Structure-Property Relationships in Dual-Phase Cu-Al Alloys: Part I . Individual Phases

Dual-phase (α + martensite) microstructures were produced in binary Cu-Al alloys by quenching from the (α + β) phase field. A wide range of martensite volume fraction V_M was obtained, depending on alloy composition and quench temperatureT. Linear dependence onT of V_M was established. Predefined values for V_M can thus be achieved by adjustment ofT and alloy composition. Furthermore, the size, shape, and distribution of component phases can be varied in a predetermined fashion by means of controlled cooling from the β range. The properties of α and martensite were tracedvia microhardness measurements. The microhardness of martensite increases with quench temperature in spite of the accompanying decrease of its solute content. This is in accord with previous work and emphasizes the dominating role of martensite ordered structure on strength. Such strength depends only on quench temperature irrespective of overall alloy composition or morphology. The α microhardness is not affected by alloy composition or quench temperature. The martensitic phase can be hardened by means of short time tempering due to order hardening or solute clustering effects. Depending on quench temperature, optimum use of such temper hardening can be achieved. Moreover, cold working of dual-phase structures followed by annealing at temperatures around 300 °C achieves substantial strengthening of both α and martensite. The strengthening of α is a consequence of anneal hardening. Although such high strength levels are accompanied by reduction of the ductility (as measured by thickness reduction achieved by cold rolling), the heat treatment schedule can be optimalized to achieve high strength while restoring ductility.