Structure-Property Relationships in Dual-Phase Cu-Al Alloys: Part II. Alloy Behavior
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INTRODUCTION
DUAL-phase steels consisting of mixtures of ferrite and martensite have become commercially attractive due to their higher ductility at the same tensile strength as compared to normal HSLA steels, e'g''l In the first part of this work, 2 referred to as Part I, it was established that dual-phase Cu-A1 alloys can have attractive mechanical properties if good use is made of the embodying strengthening mechanisms of the Cu-A1 system. A wide spectrum of controllable martensite content can be easily obtained at useful ductility levels. The measurements carried out on individual phases have indicated that martensite is hardened by means of short time annealing (temper hardening). Furthermore, the soft a phase is also liable to strengthening via "anneal hardening" where cold work is followed by low temperature annealing. It was shown that high strength levels are reached in each phase via adjustable deformation and annealing cycles. The exploitation of such hardening phenomena for both phases and the broader range of martensite volume fraction thus distinguish dual-phase Cu-A1 alloys from dual-phase steels. This work is concerned with the overall mechanical behavior of dual-phase Cu-A1 alloys. The goal is to analyze the variations of strength and ductility with relevant parameters, and to examine how far the ideas gained from studying individual phases can have practical significance.
II.
EXPERIMENTAL
The binary Cu-A1 alloys used here are those of Part I containing 9.8, 10, and 11 wt pct A1. They were produced in dual-phase form according to three schedules (treatment A): (a)"normal" form by quenching from the (a + /3) range, (b) "premartensitic" - - quenching from/3 followed
by (a + /3) quench, (c) "pre-air-cooled"-- air cooling from /3 followed by (a + /3) quench. Flat tensile specimens were machined before heat treatment with 20 mm gauge length and 5 • 1 mm 2 cross section. They were tensile tested in a Zwick tensile testing machine at a strain rate of 6.4 • 10 -4 s -1. Further treatments B, C, D, and R D were carried out on treatment (A) dual phase structures. Tempering (B) was performed in salt bath for one minute only at 450 ~ for specimens with 650 to 700 ~ quench temperature and at 370 ~ for 750 to 850 ~ range. Such temperatures were selected in view of their hardening effect on martensite (Part I). Prior deformation was performed by means of tensile straining to a prescribed amount of uniform strain. This deformation was followed by annealing for one minute at 325 ~ (C). Treatment (D) consists of one minute tempering at 370 ~ or 450 ~ depending on quench temperature, followed by deformation, and finally a second annealing for one minute at 325 ~ Only one treatment (RD) involves deformation and prolonged tempering of 10 minutes at 325 ~ and 60 minutes at 370 ~ III.
RESULTS A N D D I S C U S S I O N
Typical examples for true stress- true strain curves of the present dual-phase alloys are given in Figure 1. Such examples were selected to show effects of martensite volume fraction VM and strength,
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