(a) Make a list of the different types of fracture which occur in single crystals. (b) Now consider a polycrystalline metal. Indicate how grain boundaries add to the fracture possibilities.

Question

What will be an ideal response?

Answer 1

Solution:
(a) [1] Cleavage – It is possible for some single crystals to undergo a brittle type of fracture ,in which they split apart along a crystallographic plane of low Miller indices, such as the basal plane in zinc or the cube plane in bcc metals. [2] Failure by easy glide – At relatively high temperatures, it has been observed that the two parts of a crystal can be effectively sheared apart along a slip plane. [3] Rupture by multiple glide – If multiple glide along several intersecting slip plane occurs is concentrated in a restricted area, it is possible to obtain a necking rupture. If only two slip planes are involved, this may produce a chisel edge on each half of the crystal. If more than three slip planes are involved, the two ends may pull down to a point. [4] Twinning – Twinning may also lead to fracture of a single crystal. This is particularly true if the reoriented lattice in the twin is more conducive to slip than the matrix in which it forms. The resulting large plastic deformation in the twin may result in a tearing fracture within the boundaries of the twin.
(b) [1] The grain boundaries themselves may form a path along which fracture occurs. Such a fracture is called an intergranular fracture. Grain boundaries attract impurity atoms and this often leads to embrittlement. In some cases, brittle precipitates may form along the boundaries. A typical example is bismuth, which tends to collect along the boundaries of copper, making the copper subject to a brittle intergranular fracture. On the other hand, when FeS precipitates along the grain boundaries of polycrystalline iron and steel specimens, it can remain liquid well after the iron or steel has solidified. In such a case, the metal becomes “hot short” and cannot be hot worked. [2] Intergranular fractures are common in metal specimens, deformed at high temperature, under creep conditions. In this temperature range, plastic deformation tends to concentrate in the parts of the crystals adjacent to the grain boundaries. This concentrated plastic flow can cause a large localized increase in the vacancy concentrations, at the boundaries, so that the pores nucleate and form on the boundaries. These pores can then grow and interconnect to form intergranular fractures. [3] In general, the effect of a grain boundary is to make slip more difficult. This is because the slip planes in a given crystal are normally poorly aligned with respect to those of its neighbors. As a result, the stress level required to deform a metal increases as its grain size decreases and its grain boundary area per unit volume increases. This also increases the probability of fracture in polycrystalline metals.