Why is titanium only malleable when hot?
Because the low temperature(alpha)form has a close packed hexagonal crystal structure, which means that planes of atoms cannot glide over one another.Ductile behaviour can occur in the cubic close packed (beta)form.Zinc, beryllium cadmium and zirconium all have the hexagonal close packed crystal structure and have very poor room temperature ductility.Copper lead and iron among many other metals have a cubic crystal structure in which allows ductility.Seeing that you refer to dislocation the burgers vectors are not unity in the (slightly) distorted metallic hexagonal crystal structure and so any slight slip will strain the lattice (one atom wont quite fall into the place vacated by another)so strain energy and hardening is rapid and leads to cracking. Addition after some thought.You mention an edge dislocation and this occurs in the cubic structures because it is a line discontinuity outlining a region where part of a planar array of atoms is missind from the crystal but inside that zon
