高温高压条件下纳米材料强度的测试和分析方法

基于高温高压同步辐射X射线衍射和飞行时间中子衍射实验，我们提出了一种通过衍射研究纳米材料本征力学性能的方法.实验通过高压压缩实验对纳米晶和微米晶Ni的变形行为进行了对比研究；二者在不同压力下的晶胞体积数据表明，纳米晶Ni具有明显的弹性软化现象，体弹模量比微米晶Ni减小约10%.研究结果表明，纳米晶Ni的整体抗压缩性减小是由于该材料纳米晶壳层呈现为张应力状态，具有较低的抗压缩性，这与分子动力学模拟结果一致.基于该方法，通过定量分析衍射峰的宽化和偏移，我们可以获得晶粒之间的高应力聚集区所产生的"微观/局部"屈服，以及整个样品在应力下所产生的"宏观/整体"屈服.我们的应变/应力图形分析方法还可以研究材料的屈服强度、高温高压条件下的晶粒破碎/长大、加工硬化/软化，以及多晶体中的固有残余/表面应变.本文提出的方法可以用于岩石矿物的高温高压流变强度的定量测定，为研究岩石圈以及地幔物质的蠕变对流提供了高科技的研究手段.

A methodology for determining mechanical properties of nanomaterials at high pressure and temperature

Based on the high P-T synchrotron X-ray and time-of-flight neutron diffraction experiments, we proposed a method to investigate the intrinsic mechanical properties of nanocrystalline materials according to the diffraction profile. Properties of nano- and micro-crystalline nickel were compared in a single high-pressure experiment. The lattice volume-pressure data indicated an elastic softening and the bulk modulus is reduced by 10% in nanocrystalline Ni, compared with that in micro-Ni. The results show that the enhanced overall compressibility of nanocrystalline Ni is a consequence of the higher compressibility of the surface shell of Ni nanocrystals, which supports the results of molecular dynamics simulation and a generalized model of nanocrystals with expanded surface shell. The analytical methods we developed based on the peak profile of diffraction data, allowing us to identify the "micro/local" yield due to high stress concentration at the grain-to-grain contacts and "macro/bulk" yield due to deviatoric stress over the entire sample. The graphic approach of our strain/stress analyses can also reveal the corresponding yield strength, grain crushing/growth, work hardening/softening, and thermal relaxation under high P-T conditions, as well as the intrinsic residual/surface strains in the polycrystalline bulks. The methods proposed in this work can be used for the quantitative determination of rheological strength of rocks and minerals at high temperature and high pressure, which provides a high-tech means for the study of creep and convection of lithosphere and mantle materials.