• PDF / 2,882,656 Bytes
• 7 Pages / 612 x 792 pts (letter) Page_size
• 95 Downloads / 292 Views

DOWNLOAD

REPORT

---

I.

INTRODUCTION

THE goals

of early powder metallurgy (PM) aluminum alloy development using atomized powder were: (l) to produce alloys with tensile strength in excess of 125 ksi (862 MPa), and (2) to increase strength 10 pct above that achieved by ingot metallurgy (IM) alloys, while maintaining stress-corrosion cracking (SCC) resistance equivalent to 7075 T73 (the SCC-resistant temper). According to Haarr's I early work with PM alminum alloys, the two most promising experimental compositions for fulfilling the SCC-resistance requirement were 7xxx alloys containing either 1.4 pct Co, or 5 pct Ni + 4 pct Fe.* It *All alloy compositions in this paper are in wt pct.

should be pointed out, however, that because other compositional differences also existed among the alloys studied, the effects of Fe + Ni or of Co on SCC susceptibility were not clearly isolated. These differences included variations in both the Zn/Mg ratio and the level of minor alloy additions. In subsequent work using 3.5 pct NaCI aqueous alternate immersion tests, Lyle and Cebulak 2 showed significant improvements in the survival rate of PM alloys containing various amounts of Ni + Fe or of Co, compared with ingot metallurgy (IM) 7075 control specimens. Further work 3,4 concentrated only on the Co-containing alloys partly because of the low ductility of the iron- and nickel-containing alloys, and led to the development of MA87 and MA67, precursors to the currently available 7091 and 7090, respectively. In other work, Otto 5 examined three 7xxx alloy compositions containing 0.0, 0.4, and 0.8 pct Co, in a 3.5 pct NaC1 aqueous alternate immersion test, and he found that 25, 59, and 91 pct of the specimens survived, respectively. His finding demonstrated the beneficial effect of higher Co content, and was the clearest evidence of increased SCC resistance as a function of Co content. However, in subsequent experiments the beneficial effect of increasing Co content was not clearly observed. For in-

stance, Cebulak et al. 6 found that MA87 (0.4 pct Co) could outperform MA67 (1.5 pct Co) in survivability when extruded rod specimens were tested in the short transverse direction. However, MA67 had clearly superior survival with respect to MA66, a similar alloy having 0 pct Co. Furthermore, using a 3.5 pct NaCI aqueous solution for the alternate immersion test can introduce misleading effects. For instance, an increased propensity to pitting caused by this solution can lead to selective dissolution and subsequent failure due to tensile overload, which can be mistaken for SCC. Such severe pitting and failure has been reported by Brodie and Bakow 7 for MA87 (7091). Thus, while increased Co content appears to increase SCC resistance, its positive contribution has not always been clearly demonstrated. Furthermore, the mechanism of any beneficial effect of Co has not been adequately determined. Lyle and Cebulak 4 speculated that Co, which exists as C~AL, restricts grain boundary movement so that the grain morphology is less favorable to SCC. They also proposed th