Magnetic entropy change involving martensitic transition in NiMn-based Heusler alloys

Our recent progress on magnetic entropy change（S） involving martensitic transition in both conventional and metamagnetic NiMn-based Heusler alloys is reviewed.For the conventional alloys,where both martensite and austenite exhibit ferromagnetic（FM） behavior but show different magnetic anisotropies,a positive S as large as 4.1 J·kg^-1·K^-1 under a field change of 0-0.9 T was first observed at martensitic transition temperature T M～197 K.Through adjusting the Ni：Mn：Ga ratio to affect valence electron concentration e/a,T M was successfully tuned to room temperature,and a large negative S was observed in a single crystal.The △S attained 18.0 J·kg^-1·K^-1 under a field change of 0-5 T.We also focused on the metamagnetic alloys that show mechanisms different from the conventional ones.It was found that post-annealing in suitable conditions or introducing interstitial H atoms can shift the T M across a wide temperature range while retaining the strong metamagnetic behavior,and hence,retaining large magnetocaloric effect（MCE） and magnetoresistance（MR）.The melt-spun technique can disorder atoms and make the ribbons display a B2 structure,but the metamagnetic behavior,as well as the MCE,becomes weak due to the enhanced saturated magnetization of martensites.We also studied the effect of Fe/Co co-doping in Ni 45（Co1-xFex）5 Mn36.6In13.4 metamagnetic alloys.Introduction of Fe atoms can assist the conversion of the Mn-Mn coupling from antiferromagnetic to ferromagnetic,thus maintaining the strong metamagnetic behavior and large MCE and MR.Furthermore,a small thermal hysteresis but significant magnetic hysteresis was observed around TM in Ni51Mn49-xInx metamagnetic systems,which must be related to different nucleation mechanisms of structural transition under different external perturbations.

Determination of the magnetocaloric effect associated with martensitic transition in Ni_(46)Cu_(4)Mn_(38)Sn_(12) and Ni_(50)CoMn_(34)In_(15) Heusler alloys

This paper presents a study of the inverse magnetocaloric effect （MCE） corresponding to martensitic transition using various experimental approaches for Ni46Cu4Mn38Sn12 and NisoCoMn34In]5 Heusler alloy. Through heat capacity measurements, it is found that the ＂giant inverse MCE＂ upon martensitic transition evaluated by the Maxwell relation in these alloys are unphysical results. This is due to the coexistence of both martensitic and austenitic phases, as well as thermal hysteresis during martensitic transition. However, careful study indicates that the spurious results during martensitic transition can be removed using a Clausius Clapeyron equation based on magnetization measurements.

Tuning Martensitic Phase Transition by Non-Magnetic Atom Vacancy in MnCoGe Alloys and Related Giant Magnetocaloric Effect

The effects of non-magnetic atom vacancy on structural, martensitic phase transitions and the corresponding magnetocMoric effect in MnCoGel-x alloys are investigated using x-ray diffraction and magnetic measurements. The introduction of non-magnetic atom vacancy leads to the decrease of the martensitic transition temperature and realizes a temperature window where magnetic and martensitic phase transitions can be tuned together. Moreover, the giant magnetocaloric effect accompanied with the coupled magnetic-structural transition is ob- tained. It is observed that the peak values of magnetic entropy change of MnCoGeo.97 are about -13.9, -35.1 and -47.4J.kg-1K-1 for △H = 2, 5, 7T, respectively.

Martensitic transformation and giant magnetic entropy change in Ni_(42.8)Mn_(40.3)Co_(5.7)Sn_(11.2) alloy

The crystal structure, phase transition, and magnetocaloric effect in Ni42.8Mn40.3Co5.7Sn11.2 alloy are investigated by structure analysis and magnetic measurements. A large magnetic entropy change of 45.6 J/kg.K is obtained at 215 K under a magnetic field of 30 kOe （1 Oe = 79.5775 A.m-1）. The effective refrigerant capacity of Ni42.8Mn40.3Co5.7Sn11.2 alloy reaches 72.1 J/kg under an applied field changing from 0 to 30 kOe. The external magnetic field shifts the martensitic transition temperature about 3-4 K/10 kOe towards low temperature, indicating that magnetic field can retard the phase transition to a certain extent. The origin of large magnetic entropy change is discussed in the paper.

Regular Pattern of Morphology Transition in Mixed Martensite Structures

The morphology and substructure of mixed martensites in ferrous alloys have been examined by using optical and transmission electron microscope. The results indicated that the main formation se- quence of martensitic morphology was butterfly→ plate→lath,with decreasing forming temperatures when the plastic accommodation takes place in the parent phase,which is affected by the transforma- tion strain fiélds.It was shown that the martensite morphology is not only decided by the forming temperature alone,but also by the dislocation struc- ture in austenite before the transformation.

Transition-layer of coreerim structures andβ→a transformation in SiC ceramics

Similar to Si_(3)N_(4)ceramics,β→a phase transformation in SiC ceramics plays a key role in tailoring the microstructures thus optimizing related properties.SiC microstructures are dominated with the core erim structures by AlN-solution,and by EBSD analysis,a-lamellae were revealed as stacking-faults(SF)and twin-boundaries(TB)in b-grains,co-existing with the coreerim structures asα/β→α’/β’transformation by sintering.The structural transformation can proceed much further by gas-pressuresintering than hot-pressing with only RE2O3 agents,while the latter retain a high-density of SF/TB in the metastable b-SiC grains.By high-angle secondary-electron imaging,nanoscale transition-layer(TL)was observed as an inter-phase to fully separate the core and rim,which is created by a transitory equilibrium in the solutionereprecipitation process.The enrichment of AlN or RE in TL demonstrates their segregation to core surface until reaching the super-saturation and before the growth of rims.With higher AlN or RE solution and after sintering,a shear stress can develop from TL contour to drive the expansion of SF/TB in Martensitic transition,especially under an external isotropic pressure.The combinations ofβ→a transformation,coreerim structures and viscous liquid-phase enable the comprehensive assessment of sintering-microstructure-property-performance relationship of SiC ceramics,as demonstrated for their creep behaviors and fracture toughness.

NON-LOCAL MODELING ON MACROSCOPIC DOMAIN PATTERNS IN PHASE TRANSFORMATION OF NiTi TUBES

Recent experiments revealed many new phenomena of the macroscopic domain patterns in the stress-induced phase transformation of a superelastic polycrystalline NiTi tube during tensile loading. The new phenomena include deformation instability with the formation of a helical domain, domain topology transition from helix to cylinder, domain-front branching and loading-path dependence of domain patterns. In this paper, we model the polycrystal as an elastic continuum with nonconvex strain energy and adopt the non-local strain gradient energy to account for the energy of the diffusive domain front. We simulate the equilibrium domain patterns and their evolution in the tubes under tensile loading by a non-local Finite Element Method （FEM）. It is revealed that the observed loading-path dependence and topology transition of do- main patterns are due to the thermodynamic metastability of the tube system. The computation also shows that the tube-wall thickness has a significant effect on the domain patterns： with fixed material properties and interfacial energy density, a large tube-wall thickness leads to a long and slim helical domain and a severe branching of the cylindrical-domain front.

Influence of cooling rate on magneto-structural transition and magnetocaloric effect of Ni_(30)Cu)8Co_(12)Mn_(37)Ga_(13) alloy

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（ΔS_（mag））derived from magnetic field induced martensitic transformation is confirmed for both samples,and can be remarkably enhanced by the atomic ordering.The largestΔS_（mag） of 20.9J/（kg·K）is obtained at 307.5Kunder 5Tin the FC sample.