The low strain tensile behavior of U-7.5 wt pct Nb-2.5 wt pct Zr

Abstract

The stress-strain response of polycrystalline, γ-quenched U-7.5 wt pct Nb-2.5 wt pct Zr alloy was studied as a function of strain rate and compared to equilibrium stress-strain tests performed by allowing the strain to reach a maximum value at incrementally increasing stresses. Equilibrium stress-strain tests were also performed on prestressed samples. Sheet tensile specimens were held at various states of strain in an X-ray diffractometer to determine crystal structural changes during deformation. Prestressed tensile bars were sectioned and examined metallographically and with the X-ray diffractometer. Two linear regions were observed in the equilibrium stress-strain tests: a low stress region with a slope of 5.3 to 5.5 x 10⁶ psi, and a region above 40,000 psi with a slope of 3.3 x 10⁶ psi. Finite strain rates tended to increase both slopes. The diffractometer experiments yielded plots of lattice parameter vs strain which showed a shift from a bcc structure of the γ^s phase, to a bct structure of the γ₀ phase between 1 and 3 pct deformation. It is postulated that this is a thermoelastic martensite transformation. A semiempirical equation was developed which describes the equilibrium stress-strain behavior of this alloy in terms of a stress induced phase transformation.