对工业纯Ti板材进行异步轧制, 在板材表面获得纳米晶组织, 对显微组织演变和硬度沿板材深度的分布进行了测试分析. 结果表明, 经过异步轧制后, 工业纯Ti表面形成了等轴状、尺寸介于30-60 nm的纳米晶组织, 证明异步轧制可以使大尺寸金属板材在轧制过程中实现表面纳米化. 异步轧制板材表面纳米晶的形成过程包括: 在外力的反复作用下, 位错密度升高, 并通过滑移、湮灭和重组形成了亚微米尺度的位错胞/亚微晶. 随着轧制道次和压下量的增加, 高密度位错重复上述过程使晶粒尺寸不断减小、取向差持续增大, 最终形成等轴状、具有中等到大角度晶界的纳米晶组织. 表面梯度层在轧制初期形成, 其厚度随着压下量增加而逐渐增大, 层内硬度变化与晶粒尺寸密切相关.

Commercial pure titanium sheet was rolled by means of asymmetric rolling in order to obtain nanocrystallines in the top-surface layer, the microstructural evolution was examined by using different experimental techniques. Experimental results show that, after the asymmetric rolling, equiaxed nanocrystallines of about 30-60 nm in size form in the top-surface layer of sheet. The research work indicates that the surface nanocrystallization can be realized for large-dimensional metal sheets during the rolling process by using the asymmetric rolling. The asymmetric rolling induced surface nanocrystallization mechanism was summarized as follows: upon the application of repeated loads, dislocation cells/sub-micro-grains form through slips, annihilations and recombinations of high density of dislocations; with the increment of rolling pass and reduction,high density of dislocations in the refined cells/grains developing in above route lead to the reduction of grain size and the increment of misorientations between the refined grains, and finally equiaxed nanocrystallines with medium to large angle grain boundaries form. A gradient-structured surface layer of about 30μm thick was observed to form in the initial stage,its thickness increases with increasing rolling reduction, and the hardness variation along the depth was found to relate to the grain size.