Trong giai đoạn biến dạng dẻo của kim loại, làm việc cứng luôn luôn đi cùng với mềm. Sự tăng trưởng mầm, khoảng trống nhỏ bên trong hoặc sâu răng theo nhựa tăng là cơ chế vi làm mềm. | TẠP CHÍ PHÁT TRIỂN KH CN TẬP 10 SÓ 06 - 2007 THE SOFTENING IN PLASTIC DEFORMATION OF METAL Truong Tich Thien University of Technology VNU-HCM Manuscript received on December14th 2006 Manuscript received on June 28h 2007 ABSTRACT In the plastic deformation stage of metal work hardening always goes together with softening. The nucleation growth of small internal voids or cavities according to plastic increasing is the microscopic mechanism of softening. The voids are nucleated at the particle-matrix interface due to the agglutinate loss or the particle crack when the strain reaches critical value. The growth of voids will then occur in company with the increase of plastic deformation. The influence of void growth on material forming behaviour will be considered by softening parameter of porous material. Key words hardening constitutive softening nucleation growth porous material. 1. INTRODUCTION The nucleation growth and coalescence of small internal voids or cavities are the microscopic mechanism of ductile fracture in cold forming processes of metal. The microvoids are nucleated under the tensile loading state at the impurities and hard particles in the ductile metal. After nucleated microvoids they will be grown due to plastic deformation and coalesced together in order to create the microscopic ductile fracture when the critical state is reached. The nucleation and growth of small internal voids or cavities are interpreted as the reason of the strain softening of material. So the strain hardening and the strain softening of material are two phenomena occurring simultaneously during the plastic deformation of materials. At first the strength of material increases since the strain hardening but the material will be degraded due to the growth of microvoids. This induces the strength degradation and the stress-strain relationship will be shown by the curve with the negative slope. The initial shapes of micro-voids are multiform and complex. On the other hand their .
