stress-induced transition from PMA to FMM is realized at room temperature. Simultaneously, a large eCE marked by a temperature change ΔTad up to 9.6 K is detected. It is revealed that both the lattice vibration and magnetization change positively contribute to the large eCE. This work may open the possibility of tuning the eCE by simultaneous application of magnetic field and bias stress.Compressive (a) stress-strain curves and temperature change as a function of time (b) simultaneously tested for the Ni57Mn18Ga21In4 single crystal measured at different strain rates ranging from 0.025 mm/s to 0.4 mm/s.Download high-res image (273KB)Download full-size image

•The effect of Tb addition on magnetic transition behavior in Ni50-xTbxMn30Ga20 (x = 0−1) alloys is clarified.•Coupled magneto-structural transition were observed in Ni50-xTbxMn30Ga20 alloys with the range x = 0.1–0.8.•Sizable magnetocaloric effect was monitored from the magneto-structural transition.The magneto-structural transition, magnetic properties and magnetocaloric effect of the Ni50-xTbxMn30Ga20 (x = 0−1) alloys were studied. It was found that both the martensitic and magnetic transition temperatures were insensitive to the addition of Tb element. For the alloys with x = 0 and x = 1, the martensitic transformation and magnetic transition take place independently. For the alloys in the range of 0.1 ≤ x ≤ 0.8, coincidence of martensitic and magnetic transitions was detected, which resulted in a magneto-structural coupling transition from ferromagnetic martensite to paramagnetic austenite. The saturation magnetization of the martensite phase at 300 K marginally decreased with the addition of Tb element. Considerable and stable magnetic entropy change of approximate 6 JK−1 Kg−1 was obtained in vicinity of the magneto-structural transitions in the dual-phase alloys with 0.1 ≤ x ≤ 0.8.Download high-res image (359KB)Download full-size image

•Coupled magneto-structural transition were observed in single-phase Ni50Mn29Ga20.9Tb0.1 ribbon.•Sizable magnetocaloric effect was monitored from the magneto-structural transition.•The magnetocaloric effect was enhanced by the solid solution of Tb.Ni50Mn29Ga21−xTbx (x=0-1) ribbons with the solid solution of Tb element were synthesized by the melt spinning method. The phase transformation, magnetic properties and magnetocaloric effect were investigated. With the increasing Tb content the martensitic transformation temperatures were gradually increased while the Curie temperature was monotonously decreased. According to the detected phase transition temperatures, a phase diagram was established to describe the dependence of the magneto-structural transition on the Tb content. Three types of magneto-structural transitions were observed. Especially, the martensitic transformation was coincided with the magnetic transition in the single phase alloy with x = 0.1, giving rise to the coupled magneto-structural transition from ferromagnetic martensite to paramagnetic austenite. Sizable magnetic entropy change of 4.31 J/Kg K was induced from the coupled magneto-structural transition by the application of magnetic field of 50 kOe at 349.5 K.

•Grain size effect on martensitic transformation of Ni50Mn25Ga17Cu8 was revealed.•The martensitic transformation temperatures were decreased by the smaller grain size.•The average activation energy of transformation was higher for smaller grains.Single crystal, coarse-grained and fine-grained polycrystals of Ni50Mn25Ga17Cu8 high-temperature shape memory alloy were prepared. Grain size effect on the martensitic transformation was investigated. Compared with the single crystal, the martensitic transformation temperatures were slightly decreased by coarse grains, but were greatly decreased by fine grains. Strong grain size dependence of the kinetics of the martensitic transformation was monitored. The average activation energy of the transformation required for the fine-grained polycrystal was nearly four times of that for the single crystal.

