Wang Yinmin, Chen Mingwei, Zhou Fenghua, Ma En
Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, Maryland 21218, USA.
Nature. 2002 Oct 31;419(6910):912-5. doi: 10.1038/nature01133.
Nanocrystalline metals--with grain sizes of less than 100 nm--have strengths exceeding those of coarse-grained and even alloyed metals, and are thus expected to have many applications. For example, pure nanocrystalline Cu (refs 1-7) has a yield strength in excess of 400 MPa, which is six times higher than that of coarse-grained Cu. But nanocrystalline materials often exhibit low tensile ductility at room temperature, which limits their practical utility. The elongation to failure is typically less than a few per cent; the regime of uniform deformation is even smaller. Here we describe a thermomechanical treatment of Cu that results in a bimodal grain size distribution, with micrometre-sized grains embedded inside a matrix of nanocrystalline and ultrafine (<300 nm) grains. The matrix grains impart high strength, as expected from an extrapolation of the Hall-Petch relationship. Meanwhile, the inhomogeneous microstructure induces strain hardening mechanisms that stabilize the tensile deformation, leading to a high tensile ductility--65% elongation to failure, and 30% uniform elongation. We expect that these results will have implications in the development of tough nanostructured metals for forming operations and high-performance structural applications including microelectromechanical and biomedical systems.
纳米晶金属——晶粒尺寸小于100纳米——具有超过粗晶金属甚至合金金属的强度，因此有望有许多应用。例如，纯纳米晶铜（参考文献1 - 7）的屈服强度超过400兆帕，是粗晶铜的六倍。但纳米晶材料在室温下通常表现出低拉伸延展性，这限制了它们的实际应用。断裂伸长率通常小于百分之几；均匀变形范围甚至更小。在此，我们描述了一种对铜的热机械处理方法，该方法能产生双峰晶粒尺寸分布，在纳米晶和超细（<300纳米）晶粒的基体中嵌入微米尺寸的晶粒。正如从霍尔 - 佩奇关系推断的那样，基体晶粒赋予了高强度。同时，这种不均匀的微观结构引发了应变硬化机制，稳定了拉伸变形，从而导致高拉伸延展性——断裂伸长率达65%，均匀伸长率达30%。我们预计这些结果将对用于成型操作的韧性纳米结构金属以及包括微机电和生物医学系统在内的高性能结构应用的开发产生影响。
