Structural Transformations during Heating of a Zr-2.5% Nb Alloy Subjected to Equal-Channel Angular Pressing

The structure of a Zr-2.5% Nb alloy after equal-channel angular pressing (ECAP) at 690-700 K and annealing in the temperature range 670-1070 K is investigated. The structure of the Zr-2.5% Nb alloy deformed by ECAP is an irregular grain-subgrain oriented structure with an enhanced dislocation density, a cross-section of 30-150 nm of oriented structural elements, and an equiaxed-grain (subgrain) size of 50-200 nm. Heating after ECAP in the temperature range 720-770 K for 3-5 h is proposed for the formation of an ultrafine-grained equilibrium structure in the ECAP deformed Zr-2.5% Nb alloy. Heating of the Zr-2.5% Nb alloy after ECAP at 723 K for 5 h leads to the formation of a predominantly equiaxial submicrocrystalline structure with a grain size of 150-500 nm. Equal-channel angular pressing of the Zr-2.5% Nb alloy increases the yield strength to 622 MPa, which is higher than that in the as-delivered undeformed state by a factor of 1.6. In this case, the relative elongation decreases. Heating of the ECAP deformed Zr-2.5% Nb alloy at 723 K for 5 h decreases the yield strength to 504 MPa, but the relative elongation increases to 14%.