The addition of rare earth elements (RE) is an effective way to improve the mechanical properties of Ni–Mn–Ga alloys. Unlike previous investigations which were completely focused on the N–Mn–(Ga,RE) alloys, in this work Ni50−xTbxMn30Ga20 (x=0–1) alloys in the form of (Ni,RE)–Mn–Ga were systematically studied with the microstructure, martensitic transformation, shape memory effect and mechanical properties. Dual-phase microstructure containing body-centered tetragonal martensite and Tb-rich hexagonal precipitates were formed. With the increasing Tb content, the volume fraction of the precipitates was also increased. Yet stable martensitic transformation occurring at about 80 °C was observed over the whole composition range. Both the transformation temperatures and hysteresis (∼11 °C) were quite insensitive to the composition variation. Sizable shape memory effect is obtained in all the polycrytalline alloys. Shape memory effect of 2.68% was detected for the alloy x=1 with large amount of precipitate. With the increasing volume fraction of precipitates both the compressive strength and strain were well increased for x≤0.2 and gradually decreased for 0.2≤x≤1. The transition of fracture mode from intergranular fracture to transgranular fracture and then to interphase fracture was revealed to contribute to the evolution of the mechanical properties.

Ni30Cu20Mn37+xGa13−x (x = 0–4.5) alloys were studied with the phase transformation and mechanical properties. With the increase of Mn content, the martensitic transformation temperatures increase and the Curie temperature decreases. Simultaneously, the room temperature microstructure evolves from single phase of austenite to dual phases containing martensite and precipitation. Both the ductility and the strength of the polycrystalline alloys are significantly improved by the precipitation. Coupled magnetostructural transition from weak magnetic martensite to ferromagnetic austenite is obtained in both single-phase and ductile dual-phase alloys.

The phase stability and magnetic properties were studied in Ni50−xCuxMn31Ga19 (x = 0–4) alloys where Ni atoms were partially substituted by the non-ferromagnetic Cu atoms. The martensitic transformation temperatures decreased with the increasing Cu content. The stability of the austenite was well favored by the substitution of Cu for Ni. More interestingly, the ferromagnetic ordering temperature TC was increased by the substitution of the non-ferromagnetic Cu atoms for the ferromagnetic Ni atoms. Simultaneously, the ferromagnetism of both the austenite and martensite was enhanced by Cu addition.Highlights► The stability of the austenite was well favored by the substitution of Cu for Ni. ► The Curie temperature was increased by the addition of the non-ferromagnetic Cu. ► The ferromagnetism was enhanced by the addition of non-ferromagnetic Cu element.

A ductile Ni30Cu20Mn41.5Ga8.5 dual-phase shape memory alloy was reported with the shape memory effect of 1.8%, the compressive plasticity of more than 70%, and the tensile plasticity of 4%. Although there is only small amount of the second phase distributing along the grain boundaries, in-situ morphology observations during dynamic tensile tests revealed their key contributions to the ductility of the alloy.

The linear elasticity was studied in a martensitic alloy Ni50Mn25Ga9Cu16. A 0.4 % linear elastic strain is obtained in the polycrystalline sample under compressive stress of 745 MPa. The elastic modulus is 186 GPa. The obtained linear elastic strain and elastic modulus are much higher than that of ternary Ni–Mn–Ga martensitic alloys.

The effects of directional solidification rate on the solidified morphologies and phase transformations were studied in polycrystalline Ni50.5Mn25Ga24.5 alloy prepared by induction zone melting method. With the directional solidification rate increasing from 10 mm/h to 800 mm/h, the solidified morphologies of the austenite phase evolved from planar, to cellular, and then to dendritic. The premartensitic transformation was only observed in the planar austenite, and was suppressed in the cellular and dendritic morphologies. The martensitic transformation was observed in all austenitic morphologies. Increasing the directional solidification rate, the martensitic transformation temperatures were slightly decreased, and the transformation hysteresis was expanded. The local energy barrier related to the compositional segregation from the cellular and dendritic austenitic morphologies suppressed the premartensitic transformation and had little influence on martensitic transformation.Highlights► Evolution of the austenitic solidification morphology of Ni50.5Mn25Ga24.5 was observed. ► The martensitic transformation was monitored in all austenitic morphologies. ► The premartensitic transformation was suppressed in the cellular and dendritic austenite.