β- Cu ·Al 합금의 연속 냉각변태 (2)

Abstract

The phase transformations during continuous cooling of β-Cu-Al alloys have been studied. When the aluminum content in the alloy is varied from 11.1 wt.% to 13.3 wt.%, the ordering and the martensite transformation temperatures change from 470℃ to 550℃ and from 405℃ to 145℃, respectively, at quenching. The process of the bainite transformation appearing at slower cooling rate is clearly explained by the formation of the ordered β₁-Cu₃Al phase and its stability. This ordered phase has transformed to martensite at about 300℃. The bainite plate of α` phase is ordered at about 270℃ and its structure is found to be identical to that of the β₁` martensite and its composition is about 10 wt.% in aluminum. At very slow cooling after the ordering reaction, a pseudopearlite lamellar phase, which is often visible in an ordered matrix, is formed due to eutectoid transformation. The microhardness of the alloys with 11.1 wt.%∼12.2 wt.% Al increases with the decrease of cooling rate until abundant precipitation of the ductile a phase appears. The maximum hardness corresponds to a composite microstructure of the martensite and the α` phase. As the aluminum content in the alloy decreases, the maximum hardness increases and is obtained at high cooling rate.