A hot extruded AZ61Mg alloy was multi directionally forged (MDFed) under decreasing temperature conditions using a die. MDFing was successfully carried out, and the coarse initial grains with an average grain size of 21.6 mm were gradually fragmented to 0.3 mm in average at cumulative strain of∑De=7.2. It showed quite high hardness of 958 MPa and tensile strength of 465 MPa with ductility of 15. The tensile behavior of the ultrafine grained AZ61 Mg alloy exhibited large temperature and strain rate dependency irrespective of temperature. Superplasticity of 680 elongation to fracture could be achieved at 423 K and at strain rate of 1.0×10 -4 s -1 . Such mechanical properties depending on temperature and strain rate were reasonably understood by considering the effect of grain boundary sliding. The tensile strength of the MDFed AZ61Mg alloy was further raised to 490 MPa by additional cold rolling. Fig. 1(a)),アムスラー型万能試験機を用いて初 期ひずみ速度 3.0×10 -3 s -1 ,パス間ひずみ De=0.8 として, MDF 温度を 603~393 K まで鍛造パスごとに逐次降温しな
Synchrotron X-ray microtomography has been utilized for the in-situ observation of ductile fracture in cast Al-7%Si alloys exposed at a high temperature for 0-10 6 s. A high resolution experimental configuration and deflection contrast imaging technique have enabled the reconstruction of silicon particle images with an isotropic voxel with a 0.474 mm edge. The variations in particle shape, size and spatial distribution at the high temperature are readily observed, along with the existence and growth of high-density micro-pores. Three-dimensional image analysis is applied and its feasibility is confirmed. It is clarified that in the case of an as-cast material void nucleation and growth have been observed as has been reported for the general ductile fracture of metallic materials. When it is exposure at the high temperature for a long time, however, ductile fracture is found to be attributable mainly to the growth of pre-existing micro-pores and not the nucleation of new voids at silicon particles. Since such tendency has been also confirmed for other materials, more detailed analysis might be expected in a near future in order to understand actual ductile fracture process in practical materials.
Inspired by a success in applying a fluidized bed furnace to high-temperature solution treatments, the present authors have attempted to modify the age-hardening characteristics of Al-Mg-Si alloys by utilizing its superior heating rate. Two-step ageing treatments have been applied to five kinds of alloys with different Mg 2 Si and excess Si contents. Heating rates for the first and the second ageing treatments have been varied as well as ageing temperature for the first ageing treatment. It has been clarified that the rapid heating to the first ageing temperature enhances strength compared to the standard T6-temper for all the alloys tested, especially for a balanced alloy with a low Mg 2 Si content. It has been confirmed that small precipitates are formed in high density in such cases. In most cases, the improvements in hardness have been observed by adding the ageing treatment at 353 or 373 K for only 0.8 ks before the ordinary ageing at 450 K. We attribute this to the suppression of unfavorable clusters which ordinarily form below those temperatures during heating.
Method: Microtomography, characteristics: 3D visualization of alloying elements by dual energy subtraction imaging at the zinc K edge, conditions: A monochromatic X-ray was used at BL20XU of SPring-8 for the tomographic experiment. Isotropic voxel with a 0.474 pm edge was achieved. The material used was the cell wall of an aluminum alloy foam.
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