COMPOSITION TRIANGLE DIAGRAMS OF Ni-Mn-Ga MAGNETIC SHAPE MEMORY ALLOYS

A statistical work has been done to collect the composition ranges of Ni-Mn-Ga alloys exhibiting different structures and martensite start temperature （M,）, large magnetostrain or the co-existence of magnetic and structural transitions. The alloys with five-layered （5M）, seven-layered （7M） modulated and non-modulated （T） martensitic structures were mapped in the graph. An empirical formula has been presented to reflect the effect of elements nickel （Ni ）, manganese （ Mn ） and gallium （Ga）, on the martensite start temperature （M3）. The martensitic structure is sensitive to the composition and the martensitic transformation temperature is most drastically affected by the Ni content. The alloys with large magnetostrain or co-existence effect of the magnetic and structural transitions were also listed in a limited area.

Impact of B alloying on ductility and phase transition in the Ni–Mn-based magnetic shape memory alloys:Insights from first-principles calculation

Brittleness is a bottleneck hindering the applications of fruitful functional properties of Ni–Mn-based multiferroic alloys.Recently,experimental studies on B alloying shed new light on this issue.However,the knowledge related to B alloying is limited until now.More importantly,the mechanism of the improved ductility,which is intrinsically related to the chemical bond that is difficult to reveal by routine experiments,is still unclear.In this context,by first-principles calculations,the impact and the correlated mechanism of B alloying were systemically studied by investigating four alloying systems,i.e.,(Ni_(2-x)B_(x))MnGa,Ni_(2)(Mn_(1-x)B_(x))Ga,Ni_(2)Mn(Ga_(1-x)B_(x))and(Ni_(2)MnGa)_(1-x)B_(x).Results show that B prefers the direct occupation manner when it replaces Ni,Mn and Ga.For interstitial doping,B tends to locate at octahedral rather than tetrahedral interstice.Calculations show that the replacement of B for Ga can effectively improve(reduce)the inherent ductility(inherent strength)due to the weaker covalent strength of Ni(Mn)–B compared with Ni(Mn)–Ga.In contrast,B staying at octahedral interstice will lead to the formation of new chemical bonds between Ni(Mn)and B,bringing about a significantly improved strength and a greatly reduced ductility.Upon the substitutions for Ni and Mn,they affect both the inherent ductility and strength insignificantly.For phase transition,the replacement of B for Ga tends to destabilize the austenite,which can be understood in the picture of the band Jahn–Teller effect.Besides,the substitution for Ga would not lead to an obvious reduction of magnetization.

A Thermo-Magneto-Mechanically Coupled Constitutive Model of Magnetic Shape Memory Alloys

A macroscopic phenomenological constitutive model considering the martensite transformation and its reverse is constructed in this work to describe the thermo-magneto- mechanically coupled deformation of polycrystalline magnetic shape memory alloys （MSMAs） by referring to the existing experimental results. The proposed model is established in the frame- work of thermodynamics by introducing internal state variables. The driving force of martensite transformation, the internal heat production and the thermodynamic constraints on constitutive equations are obtained by Clausius dissipative inequality and constructed Gibbs free energy. The spatiotemporal evolution equation of temperature is deduced from the first law of thermodynam- ics. The demagnetization effect occurring in the process of magnetization is also addressed. The proposed model is verified by comparing the predictions with the corresponding experiments. It is concluded that the thermo-magneto-mechanically coupled deformation of MSMAs including the magnetostrietive and magnetocaloric effects at various temperatures can be reasonably described by the proposed model, and the magnetocaloric effect can be significantly improved over a wide range of temperature by introducing an additional applied stress.

Recent progress in Heusler-type magnetic shape memory alloys

Magnetic shape memory alloys(MSMAs), both in condensed matter physics and in material science, are one of the most extensive research subjects. They show prompt response to the external magnetic field and give rise to large strain and have fine reversibility. The well-known example is Heusler-type MSMAs, which possess excellent multifunctional properties and have potential applications in energy transducer, actuator, sensor, microelectromechanical system, and magnetic refrigerator. In this paper, it is shown the recent progress in magnetostructural transformation, magnetic properties, shape deformation, magnetocaloric effect as well as magnetic field-induced shape memory effect in Ni–Mn–Ga, Ni Mn Z(Z = In, Sn, Sb),and Ni Co Mn Z(Z = In, Sn, Sb, Al) Heusler-type MSMAs.The remaining issues and possible challenges are briefly discussed.

