The quantitative evaluation of effect of strain gradient on the strengthening and structure refinement of metals during ultra large straining

利用高压扭转机制备的不同应变和应变梯度试样，研究应变梯度对组织强化和细化的影响。对Al、Ti、Ni、Cu、Fe等纯金属的硬度测定表明，低熔点的Al、Cu等纯金属发生再结晶。在纯Ti中观察到压力诱导转变。以纯铁为材料，研究了应变和应变梯度对硬度的影响。在HPTed纯铁中观察到明显的应变梯度硬化。HPTed Fe的极限拉伸强度为2GPa，总伸长率随外加应变的增加而增加。用纯Ti考察了高压对高温热处理过程的影响，纯Ti在高压下由相转变为w相。CO相的体积分数随应变和氧含量的降低而增加。co相在环境条件下是亚稳态的，在加热时又转变回一个相。

In the present investigation, using specimens with different strain and strain gradient prepared by high pressure torsion machine, the effect of strain gradient on the strengthening and refinement of microstructure were studied. The measurement of hardness for HPTed pure metals such as Al, Ti, Ni, Cu, Fe revealed that recrystallization takes place in the pure metals with low melting point such as Al and Cu. Pressure induced transformation was observed in pure Ti. Thus using pure Fe in which only the effect of strain and strain gradient appears, the effect of strain and strain gradient on the hardness was studied. The clear strain gradient hardening was observed in the HPTed pure Fe. The HPTed Fe showed the ultimate tensile strength of 2GPa, and the total elongation increased with applied strain. The effect of high pressure during HPT was examined using pure Ti which transformed from phase to w phase when HPTed at high pressure. The volume fraction of CO phase increases with strain and with decreasing oxygen content. The co phase is metastable at ambient conditions and it transforms back to a phase during heating.