Study on Development if High Strength-Toughness- and Environmental Strength-Aluminum Alloys with Fine Grain Structure

The superposition of mode II of mode III component onto mode I loading has a different effect on SCC intiation in7075-T6series alloys，depending on the anisotropy of grain structure，namely causing a delay for the short-transverse oriented specimen，while an acceleration for the longituditinal oriented specimen。On the basis of these results，the SCC fracture loci under the combines loading conditions is determined.The effect of crystal grain size on the SCC of a high-strength alloy7475-T6sheet has been investigated using two types of tensile specimens with and without pre-crack，exposed in 3.5%sodium chloride solution。Little differences in the tensile property and fracture toughness are exhibited in the three materials A，B and C，having a fine-，medium-and coarse-grain size，respectively.The SCC rupture time of the smooth specimen is prolonged with the increase in gain size，namely in the order of material A&lt；B&lt；C.The resistance to SCC initiation and propagation of the pre-cracked specimen is also enhanced with the increase in gain size。From the test results together with the elasto-plastic FEM stress analysis，the threshold stress intensity K I D 2 ISCC文件D2 of SCC initiation is derived to be in agreement with the K I D 2 condition under that the plastic zone size R I D 2 p i D 2=(K I D 2 I D 2/σD 2 ys D 2)I D 12 D 1/πin front of crack tip expands to a width of grain ameter，where d 2 ys d 2.Thus it is supported that the SCC process of the alloy7457-T6is principally controlled by hydrogen embrittlement achieved over a critical distance corresponding to one size of grain.These results will give effective guidelines for the development of aluminum alloys with a better combination among strength，fracture toughness and resistance to SCC.

The superposition of mode II of mode III component onto mode I loading has a different effect on SCC intiation in 7075-T6 series alloys, depending on the anisotropy of grain structure, namely causing a delay for the short-transverse oriented specimen, while an acceleration for the longituditinal oriented specimen. On the basis of these results, the SCC fracture loci under the combines loading conditions is determined.The effect of crystal grain size on the SCC of a high-strength alloy 7475-T6 sheet has been investigated using two types of tensile specimens with and without pre-crack, exposed in 3.5% sodium chloride solution. Little differences in the tensile property and fracture toughness are exhibited in the three materials A, B and C, having a fine-, medium- and coarse-grain size, respectively. The SCC rupture time of the smooth specimen is prolonged with the increase in gain size, namely in the order of material A<B<C. The resistance to SCC initiation and propagation of the pre-cracked specimen is also enhanced with the increase in gain size. From the test results together with the elasto-plastic FEM stress analysis, the threshold stress intensity KィイD2ISCCィエD2 of SCC initiation is derived to be in agreement with the KィイD2IィエD2 condition under that the plastic zone size RィイD2pィエD2=(KィイD2IィエD2/ σィイD2ysィエD2)ィイD12ィエD1/π in front of crack tip expands to a width of grain diameter, where σィイD2ysィエD2 is the yield strength. Thus it is supported that the SCC process of the alloy 7457-T6 is principally controlled by hydrogen embrittlement achieved over a critical distance corresponding to one size of grain.These results will give effective guidelines for the development of aluminum alloys with a better combination among strength, fracture toughness and resistance to SCC.