NiTi合金脉冲电流退火组织及性能研究

    Microstructure and Properties of NiTi Alloy Annealed by Pulsed Current

    • 摘要: 对冷轧后的NiTi合金分别采用脉冲电流及马弗炉在不同温度下进行退火,研究了脉冲电流退火处理对合金微观结构及相变行为的影响。结果表明,采用脉冲电流退火时,450和550℃退火后合金中分别析出粒状及椭圆薄片状Ti3Ni4析出相,显微组织由冷轧态的相变马氏体孪晶向等轴状奥氏体转变,650℃退火后可完成再结晶。随脉冲电流退火温度升高,退火后合金的相变行为发生改变,在DSC相变行为测试过程中,该合金冷却时的相变类型由B2→B19'型向B2→R→B19'型转变(其中B2:奥氏体相A;B19':马氏体相M),马氏体转变及奥氏体转变的峰值转变温度MpAp逐渐降低,且650℃退火后的峰值转变温度Mp=-53.81℃、Ap=-45.15℃。随退火温度升高,NiTi合金硬度总体上呈降低趋势,但在550℃退火时析出的Ti3Ni4相作为弥散夹杂颗粒,提高了基体强度,导致出现硬度增加的异常现象;在650℃退火后再结晶完成,硬度下降至252HV0.2。与马弗炉退火试样对相比,脉冲电流处理退火使NiTi合金再结晶温度降低100℃,降低了Ti3Ni4析出相的析出温度,随脉冲电流退火温度增加合金硬度下降更快。

       

      Abstract: The cold-rolled NiTi alloy was annealed at different temperatures using pulsed current and muffle furnace,respectively, and the effect of pulsed current annealing on the microstructure and phase transformation behavior of the alloy was studied. The results show that when the pulse current annealing is used, granular and elliptical flake-like Ti3 Ni4 precipitates are precipitated in the alloy after annealing at 450 ℃ and 550 ℃, and the microstructure changes from martensite twins in cold rolled state to equiaxed austenite. Recrystallization can be completed after annealing at 650 ℃. As the pulse current annealing temperature increases, the phase transformation behavior of the alloy after annealing changes. During the cooling process of the DSC phase transformation behavior test, the phase transformation type changes from B2 →B19’ to B2→R→B19’ type transformation(where B2:austenite phase A; B19’:martensite phase M), the peak transformation temperatures of martensite transformation Mp and austenite transformation Ap gradually decrease, and when annealing at 650 ℃, the final peak transformation temperature Mp =-53.81 ℃, Ap =-45.15 ℃. With the increase of annealing temperature, the hardness of NiTi alloy generally shows a decreasing trend. Ti3 Ni4 phase precipitated during annealing at 550 ℃ acts as dispersed inclusion particles, which increases the matrix strength and resulting in an abnormal phenomenon of hardness increase. After annealing at 650 ℃, recrystallization is completed and the hardness drops to 252 HV0.2. Compared with the muffle furnace annealed sample, pulse current annealing reduces the recrystallization temperature of the NiTi alloy by 100 ℃ and reduces the precipitation temperature of the Ti3 Ni4 precipitated phase. Compared with that of the muffle furnace annealed sample, the hardness decreases faster with the increase of the pulse current annealing temperature.

       

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