Heat treatment of complex aluminium bronzes

Constitution, microstructure, cooling–transformation (CT) diagrams, and corresponding diagrams for mechanical properties are summarized for the Cu–Al–X alloy types (X = Fe, Ni, Mn, Be, Co, Si, and Sn) with β-transformation. On the basis of the cooling time, it is possible to simulate the micro-structure and the properties of various heat-treated, and cast, workpiece sections by means of laboratory tests with small specimens. For alloy types having Fe, Ni, Mn, and Be additions, the knowledge of mechanical properties as a function of thermal-treatment parameters is good. With reference to the as-cast state, yield strengths can be increased considerably by means of both quench (900°C) and temper (600°C) treatments. A finely dispersed α + K-structure is produced in the widely used Cu-Al-Fe-Ni types with this treatment. K-precipitation cannot be suppressed, even when thin plates are quenched. The mechanism of precipitation hardening contributes primarily to the increase in strength, and limits the application to workpieces having very large wall thickness. Since the martensitic and bainitic transformations in Cu-AI-X alloy systems are of minor importance, it is not beneficial to increase the ‘hardenability’ in such alloys, i.e. those in the field of martensitic and bainitic transformations in the CT diagram. The fine α + K-microstructure of quenched and tempered Cu-Al-Fe-Ni alloys is an optimum with regard to resistance to various types of corrosion, cavitation, and corrosion fatigue. Apart from cavitation resistance, only a small amount of data is available concerning the influence of heat treatment on these particular properties.