Q. Explain the following with reference to deformation of metals:
(1) Preferred orientation
(2) Elastic after effect
(3) Bauschinger effect
Ans. (1) Preferred orientation: It is a condition of polycrystalline aggregate in which crystal orientations are not random under certain conditions the crystal is a metal tend to align themselves in a common axial direction. Such a direction is known as a preferred orientation.
Preferred orientation occurs when certain crystallographic planes, tend to orient themselves in a preferred manner with respect to the direction of maximum strain.
(2) Elastic after effect: Consider a metal which is subject to sudden constant stress at a level below the elastic limit.
After an initial instantaneous strain, the metal will gradually creep until the strain reaches an essentially value.
If the load is removed suddenly, the strain drops instantaneously, but not at all a certain amount of strain still remains that solely decrease with time, approaching its original value. The time dependence of strain is called the elastic after effect.
(3) Bauschinger Effect: If in a metal the flow is started at certain stress intensity in one direction in one slip plane system, and subsequently the sign of the stress is reversed then flow in the reverse direction will initiate at a lower stress level.
Let a material has the yield stress in tension as 'a' and the same in compression as 'x'. If is a new specimen of the same material is loaded in tension past the limit yield stress to 'b' along the path 'q, a, b' and then it is unloaded it will flow the path 'b, c'.
If now, the specimen is subjected to reversed stress compressive stress the plastic flow will begin at the stress corresponding to point 'd' which is apparently lower than the original compressive yield stress of the material corresponding to point 'x'.
Thus, while the yield stress in tension was increased by strain hardening from 'a' to 'b' the yield stress in compression is decreased from 'x' to 'd'. This effect is Bauschinger effect.