Aging induced shape instability in an elastically bent uranium + 7.5 wt pct niobium + 2.5 wt pct zirconium alloy

The uranium +7.5 wt pct niobium +2.5 wt pct zirconium alloy when quenched from 1073 K was found to exist at room temperature as a metastable phase which was a slight tetragonal distortion of the elevated temperature body-centered-cubic (bcc) phase. Flat, asquenched specimens have been elastically deformed in four-point bending to maximum outer fiber stresses below the stress required for plastic deformation to occur but into a range of stress where pseudoelastic behavior has been observed. Aging of these elastically bent specimens in an oil bath at 423 K, while constrained by the bending jig, resulted in a permanent deflection and shape change. Further isothermal aging, after removal from the bending apparatus, caused increasing deflection and continued shape instability in spite of the absence of the applied load. X-ray examination of samples cut from a bent and aged specimen revealed important preferred orientation and lattice parameter differences between the tension and compression regions and the high and low stress parts of the specimen. These observations are described and compared to previous findings on quenched samples of this alloy that had been either deformed separately or aged separately. A rationalization of the shape instability is presented. Elastic twin nucleation and growth, preferred orientations, solute segregation and the interplay of all these seem to be involved.