Investigations On The Influence Of Heat Treatment On Microstructures And Mechanical Properties Of Ti₅₀Ni₄₇Fe₃ Shape Memory Alloy Rods

The Ti50Ni47Fe3 shape memory alloy is a kind of memory alloys most early used in engineering,and has been widely applied to the fastening connection of aircraft hydraulic pipeline.In order to obtain better mechanical properties and suitable martensite transformation starting temperature for practical operation,the Ti50Ni47Fe3 shape memory alloy rods are prepared by two processes of cold-drawing and hot-rolling.Meanwhile,the microstructures,mechanical properties and the phase transition temperature of the cold-drawn and the hot-rolled rods,as well as the influence of heat treatment on them were systematically investigated by optical microscope(OM),transmission electron microscope(TEM),electrical transport testing system(ETTS),Vickers-hardness tester(HV)and electronic tensile testing machine.The aim is to provide reference data for the alloy application in engineering,The dislocation density is low in the as-hot-rolled Ti50Ni47Fe3 alloy rod and they present the network shape,meanwhile,lamellar structure in some grains and microtwins with nanometer scale in lamellas are observed;As heat treatment temperature is 450℃,the recrystallization of hot-rolled rod begins and lamellar structure disappears,meanwhile,a lot of Ti2Ni phase with different shapes are precipitated at triple grain boundaries or inside grains;With continuously raising heat treatment temperature to 600℃,the growth of grains takes place and the precipitates obviously decrease.The dislocation density is higher in the as-cold-drawn Ti50Ni47Fe3 alloy rod and they are mainly in the shape of tangles,and a number of lamellar structure are formed and the boundaries between lamella and matrix are a low angle grain boundary;With raising heat treatment temperature,the lamellar structure is gradually replaced by the equiaxed sub-grains and recrystallized grains.As temperature is as high as 700℃,the lamellar structure basically disappears and the abnormal grain growth occurs.The Vickers hardness of the hot-rolled rod first increases and then decreases with raising heat treatment temperature,and reaches the maximum at 450℃.The tensile strength is the highest at 500℃,here,the yield strength is basically near to that of the as-hot-rolled;For the cold-drawn rod,as heat treatment temperature is less than 550℃ the Vickers hardness and yield strength have basically no change,and the yield strength remains about 580MPa,with continuously increasing temperature,they decrease.It can be found that the optimum heat treatment temperatures for hot-rolled and cold-drawn rods are 450℃ and 500℃,respectively.The martensitic phase transition starting temperature Ms for the hot-rolled rod treated at the optimum temperature is-133℃,which is increased by 16℃ compared to-1490C of the cold-drawn rod treated at the optimum temperature.The increase in Ms point brings convenience for the practical operation of parts made using the hot-rolled rod.