At very low temperatures where motion of SIAs or SIA clusters is limited, a crystalline to amor­phous phase transition can be induced. The phase transition usually produces large swelling (5-30%) and decreases in elastic moduli.15,91,182,209 This phase transition typically occurs for damage levels of ~0.1—1 dpa at low temperatures and has been attrib­uted to several mechanisms including direct amorphi — zation within collision cascades, and an increase in

the crystalline free energy due to point defect accu­mulation and disordering processes.86’147’210-213 The dose dependence for accumulation of the amorphous volume fraction is significantly different for the direct impact mechanism compared to point defect accumu­lation or multiple overlap mechanisms.213 As the irra­diation temperature is raised to values where long range SIA and SIA cluster migration occurs, point defect diffusion to reduce the increase in free energy occurs and the dose to induce amorphization typi­cally increases rapidly with increasing tempera­ture until a temperature is reached where it is not possible to induce amorphization. In many cases, the critical temperature for amorphization increases with increasing PKA energy. Figure 24 compares the effect of PKA energy on the temperature- dependent dose for complete amorphization for an intermetallic alloy214 and a ceramic139 material. In both materials, for all types of irradiating particles, the critical dose for amorphization increases rapidly when the irradiation temperature exceeds a critical value. The critical temperature for amorphization is significantly higher for heavy ion irradiation condi­tions compared to electron irradiation conditions.