Martensitic Transformation And Microstructures In Ti-Ni-Cu-Pt Shape Memory Alloys With Low Hysteresis

Shape memory micro-actuators have been widely applied in MEMS, andreducing the thermal hysteresis to improve the response speed has great practical andtheoretical value. The microstructures, martensitic transformation, thermal cyclingproperties and temperature memory effect(TME) of Ti50Ni35Cu15-xPtx（x=1,1.5,2.5,3at.%）and Ni-rich Ti50-xNi35+xCu13.5Pt1.5(x=1,2at.%) low hysteresisshape memory bulk alloys have been studied systematically by means of X-raydiffraction(XRD), scanning electron microscopy(SEM), transmission electronmicroscopy (TEM) and differential scanning calorimetry(DSC).The experimental results show that Ti50Ni35Cu15-xPtx(x=1,1.5,2.5,3at.%)alloys and the Ni-rich Ti50-xNi35+xCu13.5Pt1.5(x=1,2at.%) alloys after solution arecomposed of two-phases at room temperature: the B19martensite(or B2austenite)and Ti2(Ni, Cu, Pt) phase. The {011} compound twins and {111} type Ⅰ twins arefound, and the {011} compound twins dominated in Ti50Ni35Cu15-xPtx(x=1,1.5,2.5,3at.%) alloys. the {011} compound twinning martensite plates are parallel to eachother showing a single pair morphology. The {011} compound twinning interface isclear, straight and coherent well; no dislocations and other defects exist in the vicinityof the interface.The Equiatomic alloys exhibit one-step B2-B19martensitic transformationduring the process of cooling and heating. The transformation temperatures firstlydecrease and keep constant with the increasing Pt content. However the thermalhysteresis (Af-Ms) tends to reach a minimum (7.26K) with1.5at.%Pt. The Ni-richalloys exhibit one-step B2-B19martensitic transformation during cooling and heatingexcept the one aging at600℃for5h, which exhibit two-step transformation. Thethermal hysteresis of Ti50-xNi35+xCu13.5Pt1.5(x=1,2at.%) alloys get the minimum(4.32K) with x=1after solution treatment. The transformation temperatures andthermal hysteresis of all specimens after aging increase in some extend, which resultsfrom the formation of Ni-rich precipitates to decrease the Ni content in the matrix.The the multistage transformation are suggested to correspond to the regions withdifferent quantity of precipitates. The transformation temperatures of Ti50Ni35Cu15-xPtx(x=1,1.5,2.5,3at.%)alloys keep constant after60thermal cycles. Especially, the thermal cycling stabilityof Ti50Ni35Cu15-xPtx(x=1.5at.%) alloy is the best after100thermal cycles(changes nomore than1.5K). The thermal hysteresis decreases and then keeps constant duringthermal cycling. The frictional work changes in the same way with thermal hysteresisand the elastic energy stored in martensite decreases d uring thermal cycling.The TME appears when a martensitic–austenitic transformation is incomplete.This partial cycle is made by heating the sample to an intermediate arrestedtemperature (Ta) between the austenite start (As) and finish (Af) temperatures, andthen cooling down to below the martensite finish temperature (Mf). It is shown thatthe following complete heating cycle presents an additional DSC peak, closely relatedto Ta. The all arrested temperature (Ta) can be memorized when performed indecreasing order while only the maximum Tacan be memorized performed inascending order. Temperature memory effect can be trained and strengthened byhammer procedure (stops repeatedly at the same temperature). It is difficult to bewiped out after hammer procedure in Ti-Ni-Cu-Pt alloys, which is totally differentwith other alloys.