Order-disorder phenomena in α -brass - IV. Nature of order and role of atomic mobility

Abstract

On continuous heating of slowly cooled specimens of α -brass containing approximately 30, 25 and 20 at. % zinc there is a marked absorption of energy due to disordering. The range of temperature in which this absorption occurs is sensitive to the rate of heating and the zinc content. Similar characteristics are shown by a silver-zinc alloy containing 27 at. % zinc. On heating specimens of α -brass quenched from 600°C an evolution of energy results from the return of order and the rate of this evolution increases with increasing zinc content. However, for the silver-zinc alloy a constant absorption of energy occurs from approximately room temperature. The degree of disorder retained at room temperature in the quenched specimens of α -brass decreases as the zinc content increases and for the silver-zinc alloy the quenched specimen is more ordered at room temperature than a slowly cooled specimen. The influence of composition on the ΔP curves of both slowly cooled and quenched specimens of α -brass is related to the increase in atomic mobility with increase in zinc content. Similarly the difference between the copper-zinc and silver-zinc alloys is due to the higher atomic mobility in the silver-zinc alloy. It is concluded that the absorption of energy observed in the copper-zinc alloys is due to the destruction of short-range order and that the critical temperature is less than 75°C.