First-principles calculations of Ni–(Co)–Mn–Cu–Ti all-d-metal Heusler alloy on martensitic transformation,mechanical and magnetic properties

The martensitic transformation,mechanical,and magnetic properties of the Ni_(2)Mn_(1.5-x)Cu_(x)Ti_(0.5) (x=0.125,0.25,0.375,0.5) and Ni_(2-y)Co_(y)Mn_(1.5-x)Cu_(x)Ti_(0.5)[(x=0.125,y=0.125,0.25,0.375,0.5) and (x=0.125,0.25,0.375,y=0.625)]alloys were systematically studied by the first-principles calculations.For the formation energy,the martensite is smaller than the austenite,the Ni–(Co)–Mn–Cu–Ti alloys studied in this work can undergo martensitic transformation.The austenite and non-modulated (NM) martensite always present antiferromagnetic state in the Ni_(2)Mn_(1.5-x)Cu_(x)Ti_(0.5) and Ni_(2-y)Co_(y)Mn_(1.5-x)Cu_(x)Ti_(0.5) (y<0.625) alloys.When y=0.625 in the Ni_(2-y)Co_(y)Mn_(1.5-x)Cu_(x)Ti_(0.5) series,the austenite presents ferromagnetic state while the NM martensite shows antiferromagnetic state.Cu doping can decrease the thermal hysteresis and anisotropy of the Ni–(Co)–Mn–Ti alloy.Increasing Mn and decreasing Ti content can improve the shear resistance and normal stress resistance,but reduce the toughness in the Ni–Mn–Cu–Ti alloy.And the ductility of the Co–Cu co-doping alloy is inferior to that of the Ni–Mn–Cu–Ti and Ni–Co–Mn–Ti alloys.The electronic density of states was studied to reveal the essence of the mechanical and magnetic properties.

Formation of quaternary all-d-metal Heusler alloy by Co doping fcc type Ni_(2)MnV and mechanical grinding induced B2–fcc transformation

The structure of the all-d-metal alloy Ni_(50-x)Co_(x)Mn_(25)V_(25)(x=0–50)is investigated by using theoretical and experimental methods.The first-principles calculations indicate that the most stable structure of the Ni_2MnV alloy is face-centered cubic (fcc)type structure with ferrimagnetic state and the equilibrium lattice constant is 3.60A,which is in agreement with the experimental result.It is remarkable that replacing partial Ni with Co can turn the alloy from the fcc structure to the B2-type Heusler structure as Co content x>37 by using the melting spinning method,implying that the d–d hybridization between Co/Mn elements and low-valent elements V stabilizes the Heusler structure.The Curie temperature T_(C) of all-dmetal Heuser alloy Ni_(50-x)Co_(x)Mn_(25)V_(25)(x>37)increases almost linearly with the increase of Co due to that the interaction of Co–Mn is stronger than that of Ni–Mn.A magnetic transition from ferromagnetic state to weak magnetic state accompanying with grinding stress induced transformation from B2 to the dual-phase of B2 and fcc has been observed in these all-d-metal Heusler alloys.This phase transformation and magnetic change provide a guide to overcome the brittleness and make the all-d-metal Heusler alloy interesting in stress and magnetic driving structural transition.

Tailored martensitic transformation and enhanced magnetocaloric effect in all-d-metal Ni_(35)Co_(15)Mn_(33)Fe_(2)Ti_(15)alloy ribbons

The crystal structure,martensitic transformation and magnetocaloric effect have been studied in all-d-metal Ni35Co15Mn33Fe2Ti15alloy ribbons with different wheel speeds(15 m/s(S15),30 m/s(S30),and 45 m/s(S45)).All three ribbons crystalize in B2-ordered structure at room temperature with crystal constants of 5.893(2)A,5.898(4)A,and5.898(6)A,respectively.With the increase of wheel speed,the martensitic transformation temperature decreases from230 K to 210 K,the Curie temperature increases slightly from 371 K to 378 K.At the same time,magnetic entropy change(△Sm)is also enhanced,as well as refrigeration capacity(RC).The maximum△Sm of 15.6(39.7)J/kg·K and RC of85.5(212.7)J/kg under?H=20(50)k Oe(1 Oe=79.5775 A·m^(-1))appear in S45.The results indicate that the ribbons could be the candidate for solid-state magnetic refrigeration materials.

