Importance of propagation in controlling the twinning stress in Mg

To strengthen hexagonal metals one must impede twinning as well as slip. One challenge is understanding how the critical stresses required for twin nucleation, propagation and thickening relate to the observed stress at which twinning begins to impact on the response. It is argued that in magnesium, the critical stress for propagation is important. A critical condition for twin propagation is derived that agrees well with observation. It is shown that increasing the friction stress for twin propagation translates directly to an increase in the twinning stress.