Flow Stress in Al–Mg Alloy Single Crystals

Abstract

Single crystals of aluminium and its alloys containing 1.1 and 3.3 at% magnesium were investigated by tension tests at various temperatures from −196°C to 500°C, in order to determine the solution hardening mechanism in Al–Mg alloys. Magnesium addition causes not only remarkable solution hardening—the critical resolved shear stress (c.r.s.s.) increases to about ten times larger than that of pure aluminium by the addition of 1 at% magnesium—, but also produces a characteristic phenomenon in the temperature dependence of c.r.s.s.. The temperature dependence can be devided into three characteristic ranges of temperature—in the range I which lies below room temperature, the c.r.s.s. decreases with the rise of temperature, in the range II which lies between room temperature and about 300°C the c.r.s.s. increases with temperature and in the range III which lies above about 370°C the c.r.s.s. again decreases very rapidly with the rise of temperature and further in this highest temperature range occurs a so-called high temperature yield point phenomenon which has been found in polycrystalline specimens of the alloys mentioned. In the range III the temperature and strain rate dependence of the flow stress was investigated in most detail and the obtained experimental results agreed well with the theoretical predictions which are derived from the viscous motion of dislocations dragging solute atmospheres around them. The yield point phenomenon in this range of temperature was considered to come from the decrease of the average velocity of dislocations due to the increase of dislocation density during deformation, because the stress change corresponding to the change of the dislocation velocity is very large in the viscous motion concerned.