Recrystallization characteristics of Al-0.3%Zr alloys

Abstract

Two kinds of Al-0.3wt% Zr ingot were prepared; one of which was cast in permanent mould and the other in heat insulating brick mould. The recrystallization characteristics of the above alloy sheets produced from the above ingots were researched under several heat-treated conditions.The main results obtained were as follows:(1) Under the condition of practical operation, the most excellent heat resisting properties would be given by so-called supercooled solid solution produced from homogeneous liquid phase by rapid solidification. However, rather excellent properties were given even by slow solidification when subjected to suitable heat treatments.(2) The effects of heat treatments on heat resisting properties were able to be divided into two temperature stages.In recovery stage (at lower temperature), there was observed a kind of low temperature temper hardening, which probably resulted from the so-called Cottrell pinning of dislocation by Zr atoms. However; the reason of the above hardening is still ambiguous and shall be re-examined in more detail. In addition, competition between recovery and recrystallization must be taken into account, because it may effect retardation on the following recrystallization kinetics.In recrystallization stage (at higher temperature), fine precipitates of Zr-rich phase appeared by prolonged ageing at suitable temperatures. These precipitates preferentially appeared at dislocations introduced by cold working. The age hardening and locking mechanism of dislocations may be very effective in improving high temperature properties of this alloy.(3) In slowly solidified material, precipitation hardening was scarcely observed in as cast state. However, when it was solution heat treated at high temperature, fairly strong age hardening was restored.(4) The age hardening responce of the alloy was usually very sluggish. Even being aged at 350°C, hardening began to take place at about 50hr. after the start, and the hardness attained to the maximum value at about several hundred hours later. In practical, it may be required to accelerate the action by cold working. The increment of hardness by ageing was about 10 VHN.