Martensitic Transformation Behaviors And Elastocaloric Effects In Ni-Fe-Ga Foams

The demand for energy with environmental protection has led to a large global effort to find new refrigeration technologies such as solid-state refrigeration based on the elastocaloric effect（eCE）,which is really a promising cooling technology to replace conventional vapor compression owing to its high compactness,efficiency and eco-friendliness.Additionally,ferromagnetic shape memory alloys（FMSMAs）have been acquired a great attraction due to their multifunctional properties along with the capability of miniaturized eCE refrigeration because they required much lower stress to induce martensite transformation（MT）.Among FMSMAs,Ni-Fe-Ga-based single crystals exhibit excellent eCE properties with large working stability but their polycrystalline bulk alloys are ineffective eCE refrigerants due to intrinsic brittleness.In this paper,a detailed fabrication process and eCE in foams（porous materials）with micro-sized nodes/struts were studied.The vacuum induction casting was employed to fabricate Ni-Fe-Ga polycrystalline bulk alloys with a single（β-phase）and dual phases（β+γphase）followed by heat treatments T1（annealing at 1473 K for 48 h and then quenching in cold water）and T2（annealing at 1223 K for 2 h followed by furnace cooling）,respectively.The superelastic and eCE properties of single-/dual phases bulk alloys were comparatively discussed with various porosities of single and hierarchical pore foams.The single-/dual pore foams were created by using large R1（355-500μm）and small R2（70-90μm）size sodium aluminate（NaAlO₂）powders as space holders,which were subsequently removed by mixed acids under sonication.Various porosity levels,i.e.45%,53%and 61%,etc.of single-/dual pore foams can be achieved after the complete removal of NaAlO₂and partial dissolution of Ni-Fe-Ga alloy.The high-temperature annealing（T3:heating at 1453 K for 5 h and then quenching in water）was carried out to induce a single（β-phase）in polycrystalline foam samples.The annealed single pore foam with porosity 45%was utilized to elaborate the elastocaloric properties.A good reversible superelasticity with a recovery strain of 3.3%was achieved at a room temperature of 295 K（above A_f）.The maximum directly measured adiabatic temperature change(ΔT_ad（?）3.4 K)via infrared thermography revealed the inhomogeneous temperature distribution in foam samples due to their unique microstructures of nodes,struts and pores.It was also verified thatΔT_ad increased with increasing strain rate that became saturated at a strain rate of 0.02 s^-1 under compressive stress 60 MPa and large specific eCE(ΔT_ad/Δσ（?）56.7 K/GPa)with a high coefficient of performance of materials(COP_mat（?）20.6)was demonstrated.Moreover,the comparative eCE properties for Ni-Fe-Ga bulk alloy（β-phase）and single pore foam with porosity 53%were investigated.A completely reversible superelastic behavior with a large strain up to 4.7%and 3.9%was observed in bulk alloy and foam under the stress of 130 and 60 MPa,respectively.The largeΔT_ad of 5.8 K was obtained in bulk alloy while 2.8 K was calculated in single pore foam,whereas the COP_mat（?）19.2 for foam was in good agreement with bulk alloy(COP_mat（?）13).However,the annealed single pore foam exhibited better cyclic stability and reversibility up to 100cycles withΔT_ad～2.4 K,while the bulk alloy got fractured just after 43 cycles with an averageΔT_ad～4 K.Consequentially,it was stated that the single pore foams with 53%porosity had superior fatigue property than bulk alloys because the pores intensively reduced the constraints of grain boundaries under narrow hysteresis energy loss.Contrastingly,the dual pores foam（i.e.the hierarchy of pores in which the pore size may equal or smaller than the grain size）was created by tuning the material’s architecture with thin nodes and struts.The hierarchical pores foam with 53%porosity exhibited a good superelastic recovery strain of 4.9%at 296 K under 60 MPa and largeΔT_ad 4.1 K under 70 MPa.The maximum COP_mat（?）22 under work recovery was obtained with large|ΔT_ad/Δσ|58.6 K/GPa through greater stress-induced MT.Furthermore,hierarchal pore architecture maintains better materials integrity compared to single pore under cyclic loading because the cracks originating in small struts do not propagate as far as those originating in larger nodes and struts.Consequentially,the dual pores foam kept better working stability up to 194 cycle times with an averageΔT_ad～2.8 K through resisting the crack initiation/propagation under narrow hysteresis loss.The polycrystalline Ni-Fe-Ga dual-phase alloys demonstrated excellent eCE working stability(reversibleΔT_ad～2.6 K under 250 MPa for bulk alloys;constantΔT_ad～1.5 K under 80 MPa for single pore foams and constantΔT_ad～2.0 K under 130 MPa for hierarchical pore foams)over 500 cycles with insignificant degradation.The enhanced working stability can be attributed to the manipulation of microstructure via introducing the interstitialγ-precipitates through reducing the tendency of intergranular fracture via deflecting/bridging the microcracks under plastic deformation.Thus far,these facts provide a theoretical basis for future investigation of eCE and mechanical stability as promising foam（porous）e CMs.