Some Observations on the Yield Point in Zinc

Evidence is given confirming that the yield point in zinc is due to nitrogen in solution The theoretical prediction that a yield point should appear in a susceptible crystal without the necessity for any prior deformation is verified experimentally, and an explanation is given of the observation that a yield point may appear in a specimen even though free dislocations have been introduced subsequent to ageing Under certain conditions the yield point values in zinc decrease as identical strain-ageing treatments are repeated, and a smooth stress-strain curve is ultmately obtained This is attributed to an accumulation of work hardening which causes the dislocation density to increase to such an extent that the dislocations can no longer be effectively anchored by the available nitrogen atoms The difference between this behaviour and that of iron on repeated strain ageing is discussed