A Study on the Strain Rate and Temperature Dependence of Yield Stress of Al-Li Alloy

Abstract

Abstract The effect of strain rate on the yield stress of an Al-Li alloy has been investigated at temperaturesbetween 77 and 523 K and over the strain rate range from 1.77×10 Š4 s 1 to 1.77×10 Š 2 s . At testing temperaturesbelow 373 K, the yield stress is almost independent of strain rate at any aging stage. At testing temperaturesabove 373 K, the yield stress increases linearly with the logarithm of strain rate, and the strain rate dependenceincreases with increasing testing temperature. The yield stresses of under-aged alloy at temperatures between373 and 473 K at high strain rates are greater than the yield stress at 77 K. For the alloy under-aged or agednearly to its peak strength, the temperature range within which the positive temperature dependence of yieldstress appears expands to the higher temperature side with increasing strain rate. The strain rate dependence ofthe yield stress is slightly negative at this aging stage. The yield stress of the over-aged alloy decreases mono-tonically with decreasing strain rate and with increasing testing temperature above 373 K. The modulus normal-ized yield stress is nearly constant at testing temperatures below 373 K at any strain rate investigated. And,strength depends largely both on the aging conditions and on the testing temperature. The peak positions instrength vs. aging time curves shift to the side of shorter aging time with increasing testing temperature. For thespecimens aged nearly to the peak strength, the positive temperature dependence of yield stress is observed inthe temperature range. The shift of peak positions in the aging curves are explained in terms of the positive tem-perature dependence of cutting stress and the negative temperature dependence of by-passing stress.(Received August 30, 2011; Revised September 15, 2011; Accepted October 4, 2011)Key words: aluminum-lithium alloy, mechanical property, strain rate dependence, temperature dependence