Split-age hardening and temperature dependence of yield stress of chill cast Al-Li-Zr alloys.

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Experimental studies were made on split-age hardening and temperature dependence of yield stress in the chill cast Al-Li-Zr alloys.Results are as follows.1.Chill cast condition:It is possible to dissolve lmass% Zr in Al by chill casting in a water colled copper mold of 3mm gap.2.Optimum heat treatment condition : The first stage aging at 673K for 1.08x10^6s and the successively second stage aging at 473K for 1.08x10^4s provides a rather increase in the yield stress of chill cast Al-Li-Zr alloys than that by aging only at 473K.3.Temperature dependence of yield stress : The binary Al-Li alloys aged to its peak strength show positive temperature dependence of yield stress. On the other hand,the yield stress of binary Al-Zr alloys decreases monotonically with increasing temperature. With increasing the Zr content of Al-Li-Zr alloy,the negative temperature dependence of the yield stress becomes apparent.4.Observation of strain-induced dislocations by transmission electron microscopy: Dislocations move in pairs cutting through the delta'-Al_3Li particles during deformation at the temperature range showing positive temperature dependence of yield strength in the binary Al_3-Li alloys.Dislocations in the binary Al-Zr alloys by-pass,cross slip or climb over the beta'precipitates in the Al-Li-Zr alloys promote the cross slipping or climing over of dislocations and then make the alloy to deform more easily at elevated temperature.