The Ductility Reversal During Age Hardening of Beryllium Nickel Contact Spring Material

Greater insight into the metallurgical properties of existing contact spring materials is necessary so that material selection can be optimized and processing innovations planned and introduced. Age hardenable beryllium nickel has been studied and a unique property of the material identified. Consequently, a significant processing innovation is suggested. Other information provided will help the designer in selecting proper applications for beryllium nickel with greater precision and confidence. A good spring material requires a high modulus of elasticity for stiffness and high yield and tensile strengths to sustain spring loads. The material must also have adequate ductility (approximately 10%) for forming, so that brittle fracture can be avoided. A normal consequence of all hardening mechanisms is an accompanying reduction in ductility; consequently, most age hardenable metals have high strength but low ductility in the hardened state. For this reason it is necessary to first form the material and then age harden it. Beryllium nickel strip of full hard temper was investigated. Ductilities of nonheat treated and heat treated (250-1300°F) strip were determined at room temperature, along with tensile (yield and ultimate strengths) and hardness properties. Transmission electron microscopy was used to study the microstructural changes resulting from the heat treatments, and the observations were correlated with changes in ductility and mechanical strengths. Our experiments have shown that, unlike other age hardenable materials, beryllium nickel increased both in ductility and strength in the range from 250 to 850°F. We have termed this unique phenomenon "ductility reversal." Ductility reversal is shown to be the result of the thermally activated recovery state of annealing. Beryllium nickel heat treated in the range 875 to 975°F shows a drastic drop in elongation values and a corresponding increase in strength. In the range 1000 to 1300°F there is a severe drop in strength and an increase in ductility. This behavior of beryllium nickel (875 to 1300°F) is similar to the traditional age hardenable alloys. The significance of ductility reversal lies in the fact that optimization of ductility at high strength'can yield up to 8 times the ductility usually found in normal coherent precipitation hardened spring materials. Thus beryllium nickel, age hardened by the material supplier at the recommended temperature and possessing the desired properties, can be purchased and directly fabricated without the need for an "inhouse" heat treatment, thereby eliminating attendant processing uncertainties and costs. Further, the microstructural and mechanical property data will help the designer select realistic strength-ductility combinations for the design from within the range of the ductility reversal.