【作者】 刘洪涛；

【导师】 王沿东； 陈波；

【作者基本信息】 东北大学 ， 材料学， 2013， 博士

【摘要】 作为非常重要的功能材料,NiTi基形状记忆合金具有良好的形状记忆效应、超弹性以及优异的力学性能,在生物医学、机械电子、航空航天等方面具有重要的应用。目前,对NiTi基形状记忆合金的研究大多集中在常规实验条件下。然而,NiTi基形状记忆合金在实际应用过程中,经常还会处于高速率、动高压、高低温等极端情况,如：盔甲失效、汽车碰撞测试、地震防护、外太空应用等。基于上述原因,本文利用轻气炮、高速拉伸、高低温拉伸装置对NiTi和NiTi形状记忆合金在极端条件下的宏观力学性能、相变行为以及微观组织演化规律进行了系统的研究,主要取得了以下研究成果：NiTi合金受冲击后,合金样品在第一次DSC热循环中除热诱发马氏体吸热峰外还发现了两个应力诱发马氏体吸热峰,表现为三步逆马氏体相变,在第二次热循环中两个应力诱发的马氏体吸热峰消失。冲击之后NiTi合金样品在放热峰上有R相产生,显示马氏体相变由一步相变变为两步相变。在受冲击样品的微观组织中存在大量马氏体孪晶,孪晶类型主要为<011>Ⅱ型和(11-1)Ⅰ型孪晶,并在<011>Ⅱ型孪晶解孪晶区域发现了更为细小的(001)混合型形变孪晶。Ni47Ti44Nb9合金冲击之后,合金样品由最初的{111}<1-10>和{111}<0-11>板织构转变为{111}面织构。此外,在受冲击样品中还出现了较弱的{001}织构,表明样品的可恢复应变减小。在冲击过程中,位错等缺陷最先在β-Nb粒子相周边聚集。p-Nb粒子首先通过自身变形吸收冲击能量,当冲击能量超过p-Nb粒子承载极限时才会使形变过程蔓延至NiTi基体相。随着冲击速率进一步增大到958m/s时,最终在NiTi基体上产生了B2奥氏体相形变孪晶。在NiTi合金不同应变率下的拉伸实验中发现,NiTi合金马氏体解孪晶应力具有明显的正向应变率相关性。10/s应变率下拉伸的样品中存在大量<011>Ⅱ型孪晶的解孪晶区域,而在100/s和1200/s应变率下拉伸的样品中,没有发现解孪晶区域,这表明NiTi合金马氏体孪晶的解孪晶速率应在10/s～100/s之间。在高应变率下(≥10/s)拉伸样品的微观组织中还发现了热引发B2相的存在,表明拉伸应变率增加到一定程度后,拉伸过程将由等温过程变为绝热过程。在NiTi合金不同温度下的拉伸实验中发现,马氏体状态NiTi合金的解孪晶应力、抗拉强度和断裂时的最大应变量均具有明显的温度相关性。在450℃下的拉伸曲线中,当应力值增加到50MPa时,出现了一段长度约为13%的应力平台区,这与高温拉伸过程中触发了动态再结晶过程有关。在450℃下拉伸样品的微观组织中观察到,在<011>Ⅱ型马氏体孪晶上产生了微小的(001)混合型孪晶,它与<011>Ⅱ型孪晶约成60°夹角,<011>Ⅱ型孪晶的解孪晶过程与(001)混合型孪晶的生成过程可以同时进行。与NiTi合金在低温下拉伸曲线不同,在Ni47Ti44Nb9合金低温拉伸曲线中没有出现马氏体解孪晶应力平台。这是由于Nb粒子在低温下由软相变为硬相,对马氏体解孪晶过程产生严重的阻碍作用。在Ni47Ti44Nb9合金的DSC曲线中,只有在450℃下拉伸后的样品中还存在相变峰,在其他温度下拉伸样品的相变峰消失。而NiTi合金在各个温度拉伸后均存在明显的DSC相变峰,这种差异是由于在拉伸过程中,位错会在β-Nb粒子处聚集,对β-Nb粒子起到钉扎作用,使得β-Nb粒子变得更加稳定不易变形,从而阻碍了热循环过程中的相变,导致DSC相变峰消失。而在450℃下拉伸时,因温度较高会消除部分位错,因此对相变的阻碍作用减小。更多还原

