Hydrogen effects on beryllium-copper alloys

Abstract

Beryllium-copper is a precipitation-hardening alloy system containing from 0.5 to 2.0% beryllium and minor amounts of nickel and cobalt. Depending on the beryllium content, it is divided into two categories based on high strength or high conductivity. However, the alloy system is the most widely used of all beryllium-containing alloys, primarily because it can be precipitation heat-treated to the highest strength levels attainable in copper-based alloys. These alloys are used for applications requiring a combination of high electrical conductivity and mechanical strength, and for applications requiring wear and even corrosion resistance [1, 2]. The alloys have also been chosen because of their non-magnetic characteristics or their hardness, and most often for their innate ability to be worked in a soft condition and then hardened by simple heat-treatments. The non-magnetic nature of the alloy makes it an extremely attractive candidate for precision instrumentation [2]. Heat-treatment is the most important process for the alloy system. Although almost all copper alloys are hardenable by cold working, beryllium-copper alloy is unique because it is hardenable by a single low-temperature thermal treatment. This facilitates the development of desirable properties in these alloys. The phase diagram of the beryllium-copper alloy system is shown in Fig. 1. Age-hardening or precipitation-strengthening is a lower thermal cycle treatment designed to enhance the strength of the material appreciably. During age-hardening the dissolved metastable beryllium comes out of solution and precipitates as the beryllium-rich y-phase, both in the copper matrix and along the grain boundaries. It is the formation of y-precipitates which is responsible for the large increase in material strength. Depending on the alloy composition (beryllium content), ageing temperature and time at temperature, the precipitates can be either fully coherent or incoherent with the matrix. Since precipitation is a nucleation and growth process, the coherent precipitate forms during the early stages of growth and the incoherent precipitate forms during continued ageing at the temperature or while ageing at higher temperatures. The ageing sequence follows