Spontaneous deformation during aging under stress in a copper-beryllium alloy

An anomalous phenomenon has been found in a Cu-Be alloy during an investiga tion of the aging behavior. A platelike testpiece of a cold-rolled Cu-Be al loy (Japanese Industrial Standard No. C1720) was held at 1073 K for 2 hours , followed by quenching to solutionize. The testpiece was set in a hand-mad e cantilever jig and aged by applying an external stress with a screw at th e free end of the cantilever. The free end of the cantilever leaves the scr ew in 30 minutes and warps further, within 1 hour, during aging. The sponta neous warping deflection is closely related to the hardness change through aging and is dependent upon the elastic strain introduced initially by the external load. Transmission electron microscope (TEM) observation makes cle ar that matastable gamma' precipitates, which bring about the most effectiv e hardening and shrinkage, are predominant at the compression side of the c antilever, while extremely fine Guinier-Preston (G-P) zones are observed at an early stage of aging and coarse gamma -phase precipitates are observed at a prolonged aging time in the tension side. The spontaneous warping resu lts from the difference of the precipitation sequence between the two sides .