【Abstract】 As the important functional materials, NiTi-based shape memory alloys (SMAs) have the unique shape memory effect, superelastic and the excellent mechanical properties. They are very suitable for engineering applications, involving biomedicine, machine and electron, aerospace, and many other areas. At present, NiTi-based SMAs mostly focus on research under conventional experimental conditions. While, they are also under extreme conditions (e.g., high speed-rate, high pressure, high and low temperatures) during practical applications, such as armour defeat mechanisms, crashworthiness testing and satellite protection. According to the above reasons, the single gas gun, high-speed tensile machine and variable-temperature tensile machine were used to study the NiTi and Ni47Ti44Nb9 SMAs under extreme conditions. Their mechanical properties, phase transformation behaviors and microstructural evolution were systematic investigated. The main research results were obtained as follows:After shock loading on NiTi SMA, three endotherms are observed in the first heating cycle, showing the presence of three-step reverse phase transformation; whereas during the second heating only one endotherm is seen, because the other two endotherms attributed to stress-induced martensite have disappeared. A small shoulder is detected in exothermic peak, indicating that the intermediate phase (R-phase) results in two-step phase transformation. In shocked NiTi specimens, the main martensitic twin type are<011> type Ⅱ twin and (11-1) type Ⅰ twin. Moreover, (001) compound twin is also observed in detwinning area of <011> type Ⅱ.After shock loading on Ni47Ti44Nb9 SMA, the initial sheet textures of{111} <1-10> and {111}<0-11> gradually evolve into{111} plane texture. Meanwhile, a weak texture component{001} is also observed in all the shocked specimens, indicating that the recovery strains in shocked specimens decrease. During the shock loading, the dislocations first gathered around the β-Nb particles phase. The β-Nb particles first absorb the shock energy by their own deformation, only when the shock energy is more than the critical bearing capacity of β-Nb particles, the deformation process would diffuse into NiTi matrix phase. As the shock velocity up to 958m/s, the deformation twins are formed in NiTi matrix of Ni47Ti44Nb9 SMA.During the tensile experiments of NiTi SMA under various strain-rates, it is found that the martensitic detwinning stress has the positive strain-rate dependence. A large number of detwinning regions were found in the NiTi specimen, which was deformed at the strain-rate of 10/s under tension. While, with the strain-rate further up to 100/s and 1200/s, no detwinning region was detected. It indicates that the detwinning rate of martensitic twins are in the range of 10/s-100/s. Meanwhile, thermal-induced austenite was detected in the NiTi specimens deformed at high strain-rates (≥10/s). It is ascribed to that there is a change from the isothermal process to the adiabatic process when the tensile strain-rate is up to a critical value.The detwinning stress, tensile strength and max strain of NiTi alloy have the temperature dependence, when the NiTi alloy is in martensite. During the tensile test of NiTi alloy at 450 ℃, with the stress is up to 50MPa, a stress-plateau of a length about 13% is observed, which is related to the process of dynamic recrystallization. The detwinning process of <011> type II twin and the formation of (001) compound twin could proceed simultaneously.Differ from the NiTi SMA, no detwinning stress-plateau was detected in the low temperature tensile curves of Ni47Ti44Nb9 SMA. It is due to that the β-Nb particles phase becomes hard at low temperature, so the resistence of detwinning is increased. The DSC peaks only appear in the Ni47Ti44Nb9 specimen deformed at 450℃ and the DSC peaks in the others are all not observed. While, the DSC peaks are always presence in the NiTi specimens deformed at various temperatures. This difference is also caused by β-Nb particles. During the tensile tests, the dislocations gather around the β-Nb particles and their deformation stability could be enhanced, so the phase transformation was prevented. Owing to the 450℃ is high enough to eliminate some dislocations, the DSC peaks exist in this specimen.更多还原