Porosity and tensile ductility in Al-Zn alloys

Abstract

The tensile ductility of Al-Zn alloys processed in two different ways was examined over a broad range of temperature and strain rates. The different processing conditions produced materials with differing initial void andJor inclusion contents. If all the defects present are assumed to be in the form of voids, their respective volume fractions in the two alloys are 0.5 and 1.5 pct. Testing at temperatures from 77 to 480 K encompasses a range of mechanical behavior, ranging from brittle to superplastic. The poorly processed material has markedly less ductility than the one containing fewer defects both at high temperatures and at near cryogenic ones (196 K), where the alloys are marginally ductile. Increasing strain rate embrittles alloys in the temperature range 196 K to 300 K. At 196 K the strain rate needed to induce brittleness in the low defect alloy is over two orders of magnitude higher than in the alloy with a greater voidJinclusion content. Critical embrittling strain rates at room temperature are in the range common to conventional metal-forming operations for the poorly processed alloy, and thus cold formability devolves importantly on initial processing. In the vicinity of room temperature and at low strain rates, tensile ductility is greater for alloys with a higher defect concentration; this behavior is related to the development of multiple necks in the poorly processed materials. At the highest temperatures investigated, tensile ductility correlates directly with strain rate sensitivity and material defect concentration. Materials with low defect concentrations resist neck development during quasi-uniform flow to a much greater degree than do those with a high defect concentration.