利用高速拉伸实验机在宽的应变率范围内(0.001—1200 s 1),研究了NiTi形状记忆合金的宏观力学性能随应变率的变化规律,并借助透射电子显微镜深入研究了微观结构在不同应变率下的演变机制.研究发现:NiTi合金马氏体(B19′相)孪晶的解孪晶应力随应变率的升高而近乎线性增大,表明NiTi合金解孪晶应力具有正向应变率相关性.在拉伸应变率为10 s 1的样品微观结构中发现了大量的解孪晶区域,而当应变率进一步增大到100 s 1和1200 s 1时,在样品中没有发现解孪晶区域的存在,样品微观组织以孪晶形式存在.该结果表明,NiTi合金的马氏体解孪晶速率应在10—100 s 1范围内.在高应变率下(10 s 1)均发现了热引发奥氏体相(B2)的存在,表明随应变率的增加,拉伸过程由等温过程逐渐变为绝热过程.此外,在1200 s 1的样品差示扫描热量曲线中还发现了一个小肩峰,表明相变过程由一步相变变为两步相变.
High pressure pyrolysis of melamine has been attracting great interest recently, due to it being considered as a suitable precursor to realize the g-C3N4 and even superhard C3N4. In this work, we studied the detailed pyrolysis behavior of melamine at 22 GPa. Melamine was stable at 800℃, and decomposed to diamond in the form of powder at 1500-2000 ℃ under this pressure condition. At 2000℃, the pure cubic diamond powders with 0.1 0.5 μm grain size were obtained. The diamond particles exhibited euhedral forms and dispersed to each other, we proposed that these novel features were caused by the presence of liquid N2 and NH3 during diamond formation. The high pressure pyrolysis of melamine may provide a new means of producing micrometer-sized diamond powders.