INFLUENCE OF MAGNETIZATION ROTATION ON MARTENSITE REORIENTATION IN MAGNETIC SHAPE MEMORY ALLOY

A large field-induced strain of magnetic shape memory alloy is developed by the martensite variant reorientation. It is widely recognized that the martensite reorientation in a magnetic shape memory alloy （MSMA） can develop if the magnetic field is large enough. However, it has been shown in the literature that the magnetization rotation may block variant reorientation via energy minimization approach. In this paper, based on a micromechanicat model associated with the thermodynamic theory, authors show that there are some limits for the martensite reorientation, which is hindered by the magnetization rotation. Some useful conclusions are obtained.

A first-principle assisted framework for designing high elastocaloric Ni-Mn-based magnetic shape memory alloy

A large adiabatic temperature change(△T_(ad))is a prerequisite for the application of elastocaloric refriger-ation.Theoretically,a large volume change ratio(△V/V_(0))during martensitic transformation is favorable to enhance△T_(ad).However,the design or prediction of△V/V_(0)in experiments is a complex task because the structure of martensite changes simultaneously when the lattice parameter of austenite is tuned by mod-ifying chemical composition.So far,the solid strategy to tailor△V/V_(0)is still urgently desirable.In this work,a first-principles-based method was proposed to estimate△V/V_(0)for Ni-Mn-based alloys.With this method,the substitution of Ga for In is found to be an effective method to increase the value of△V/V_(0)for Ni-Mn-In alloys.Combined with the strategies of reducing the negative contribution of magnetic en-tropy change(via the substitution of Cu for Mn)and introducing strong crystallographic texture(through directional solidification),an outstanding elastocaloric prototype alloy of Ni_(50)(Mn_(28.5)Cu_(4.5))(In_(14)Ga_(3))was fabricated experimentally.At room temperature,a huge△T_(ad)of-19 K and a large specific adiabatic temperature change of 67.8 K/GPa are obtained.The proposed first-principle-assisted framework opens up the possibility of efficiently tailoring△V/V_(0)to promote the design of advanced elastocaloric refrigerants.

Quantitative reorientation behaviors of macro-twin interfaces in shape-memory alloy under compression stimulus in situ TEM

Twinning stress is known to be a critical factor for the actuating performance of magnetic shape memory alloys because of the harmful deterioration of their magnetic field-induced strain effect.However,the intrinsic origin of the high twinning stress is still in debate.In this work,we firstly fill this gap by precisely probing the reorientation behaviors of A-C and A-B two common macro-twin interfaces under the stimulus of uniaxial compression in-situ transmission electron microscope.The grain boundary is proved to be the main reason for large twinning stress.The twinning stress of the A-C and A-B type interfaces quantitatively are~0.69 and 1.27 MPa within the plate respectively.The A-C type interface evidently has smaller twinning stress and larger deformation variable than the A-B interface.Under the action of compression,not only the orientations of the crystals have changed,but also the roles of the major and minor lamellae have changed for both interfaces due to the movements of twinning dislocations.Combining insitu and quasi in-situ electron diffraction data,the reorientation process is clearly and intuitively shown by the stereographic projection.Atomic models and the theory of dislocation motion are proposed to phenomenologically clarify the intrinsic mechanism.This work is believed to not only provide a deeper understanding of the deformation mechanism of magnetic shape memory alloys under uniaxial compression testing,but also discover that compression training is not the mechanical training way to decrease the twinning stress of non-modulated martensite in single crystal shape memory alloys.

Strain-magnetization property of Ni-Mn-Ga(Co,Cu) microwires

Magnetization associated with reversible phase transformation or rearrangement of martensite variants of two kinds of shape memory alloys under the coupling of tensile stress were investigated.One is the austenitic Ni_(46)Mn_(28)Ga_(20)Co_(3)Cu_(3)micro wire with the [001] preferred orientation,which exhibits enhanced cyclic stability and large fully recoverable strain(> 8%) due to the stress-induced reversible martensitic transformation at room temperature.The other is the Ni_(54)Mn_(24)Ga_(22)microwire with ferromagnetic martensitic phase,which has preferential orientation and also exhibits large tensile strain.Based on the improved mechanical properties,the strain-magnetization effect of the two kinds of microwire under the coupling of orthogonal magnetic field and tensile stress was performed and the results indicate that the magnetization decreases with the increase of tensile strains.Furthermore,the magnetization mechanism related to the magnetostructural evolution under stress-magnetic coupling was discussed.This study provides a new way for smart magnetic microwires for novel non-contact and non-destructive detection.