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.

Microstructural,crystallographic,and mechanical characteristics in Ni-Mn-Ga alloys directionally solidified under a transverse magnetic field

In this study,the effect of transverse magnetic field-assisted directional solidification(MFADS)on the microstructures in Ni-Mn-Ga alloys has been investigated.The results show that the magnetic field is capable of inducing transversal macrosegregation perpendicular to the magnetic field,causing the emergence of martensite clusters in the austenite matrix.Moreover,the magnetic field alleviates the microseg-regation on a dendritic scale and promotes the preferred growth of austenite dendrites.On the basis of the above investigation,several special samples are designed using the MFADS to study the crystallographic evolution and mechanical behavior during thermal/stress-induced martensite transformation.The martensite cluster in the austenite matrix is used to investigate the martensite transformation and growth under cooling-heating cycles.The crystallographic relationship and phase boundary microstructure between martensite and austenite have been characterized.In addition,the microsegregation on a dendritic scale can significantly influence the martensite variant distribution,corresponding to the performance during compressive circles based on the analysis of the deformation gradient tensor.The stress-induced superelasticity is closely dependent on orientation,well explained from the perspective of different resolved shear stress factors and correspondence variant pair formation transformation strain.The crystallographic evolution has been characterized during in-situ stress-induced transformation.The findings not only deepen the understanding of martensite transformation and mechanical behavior under a thermal/stress field in Ni-Mn-Ga alloys but also propose a promising strategy to obtain microstructure-controllable functional alloys by MFADS.

Successive inverse and normal magnetocaloric effects in the Mn-vacancy compound Mn0.95Co0.75Cu0.25Ge

Ferromagnetic-structural transformation has been studied widely in MnCoGe-based materials. However, the magnetostructural transition(MST) from antiferromagnetic(AFM) orthorhombic phase to ferromagnetic(FM) hexagonal phase, which may lead to a large inverse magnetocaloric effect(MCE), has rarely been reported. Here, the introduction of Mn vacancy lowers the structural transition temperature while retains the AFM state in the orthorhombic phase, thus successfully realizing the AFM-FM MST in Mn0.95Co0.75Cu0.25Ge. Moreover, successive inverse and normal MCEs are observed around the first-order AFM-FM MST and the second-order FM-paramagnetic(PM) transition, respectively. A thermostat is proposed based on this special feature, which could release heat above the critical temperature while absorb heat below the critical temperature by simply applying the same magnetization/demagnetization cycles. This thermostat can be very useful in many applications where a constant temperature is required, such as cryostats and incubators.

Machine-learning-assisted discovery of empirical rule for inherent brittleness of full Heusler alloys

Brittleness is a critical issue hindering the potential application of the X_2YZ-type full Heusler alloys in several fields of state-of-the-art technologies.To realize optimization of brittleness or design a ductile Heuser alloy,it is greatly urgent to identify the key materials factors deciding brittleness and establish an empirical rule to effectively evaluate ductility.For this purpose,by using a machine learning and human analysis cooperation approach,the brittleness of the X_2YZ-type Heusler alloys was systematically studied.Results showed that the ductility is majorly decided by 6 key materials factors in the studied alloys.Using these 6 factors,a machine learning model to predict the Pugh's ratio k was constructed.Further analyses showed that the crystal structure of the X component could be the most critical factor deciding the ductility.The X component has the face-centered cubic(FCC)structure for most of the alloys with superior ductility.To effectively estimate ductility and guide materials design,an empirical formula of k=mEWF_(m+n)G_(m)+k_(0)was established based on the known information of electron work function(EWF)and shear modulus(G)of the X,Y,and Z elements where the subscript m represents the weight-average value.The coefficients of m(negative)and n(positive)were confirmed to have opposite signs,which can be explained based on the relations between the ductility and the deformation/fracture resistance.This work is expected to deepen the understanding in ductility and promote the design of advanced ductile Heusler alloys.

Large enhancement of magnetocaloric effect induced by dual regulation effects of hydrostatic pressure in Mn_(0.94)Fe_(0.06)NiGe compound

MM'X(M,M'=transition metals,X=carbon or boron group elements)compounds could exhibit large magnetocaloric effect due to the magnetostructural transition,and the composition regulation has been widely studied to realize the magnetostructural transition.Moreover,the magnetostructural transition is also sensitive to the pressure.Herein,the effect of hydrostatic pressure on magnetostructural transformation and magnetocaloric effect has been investigated in Mn_(0.94)Fe_(0.06)NiGe compound.Dual regulation effect of lowering structural transition temperature and strengthening ferromagnetic(FM)state of martensite is realized by applying hydrostatic pressure,which would greatly improve the magnetocaloric effect of Mn_(0.94)Fe_(0.06)NiGe compound.Moreover,the first-principles calculations have also been performed to discuss the origin of the regulation effect under hydrostatic pressure,and it indicates that the hydrostatic pressure can stabilize the hexagonal structure and decrease the structural transition temperature.The maximum isothermal entropy change increases by 109%from 4.3 J/(kg K)under 0 GPa to 9.0 J/(kg K)under 0.402 GPa for a magnetic field change of 0-3 T.This work proves that the hydrostatic pressure is an effective method to regulate the magnetostructural transition and enhance magnetocaloric effect in MM'X compounds.