Tunable Martensitic Transformation and Magnetic Properties of Sm-Doped NiMnSn Ferromagnetic Shape Memory Alloys

NiMnSn ferromagnetic shape memory alloys exhibit martensitic transformation at low temperatures, restricting their applications. Therefore, this is a key factor in improving the martensitic transformation temperature, which is effectively carried out by proper element doping. In this research, we investigated the martensitic transformation and magnetic properties of Ni43Mn46-x SmxSn11 (x = 0, 1, 2, 3) alloys on the basis of structural and magnetic measurements. X-ray diffraction showed that the crystal structure transforms from the cubic L21 to the orthorhombic martensite and gamma (γ) phases. The reverse martensitic and martensitic transformations were indicated by exothermic and endothermic peaks in differential scanning calorimetry. The martensitic transformation temperature increased considerably with Sm doping and exceeded room temperature for Sm = 3 at. %. The Ni43Mn45SmSn11 alloy exhibited magnetostructural transformation, leading to a large magnetocaloric effect near room temperature. The existence of thermal hysteresis and the metamagnetic behavior of Ni43Mn45SmSn11 confirm the first-order magnetostructural transition. The magnetic entropy change reached 20 J·kg−1·K−1 at 266 K, and the refrigeration capacity reached ~162 J·Kg−1, for Ni43Mn45SmSn11 under a magnetic field variation of 0–5 T.

MICROSTRUCTURE AND MARTENSITIC TRANSFORMATION OF Ni_{50}Ti_{50-x}Er_{x} SHAPE MEMORY ALLOYS

The effect of rare earth element Er addition on the microstructure and phase transformation behavior of Ni_{50}Ti_{50-x}Er_{x} (x =0, 1, 5) shape memory alloy was investigated experimentally. The results showed that the microstructure of Ni-Ti-Er ternary alloys consists of the NiEr precipitate and the NiTi matrix. A one-step martensitic transformation was observed in all alloys. The martensitic transformation temperature M_s increased gradually with increasing Er content.

Full Variation of Site Substitution in Ni-Mn-Ga by Ferromagnetic Transition Metals

Systematic doping by transition elements Fe, Co and Ni on each site of Ni2MnGa alloy reveal that in bulk material the increase in martensitic transformation temperature is usually accompanied by the decrease in ferromagnetic Curie temperature, and vice versa. The highest martensitic transformation temperature (571 K) was found for Ni50.0Mn25.4(Ga20.3Ni4.3) with the result of a reduction in Curie temperature by 55 K. The highest Curie point (444 K) was found in alloy (Ni44.9Co5.1)Mn25.1Ga24.9; however, the transition temperature was reduced to 77 K. The dependence of transition temperature is better scaled with the Ne/a parameter (number of non-bonding electrons per atom) compared to usual e/a (valence electrons per atom). Ne/a dependence predicts a disappearance of martensitic transformation in (Ni45.3Fe5.3)Mn23.8Ga25.6, in agreement with our experiment. Although Curie temperature usually slightly decreases while the martensitic transition increases, there is no significant correlation of Curie temperature with e/a or Ne/a parameters. The doping effect of the same element is different for each compositional site. The cascade substitution is discussed and related to the experimental data.

Microstructure and Magnetic Field-Induced Strain of a Ni-Mn-Ga-Co-Gd High-Entropy Alloy

The effect of a high-entropy design on martensitic transformation and magnetic field-induced strain has been investigated in the present study for Ni-Mn-Ga-Co-Gd ferromagnetic shape-memory alloys. The purpose was to increase the martensitic transition temperature, as well as the magnetic field-induced strain, of these materials. The results show that there is a co-existence of β, γ, and martensite phases in the microstructure of the alloy samples. Additionally, the martensitic transformation temperature shows a markedly increasing trend for these high-entropy samples, with the largest value being approximately 500 °C. The morphology of the martensite exhibits typical twin characteristics of type L10. Moreover, the magnetic field-induced strain shows an increasing trend, which is caused by the driving force of the twin martensite re-arrangement strengthening.

Structure, phase transformation, and hardness of NiTiHfNd alloys

Ni50Ti29Hf21−xNdx (x = 0, 1, 2 at.%) and Ni50Ti29−xHf21Ndx (x = 1, 2 at.%) alloys were fabricated via arc melting. For the first time, the influence of Nd addition on structure, phase transformation, and hardness of NiTiHf alloy was investigated experimentally. It is found that the NiTiHfNd alloys consist of NiTiHf matrix and Nd-rich precipitates. Ni50Ti29Hf21 alloy demonstrates a martensitic transformation temperature as high as 314.1 °C, a thermal hysteresis as narrow as 37.7 °C, and a Vickers hardness as high as 500 HV. Nd addition obviously decreases the martensitic transformation temperature of NiTiHf alloys but still maintains a relatively narrow thermal hysteresis and a relatively high Vickers hardness compared with most other components of NiTiHf-based alloys.

TiPd- and TiPt-Based High-Temperature Shape Memory Alloys: A Review on Recent Advances

In this paper high-temperature shape memory alloys based on TiPd and TiPt are reviewed. The effect of the alloying elements in ternary TiPd and TiPt alloys on phase transformation and strain recovery is also discussed. Generally, the addition of alloying elements decreases the martensitic transformation temperature and improves the strength of the martensite and austenite phases. Additionally, it also decreases irrecoverable strain, but without perfect recovery due to plastic deformation. With the aim to improve the strength of high-temperature shape memory alloys, multi-component alloys, including medium- and high-entropy alloys, have been investigated and proposed as new structural materials. Notably, it was discovered that the martensitic transformation temperature could be controlled through a combination of the constituent elements and alloys with high austenite finish temperatures above 500 °C. The irrecoverable strain decreased in the multi-component alloys compared with the ternary alloys. The repeated thermal cyclic test was effective toward obtaining perfect strain recoveries in multi-component alloys, which could be good candidates for high-temperature shape memory alloys.