Cr含量对近等原子比NiTi形状记忆合金相变特性和显微硬度的影响

Effect of Cr Content on Phase Transition Characteristics and Microhardness of Near Equiatomic NiTi SMA

采用真空非自耗电弧熔炼法制备NiTiCr合金铸锭,利用电子扫描电镜(SEM),X射线衍射仪(XRD),差示扫描量热法(DSC)和显微硬度仪等多种表征方法研究了Cr含量对NiTi形状记忆合金相变行为和显微硬度的影响规律。结果表明,随着Cr含量的增加,马氏体逐渐转化为奥氏体,同时有hcp结构的高强高硬Cr_(2)Ti相沿奥氏体晶界析出。Cr原子对NiTi合金相变行为影响显著,在一定范围内,Cr原子可显著降低NiTi合金的相转变温度,且随着Cr含量的增加,合金相转变温度显著降低。合金硬度与Cr原子含量呈非线性相关性,在等原子比NiTi合金中微量Cr原子的加入会降低合金硬度,且随着Cr含量的增加,合金硬度逐渐增大。

NiTi shape memory alloy with the near equal atomic ratio is a new functional material which has attracted much attention due to its unique properties.By actively adjusting the composition of the alloy to meet the requirements of use in extreme environments,it has become an important way to prepare high performance NiTi-based shape memory alloys.NiTiCr alloy is a typical shape memory alloy which has been studied widely on its microscopic mechanism.However,the effect of Cr content on the microstructure and properties of near-isoatomic NiTi alloys has not been studied systematically.Advanced technologies such as scanning electron microscope(SEM),X-ray diffractometer(XRD),differential scanning calorimeter(DSC)and microhardness tester were used to study the effects of Cr content on NiTi alloy phase transition behavior and microhardness.Ni_(50-x/2)Ti_(50-x/2)Crx(x=0,0.5,1.5,2.5,3.5)(%,atom fraction)alloy ingots were prepared by the vacuum non-consumable arc melting method by using high-purity metals Ni,Ti and Cr.The electrical discharge machining(EDM)was used to prepare test samples.The JEOL JXA-8100 SEM equipped with energy dispersive spectrometer(EDS)was employed to observe the microstructure and analyze the composition of the as-cast alloy.The phase composition of the as-cast alloy was analyzed by D8 ADVANCE XRD.The phase transition behavior of the as-cast alloy was detected by DSC6000 differential scanning calorimeter,and the phase transition temperature and enthalpy value were calculated in the Pyris Date Analysis software by using the tangent method.The alloy microhardness was measured with EM-1500 L Vickers microhardness tester.When the Cr content was 0.5%,in the equal atomic ratio NiTi SMA,B2 austenite and B19’ martensite coexisted in the matrix,and there were dendritic grains,massive structure and linear and point-like Ti_(2)Ni precipitates.The trans-temperatures of start(M_(s))and end(M_(f))for the martensite were 46.03 and 12.09 ℃ respectively.The trans-temperatures of start(A_(s))and end(A_(f))for the austenite were 44.27 and 64.29 ℃,respectively.As the Cr content increased,the alloy transformation temperature continually decreased,and martensite transformed gradually into austenite,and Cr_(2)Ti phase precipitated along the austenite grain boundary.When the Cr contents were 1.5% and 2.5%,the critical temperature of the phase transition of B2?B19’ was significantly reduced,and the two-stage transformation of B2→R→B19’ apparently occured during the cooling process.When the Cr content reached 2.5%,the trans-temperatures of start(M_(s))and end(M_(f))for martensite decreased to-98.30 and-111.90 ℃ respectively,and the trans-temperatures of start(A_(s))and end(A_(f))for austenite decreased to-59.30 and-43.80 ℃.The microhardness of the alloy decreased first and then increased with the increase of Cr content.When the Cr content reached 3.5%,the microhardness of the alloy increased by 27% to HV 357.687.In the equal atomic ratio NiTi alloy,there was a large amount of Ti_(2)Ni precipitated phase,which promoted the nucleation of martensite,and in turn made the phase transformation temperature higher,and at the same time formed the second phase strengthening to make the alloy harder.On the one hand,the addition of Cr atoms dissolved in the matrix to form a interstitial solid solution,and the matrix grains underwent lattice distortion,which increased the stability of the grains and reduced the phase transition temperature,thereby transforming the matrix martensite into austenite,the microhardness of the alloy decreased significantly.But as the content of Cr atoms continued to increase,the solid solution strengthening effect increased.As a result,the phase transition temperature gradually decreased,and the microhardness of the alloy increased.On the other hand,hcp structure Cr_(2)Ti point-like precipitation phase was formed on the grain boundary,which formed the precipitation strengthening effect to further reduce the phase transition temperature and increase the alloy microhardness.The influence of Cr content on phase transformation behavior and microhardness of NiTi shape memory alloy was studied by using a variety of characterization methods.It could be found that Cr atom had a significant effect on the phase transformation behavior of NiTi alloys.Cr atom in a certain range could significantly reduce the phase transformation temperature of NiTi alloys,and as the Cr content increased,the alloy phase transformation temperature significantly decreased.Furthermore,the microhardness of alloy was nonlinearly related to the Cr atom content.Adding a small amount of Cr atom to the equal atomic ratio NiTi alloy would reduce the microhardness of the alloy.However,as the content of Cr increased,the microhardness of the alloy gradually increased.