Modern Physical Metallurgy and Materials Engineering Part 9

Tham khảo tài liệu 'modern physical metallurgy and materials engineering part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Mechanical behaviour of materials 231 Grain boundaries affect work-hardening by acting as barriers to slip from one grain to the next. In addition the continuity criterion of polycrystals enforces complex slip in the neighbourhood of the boundaries which spreads across the grains with increasing deformation. This introduces a dependence of work-hardening rate on grain size which extends to several per cent elongation. After this stage however the work-hardening rate is independent of grain size and for fcc polycrystals is about 40 which allowing for the orientation factors is roughly comparable with that found in single crystals deforming in multiple slip. Thus from the relations Ơ mĩ and e y m the average resolved shear stress on a slip plane is rather less than half the applied tensile stress and the average shear strain parallel to the slip plane is rather more than twice the tensile elongation. The polycrystal work-hardening rate is thus related to the single-crystal work-hardening rate by the relation dơ de m2dr dy For bcc metals with the multiplicity of slip systems and the ease of cross-slip m is more nearly 2 so that the work-hardening rate is low. In polycrystalline cph metals the deformation is complicated by twinning but in the absence of twinning m and hence the work-hardening rate is expected to be more than an order of magnitude greater than for single crystals and also higher than the rate observed in fcc polycrystals for which m 3. Dispersion-hardened alloys On deforming an alloy containing incoherent non-deformable particles the rate of work-hardening is much greater than that shown by the matrix alone see Figure . The dislocation density increases very rapidly with strain because the particles produce a turbulent and complex deformation pattern around them. The dislocations gliding in the matrix leave loops around particles either by bowing between the particles or by cross-slipping around them both these mechanisms are .

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