Deformation Mechanisms, Microstructure and Mechanical Properties of Nanoscale Crystalline and Noncrystalline Materials i
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Deformation Mechanisms, Microstructure and Mechanical Properties of Nanoscale Crystalline and Noncrystalline Materials in Different Temperature Ranges Yuly V. Milman Institute for Problems of Materials Science, 3, Krzhizhanovsky Str., 03142, Kyiv, Ukraine ABSTRACT A review of the influence of nanoscale structural elements on the mechanical properties of crystals, quasicrystals, and metallic glasses (MG) is presented. Temperature ranges of cold, warm, and hot deformation are distinguished for crystalline materials, but a nanocrystalline (NC) structure may be formed by severe plastic deformation in the temperature ranges of warm and hot deformation. The plasticity characteristic obtained by indentation can be used for the characterization of low-ductile NC materials. The main features of the plastic deformation mechanisms of NC materials, including results obtained by molecular dynamic simulation, are considered. For MG, the following two problems are discussed: the comparison of the yield stresses for NC and MG and the possibility of strengthening of MG by disperse crystalline nanoscale particles. Quasicrystals with nanosize grains, which are also called nanoquasicrystals (NQC), form a separate class of materials. The mechanical properties of NQC and crystalline materials strengthened by NQC particles are analyzed. Dispersion hardening of metals by NC particles was the first application of nanoscale structures for structural materials. New possibilities of such strengthening are considered. INTRODUCTION Features of formation of nanocrystalline (NC) structures during plastic deformation in the temperature ranges of warm and hot deformation, the mechanism of plastic deformation of NC, the strength of MG in comparison with NC materials and the possibility of strengthening of MG by disperse NC particles, features of mechanical behavior of nanoquasicrystals, and some new possibilities of strengthening crystalline materials by NC particles are the subject matter of the present review. EXPERIMENT AND DISCUSSION Features of NC structures obtained in different temperature ranges Severe plastic deformation (SPD) is the most effective method for production of NC materials [1]. Temperature ranges of cold, warm and hot deformation can be distinguished during plastic deformation [2, 3]. The boundary between temperature ranges of hot and warm deformation is the recrystallization temperature Tr, and the boundary between warm and cold deformation is the characteristic deformation temperature T* (Fig. 1). The characteristic deformation temperature
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T* is the temperature below which the Peierls–Nabarro stress becomes significant, and an abrupt increase in the yield stress σs (and hardness HV) is observed as temperature decreases.
Figure 1. Temperature dependence of yield stress σs and hardness HV, and temperature ranges of cold, warm, and hot deformation for BCC metals. In this case, the temperature ranges are characterized by the grain structure and dislocation substructure that developed during deformation (Fig. 2) [4].
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