Constitution and Precipitation-Hardening Properties of Copper-Rich Copper-Tin-Beryllium Alloys

Summary and Conclusions

Isothermal sections of the ternary Cu-Sn-Be system have been determined by microscopic methods for alloys in equilibrium at 700°, 600°, 580°, 500°, 400°, and 300°C and for the composition range 0.25 to 3.0 pct Be and, 1.5 to 13.5 pct Sn. The decrease in the limit of α solid solubility with fall in temperature is found to be more marked than is indicated by the work of previous investigators, particularly at temperatures below 350°C, where the field is restricted by the eutectoid transformation of the Cu-Sn system. The presence of the ε phase of the Cu-Sn system in specimens that had been annealed at 300 °C for 8 weeks was confirmed by X-ray examination.

Alloys containing 0.25 to 0.75 pct Be and, 2.0 to 13.5 pct S. have, been shown to possess age-hardening properties. The extent of precipitation hardening obtainable depends primarily on the beryllium content of the alloy. After appropriate aging of rolled and solution-treated material, the maximum hardness is obtained with a nominal composition of 10 pct Sn, 0.75 pct Be. For the same pretreatment, the maximum tensile strength is obtained with a nominal composition of 8 pct Sn, 0.75 pct Be; this alloy can show an ultimate tensile strength of 110,000 psi with an elongation of 23 pct on 2 in. The hardness of the ternary alloys, aged from the solution-treated condition, does not reach that of heat-treated binary Be-Cu alloys with around 2 pct Be, but by cold working solution-treated alloys prior to aging, hardness figures comparable with those exhibited by commercial Cu-Be alloys can be obtained from ternary alloys of suitable composition.

Annealed alloys containing a large amount of second phase tend to crack on cold rolling. These alloys can be heat treated, however, to give high hardness values, a hardness of 260 being attainable for alloys of high tin and high beryllium content.

The constitution changes taking place during precipitation heat treatment have been examined. In is, suggested that, on aging three-phase alloys below 350°C, the observed initial hardening is attributable to precipitation of the ß’ phase of the Cu-Be system and that the subsequent hardening is explained by slow precipitation of the ε phase of the Cu-Sn system.

In view of the high cost of beryllium, the fact that approximately the same properties can be obtained in a ternary alloy containing 0.75 pct Be as, in a binary alloy containing 2.0 pct Be is, noteworthy, and there seems no reason to believe that the cheaper alloy might not function satisfactorily in some of the applications where the binary alloy is now used.