Explanation:

When a metal crystal undergoes plastic deformation, dislocations move through the crystal lattice causing slip. The slip lines formed during deformation can have various patterns depending on the crystal structure, temperature, and other factors. In the case of a plastically deformed metal crystal at low temperature, the slip lines exhibit a wavy pattern due to the following reasons:

Dislocation pile-up:
When dislocations move through the crystal lattice, they can pile up at obstacles such as grain boundaries or other dislocations. This can result in a pile-up of dislocations at certain points, leading to a wavy slip line pattern.

Large number of slip systems:
Slip in a crystal lattice occurs along specific planes and in specific directions, known as slip systems. If there are multiple slip systems available in a crystal, the dislocations can move along different paths, leading to a wavy slip line pattern.

Low stacking fault energy:
Stacking fault energy (SFE) is a measure of the energy needed to create a defect in a crystal lattice. A low SFE means that it is easier for dislocations to move through the crystal lattice, resulting in a wavy slip line pattern.

Dislocation climb:
Dislocation climb occurs when a dislocation moves through the crystal lattice by breaking and reforming bonds with neighboring atoms. This can result in a wavy slip line pattern.

Summary:
A plastically deformed metal crystal at low temperature exhibits a wavy slip line pattern due to dislocation pile-up, a large number of slip systems, a low stacking fault energy, and dislocation climb. These factors can cause dislocations to move through the crystal lattice in a wavy pattern, resulting in the observed slip line pattern.