Fabrication Of Functionally Graded Nickel-Titanium By Aging

Near-equiatomic nickel-titanium alloys have broad applications in many fields owing to their excellent properties.However in some practical cases, because of the effect of the Lüders-type deformation that appears in the stressstrain curves of the Ni-Ti alloys,when the load reaches the "yielding" area,a slight increase of the stress will result in an unstable apparent upper-lower change of the strain,which can not be controlled by the adjustment of stress input.This disadvantage has strongly reduced the functional applications of the Ni-Ti alloys in some engineering field,such as the accuracy controlling.A Ti-50.9at.%Ni wire material of 1.0mm in diameter was used in this study. According to the transformation mechanism and the transformation behavior of this alloy under different temperature conditions,the per-solution treated wire was graded aged using a tube furnace which was controlled to create a temperature gradient along its length following with quenching in water in room temperature. The obtained Ni-Ti wire has a continuous changing along the length direction of the content of a precipitate which has an important effect on the transformation and the mechanical behavior,that is,after the aging process,the mechanical behavior of the wire changed,and a graded change of the mechanical parameter occurs.Furthermore,the wire of the same specifications was aging under some chosen unchanging temperature to compare with the graded aged wire(the other parameters are same as the graded aged wire).Some of the graded aged wire samples were cut into short sections,the aging temperature of each section can be exactly confirmed.The Differential Scanning Calorimeter(DSC) was employed to analysis the transformation behavior of the sections and a tensile testing machine was used to describe and record the stress- strain relationship of the samples.In succession,the data about the transformation behavior and the mechanical property of the graded aging wires and the isothermal aged wire were compared and discussed.The DSC analysis shows,the section of about 3.0mm in length that was cut from the graded aged wire has an approximate transformation behavior to the wires which was invariably aged under the temperature that corresponding to the temperature under which the former section was aged.So it is proved that when the Ni-Ti alloy was aged in a temperature graded changing environment,the content of the precipitate changes along the wire's length direction.The tensile testing of the isothermal aged wire shows,the Lüders-type deformation behavior which is characterized by an apparent upper-lower "yielding" at the onset of the stress-induced martensitic transformation occurs.It is clearly showed that the wires aged under different temperature have different transformation behaviors,the yielding onset point and the timing stress or the strain when the wire ruptures change along with the change of the aging temperature.Meanwhile,the testing condition(such as temperature,etc.) also visibly influences the mechanical behavior of the specimens.However,the tensile testing of the graded aged Ni-Ti alloy wire represents,the Lüders-type deformation phenomenon has not been seen after the stress reaches the onset point of yielding,the strain increases linearly with the increase of the stress.Because the Ni-Ti alloy wire was graded aged along its length,the differences of the aging temperature between the different sections cause the differences of the onset stress of the stress-induced martensitic transformation,result in achieving the functionally structural gradient.The graded aged wires through tensile tests under different environments also have different mechanical properties.As obviously shows in the stress-strain curves,the onset of the stress-induced martensitic transformation which indicates the start-up of the gradient effect rises along with the increases of the testing environment temperature.Tensile testing of different length of the tailored sections cut off from the lower aging temperature end of the wires shows that the slope of gradient stress plateau(the finish area of the gradient stress plateau) changes as the aging temperature range(namely,the testing sample length) widened,while the starts of all the gradient stress plateaus almost the same. the termination point of graded effect changes with the prolong of the samples. Meanwhile,the results show the NiTi wires have the same mechanical behaviors between the graded aged process and the isothermal aged process.The stressstrain curve changes accordingly with the prolong of the samples.To sum up,a functionally gradient Ni-Ti alloy was successfully prepared by aging the alloy under a temperature-gradient condition.Furthermore,the mechanical property of the Ni-Ti alloy can be controllably acquired by changing the parameters such as the graded aging parameters.This novel heat treatment process exploits a newly method for the preparation of the FGMs and provides a potential promising avenue to improve controllability of near-equiatomic Ni-Ti alloys for accuracy applications,etc.