Segregation Stabilizes Nanocrystalline Bulk Steel with Near Theoretical Strength
Segregation Stabilizes Nanocrystalline Bulk Steel with Near Theoretical Strength
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DOI：
10.1103/physrevlett.113.106104
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发表时间：
2014-09-05
影响因子：
8.6
通讯作者：
Kirchheim, Reiner
中科院分区：
文献类型：
--
作者：
Li, Yujiao;Raabe, Dierk;Kirchheim, Reiner
通过严重的塑性变形细化晶粒可以合成超高强度的纳米结构材料。在这方面存在两个挑战：第一，变形驱动的晶粒细化受到动态位错恢复和毛细驱动力导致的晶体粗化的限制；第二，当晶粒尺寸接近几个纳米时，晶界滑动，从而发生软化。在这里，通过严格拉拔珠光体钢丝(珠光体：交替的铁层和碳化铁层的片层结构)克服了这两个挑战。首先，在大应变下，碳化物相通过机械合金化溶解，使最初的两相珠光体结构变成碳过饱和的铁相。这种富碳的铁相演变成柱状纳米尺度的亚晶结构，从拓扑上阻止了晶界的滑动。第二，过饱和碳向铁亚晶界的吉布斯偏析降低了它们的界面能，从而降低了动态回复和晶体粗化的驱动力。从而获得了稳定的亚晶横截面尺寸<；10 nm。这两种效应导致了稳定的柱状纳米晶结构，阻止了位错的运动，并使极限拉伸强度达到7 GPA，使该合金成为已知最强的延展性块体材料。
Grain refinement through severe plastic deformation enables synthesis of ultrahigh-strength nano-structured materials. Two challenges exist in that context: First, deformation-driven grain refinement is limited by dynamic dislocation recovery and crystal coarsening due to capillary driving forces; second, grain boundary sliding and hence softening occur when the grain size approaches several nanometers. Here, both challenges have been overcome by severe drawing of a pearlitic steel wire (pearlite: lamellar structure of alternating iron and iron carbide layers). First, at large strains the carbide phase dissolves via mechanical alloying, rendering the initially two-phase pearlite structure into a carbon-supersaturated iron phase. This carbon-rich iron phase evolves into a columnar nanoscaled subgrain structure which topologically prevents grain boundary sliding. Second, Gibbs segregation of the supersaturated carbon to the iron subgrain boundaries reduces their interface energy, hence reducing the driving force for dynamic recovery and crystal coarsening. Thus, a stable cross-sectional subgrain size < 10 nm is achieved. These two effects lead to a stable columnar nanosized grain structure that impedes dislocation motion and enables an extreme tensile strength of 7 GPa, making this alloy the strongest ductile bulk material known.