Structure-Property Relationships in Dual-Phase Cu-Al Alloys: Part I . Individual Phases
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I.
INTRODUCTION
THE term "dual-phase" as applied to steels implies a duplex microstructure consisting of ferrite and martensite. This microstructure can be obtained by quenching steel from the (a + 3') phase field so that 3/transforms to a martensite phase (M) of high strength embedded in the ductile ot phase. The latter is strongly work hardened due to the T-,M transformation strains. 1 After several years of research work e g.,2,3,4 dual-phase steels now have become of technological interest. They can offer at the same tensile strength level superior formability, compared to standard high strength low alloy steels, e g ,5,6 Martensitic transformations in nonferrous alloy systems can in principle yield dual-phase structures. Of considerable technological importance are the multiphase Cu-A1 alloys (aluminum bronze) which bear some similarities to steels. They undergo the eutectoidal transformation: /3 (11.8 pct Al,bcc) 565_ LC c~ (9.4 pct Al,fcc) + 3/2(15.6 pct A1, complex cubic). The /3phase transforms to its ordered version /31 (Fe3A1 structure) upon cooling. The critical temperature for ordering Tc depends on composition, as shown in Figure l(a). Upon rapid cooling, the/3 phase transforms to martensite with a close packed structure. On the lower A1 side, the martensitic structure is designated /3' and is disordered. At higher A1 content, martensite is designated /3'1 and has an ordered orthorhombic lattice with 18 close packed layers along the c-axis and a high stacking fault density]-12 It inherits its superlattice from the very rapid intervening ordering reaction. A . A . HUSSEIN is Professor, Department of Metallurgy, Faculty of Engineering, Cairo University, Giza, Egypt. Manuscript submitted May 29, 1981. METALLURGICAL TRANSACTIONS A
Although some commercial aluminum bronzes contain the martensitic phase, no systematic study has appeared in the literature dealing with strength and ductility of dualphase aluminum bronzes under the specific consideration of recent achievements in understanding the behavior of both martensite and c~ phases. A great deal of work has been carried out on structure, microstructure, and tempering characteristics of Cu-A1 martensites, e'g' 7-22 In addition to its as-quenched strength, martensite reveals affinity to hardening upon tempering. 12'13'~8'21'z2 Attempts to explain this hardening tendency were based on the formation of dislocations, coherency strains, metastable, and preeutectoid phases. Recently, 2z substantial hardening of martensite was reported as a result of short time tempering at relatively low temperatures. This hardening is attributed to an ordering mechanism. Furthermore, studies carried out on single phase c~ Cu-A1 alloys revealed their strengthening via the phenomenon of anneal hardening. 13-28 In view of those accomplishments, it is felt that more effort has to be devoted to studying Cu-A1 dual-phase alloys. This study is therefore concerned with experimental work carried out on such alloys, in an attempt to reveal their potential. The interest was focused o
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