Mechanical And Elastocaloric Properties Of Mn-Ni-Sn-X Magnetic Shape Memory Alloys

The traditional vapor compression refrigeration technology depends on a variety of halogenated hydrocarbon refrigerants as the working medium,whose leakage will seriously damage the environment,and the refrigeration efficiency of this technology has approached the theoretical limit.Therefore,it is very urgent to develop more efficient and environmentally friendly refrigeration technology.Solid state refrigeration is a new refrigeration technology based on the elastocaloric,magnetocaloric and electrocaloric effects of solid medium under the action of external field.Elastocaloric refrigeration has been widely concerned because of its remarkable thermal effect and simple-realization conditions.Mn-Ni-X(Sn,In,Ga)ferromagnetic shape memory alloy shows rich physical properties such as elastocaloric effect,magnetocaloric effect and shape memory effect near the phase transition temperature,so it is considered as a novel and promising solid cooling material.But the intrinsic brittleness of these alloys limits their applications in elastocaloric refrigeration.In this paper,we mainly try to improve the mechanical properties of Mn-Ni-Sn ferromagnetic shape memory alloy by element doping method,and adjust the alloy phase transition temperature to near room temperature to study the elastocaloric effect of the alloy near room temperature.The main research contents include the following aspects:In Mn49Ni4iSn10-xGdx alloys,the martensitic transformation temperature can be controlled by replacing Sn with Gd,and a significant elastocaloric effect at room temperature has been obtained.The martensitic transformation temperature of the alloy increases with the increase of x.Mn49Ni4iSn9.4Gd0.6 alloy exihbits excellent superelastic properties,reversible elastocaloric properties and magnetocaloric properties at room temperature.After unloading at 500 MPa,the temperature drop of the alloy can reach-11.2 K.At the same time,the elastocaloric properties of the alloy almost did not decrease after 200 cycles under 400 MPa stress.In addition,the magneto-thermal effect of Mn49Ni4iSn9.4Gd0.6 is significant,and the magnetic entropy change can reach 8.6 J kg-1 k-1 when the external magnetic field intensity is 3 T.Gd doping will introduce a fine,diffuse second phase(y phase)in the alloys,and y phase increases with the increase of x.The fracture strength of Mn49Ni4iSn10 is only 400 MPa,while the fracture strength of Mn49Ni4iSn9.4Gd0.9 is 1550 MPa.By replacing Sn with Tb,Mn50Ni40Sn10-xTbx alloy can obtain large and reversible elastocaloric effects at room temperature.The martensitic transformation temperature of the alloy is adjusted to room temperature by adding Tb element.Mn50Ni40Sn9.3Tb0.7 alloy at room temperature mainly consists of austenite(L2I type)and a small amount of martensite(5M type),showing good room temperature superelastic properties.The maximum elastocaloric temperature change of the alloy is-13.6 K after unloading the stress of 600 MPa at room temperature.The fracture strength of Mn50Ni40Sn9.iTb0.9 alloy increases obviously with the increase of x,and the fracture strength of Mn50Ni40Sn9.1Tb0.9 alloy can reach 1550 MPa.Mn50Ni40Sn10-xTbx alloy is sensitive to heat treatment conditions,and there are significant differences in microstructure after solution at different temperatures.The martensitic transformation temperature and mechanical properties of Mn50Ni40Sn10-xNbx alloy are effectively adjusted by the transition element Nb.The increase of Nb significantly inhibits the precipitated phase.With the increase of x,the number of the second phase in the alloy decreases,and the shape and distribution of the second phase also change significantly.The composition phase of Mn50Ni40Sn9.4Nb0.9 alloy is mainly austenite at room temperature.The alloy has good elastocaloric properties at room temperature,and the temperature drop reaches-7.7K after unloading the stress at 700 MPa.The mechanical properties of Mn50Ni40Sn9.4Nb0.6 alloy are significantly improved by the doping of Nb element.The fracture strength of Mn50Ni40Sn9.4Nb0.6 alloy can reach 1600 MPa.By doping Ti element,Mn50Ni40Sn10-xTix(x=2.0,2.5,3.0)alloys have obtained good mechanical properties and elastocaloric effect at room temperature.When x=2.5,the elastocaloric effect is the most significant,and the temperature drop of the alloy reaches-8.4 K after unloading the stress at 700 MPa.The fracture strength of Mn50Ni40Sn8.0Ti2.0 alloy reaches 1600 MPa.With the increase of Ti element,the cross-section microstructure of the alloy changes obviously.