Heat treatment of zirconium and hafnium alloys

Heat treatments are used throughout the processing range of zirconium and hafnium alloys to modify the microstructure, enable further processing and obtain the final properties, which vary significantly from one application to another. It is therefore essential to master the thermal aspects of these processes and to understand their effects on microstructural changes and their consequences on properties, as demonstrated by the numerous documents in the literature.

This article is a summary of the subject, with the first part devoted to a reminder of thermics, the presentation of quantities relating to zirconium alloys, industrial practices in terms of heating and cooling rates, and the atmospheres used during heat treatments. In the second part, we describe the influence of all these aspects on microstructures and properties in relation to industrial practices: the β → α phase transformation on cooling (quenching) and associated microstructural characteristics, but also recrystallization kinetics. Microstructural evolutions (such as precipitated phases at thermodynamic equilibrium and precipitate coarsening in connection with corrosion) are specified as a function of time and temperature. Texture evolutions are discussed, as is the influence of heat treatments on properties. A final paragraph is devoted to hafnium.

At the end of the article, readers will find a glossary and a table of symbols used. See also [M 2360] (Metallurgy of zirconium and hafnium) and [M 4 785] (Properties of zirconium and hafnium).