Electronic structures,magnetic properties,and martensitic transformation in all-d-metal Heusler-like alloys Cd2MnTM(TM=Fe,Ni,Cu)

The electronic structures,magnetic properties,and martensitic transformation in all-d-metal Heusler-like alloys Cd2MnTM(TM=Fe,Ni,Cu)were investigated by the first-principles calculations based on density-functional theory.The results indicate that all three alloys are stabilized in the ferromagnetic L21-type structure.The total magnetic moments mainly come from Mn and Fe atoms for Cd2MnFe,whereas,only from Mn atoms for Cd2MnNi and Cd2MnCu.The magnetic moment at equilibrium lattice constant of Cd2MnFe(6.36μB)is obviously larger than that of Cd2MnNi(3.95μB)and Cd2MnCu(3.82μB).The large negative energy differences(ΔE)between martensite and austenite in Cd2MnFe and Cd2MnNi under tetragonal distortion and different uniform strains indicate the possible occurrence of ferromagnetic martensitic transformation(FMMT).The minimum total energies in martensitic phase are located with the c/a ratios of 1.41 and 1.33 for Cd2MnFe and Cd2MnNi,respectively.The total moments in martensitic state still maintain large values compared with those in cubic state.The study is useful to find the new all-d-metal Heusler alloys with FMMT.

Enhanced magnetocaloric performances and tunable martensitic transformation in Ni_(35)Co_(15)Mn_(35-x)Fe_(x)Ti_(15) all-d-metal Heusler alloys by chemical and physical pressures

The solid-state magnetic cooling(MC)method based on the magnetocaloric effect(MCE)is recognized as an environmentally friendly and high-energy-efficiency technology.The search or design of suitable magnetic materials with large MCEs is one of the main targets at present.In this work,we apply the chemical and hydrostatic pressures in the Ni_(35)Co_(15)Mn_(35-x)Fe_(x)Ti_(15) all-d-metal Heusler alloys and systematically investigate their crystal structures,phases,and magnetocaloric performances experimentally and theoretically.All the alloys are found to crystallize in an ordered B2-type structure at room temperature and the atoms of Fe are confirmed to all occupy at sites Mn(B).The total magnetic moments decrease gradually with increasing Fe content and decreasing of volume as well.The martensitic transformation temperature decreases with the increase of Fe content,whereas increases with increasing hydrostatic pressure.Moreover,obviously enhanced magnetocaloric performances can also be obtained by applied pressures.The maximum values of magnetic entropy change and refrigeration capacity are as high as 15.61(24.20)J(kg K)^(−1) and 109.91(347.26)J kg^(−1) withΔH=20(50)kOe,respectively.These magnetocaloric performances are superior to most of the recently reported famous materials,indicating the potential application for active MC.

X_(2)MnSc(X=Ni,Cu,Pd,Pt,Ag和Au)合金晶体结构和弹性常数的第一性原理计算

全d族Heusler合金作为一种新型的多功能材料,有望改善传统非全d族Heusler合金Ni_(2)MnGa易脆、不易加工等不良的力学性能。基于第一性原理密度泛函理论,采用精确Muffin-Tin轨道方法,系统计算研究了全d族Heusler合金X_(2)MnSc(X=Ni,Cu,Pd,Pt,Ag和Au)在0 K下的晶体结构、四方晶格变形、磁矩和弹性常数。研究结果表明,相比XA和fcc结构,这些全d族Heusler合金在L21结构下的电子总能均更低,表明它们均具有该立方稳定结构。当L2_(1)-X_(2)MnSc合金发生四方晶格变形时,体系的电子总能总是高于该立方结构,因而它们均不能发生马氏体相变。L2_(1)-X_(2)MnSc合金的总磁矩均主要源于Mn原子;当X=Ni,Pd和Pt时,它们也贡献较小磁矩,而与Sc原子类似,X=Cu,Ag和Au则均可视为非自旋极化的。L2_(1)-X_(2)MnSc合金的弹性常数均符合立方晶系力学稳定性要求,且Pt_(2)MnSc合金的体弹性模量与剪切模量的比值(B/G)、泊松比(ν)均明显高于Ni_(2)MnGa合金,因而其有望具有更良好的力学性能,从而在工程上具有更广泛的应用潜能。研究结果有望为全d族Heusler合金X2MnSc力学性能的优化设计提供理论参考。