Repeated yielding in tin bronze alloys

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Abstract
A method is proposed to evaluate the index m in Van Bueren's (1955) equation C v = Bεm , which relates the concentration of vacancies (C v) in a strained metal to the instantaneous plastic strain (ε). The magnitude of m is 2•2. Two distinct types of repeated yielding, each having an activation energy of 0•79 ev, are recognized in solid solutions of 1—7 at.% tin in copper. Type A yielding is characterized by widely spaced yield points which increase in size and spacing as the plastic strain increases. This type of yielding begins when the diffusion coefficient reaches a minimum value of 5•7 x 10-9ε* cm2 sec-1, where ε* is the strain rate. The magnitude of individual yield points depends on the instantaneous diffusion coefficient, the time between consecutive yield points and the temperature of testing, in accordance with the Cottrell and Bilby (1949) equation for strain ageing. Type B yielding, which is superimposed on Type A yielding, is typical of unstable dislocation motion and produces rapid fluctuations of flow stress during a test. The minimum diffusion coefficient required to initiate Type B yielding is 5•3 x 10-9ε* cm2 sec-1, which compares favourably with an approximate value of 10-9ε* cm2 sec-1 predicted theoretically. A mechanism is proposed by means of which both types of yielding may be explained.

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