ORIGINAL RESEARCH ARTICLE Evolution of Texture and Deformation Mechanisms During Repeated Deformation and Heat Treating Cycles of U-6Nb DONALD W. BROWN, KESTER D. CLARKE, BJORN CLAUSEN, CATHERINE N. TUPPER, SVEN VOGEL, and ELOISA ZEPEDA-ALARCON The evolution of the crystallographic texture and lattice strain of uranium 6-weight percent niobium alloy samples are tracked during multiple deformation and heat treating cycles in an effort to understand and control the mechanical properties of the material following thermo-mechanical processing. The heavily twinned microstructure and low-symmetry crystal structure of U-6Nb result in multiple sequential active deformation mechanisms associated with distinctive deformation textures in strain ranges from 0-0.15 true strain. It is found that heating into the high-temperature c-phase erases much of the texture formed during deformation at room temperature in the a¢¢-phase and resets the active deformation mechanisms. Through a small number of deformation/heat treat cycles to moderate strains, i.e., ~ 0.13 per cycle, the flow strength of the material is recovered to its original value. However, on the fourth such cycle, a reduction of strength is observed and the sample failed. https://doi.org/10.1007/s11661-021-06210-y Ó The Minerals, Metals & Materials Society and ASM International 2021 I. INTRODUCTION METALLIC uranium finds application primarily due to its high density and nuclear properties. However, pure uranium has poor corrosion resistance and low ductility at room temperature. Transition metals such as Mo, [1,2] Nb, [3] Zr, [4] and Ti [5,6] are often used as alloying elements to improve these properties while maintaining high density. While these elements have minimal solu- bility in the b (tetragonal) or a (orthorhombic) phases of uranium, metastable alloys can be formed through rapid quenching from the high-temperature c (body-centered cubic) phase where they are soluble. The addition of niobium to uranium, in particular, near the monotectoid composition of 6 wt pct Nb (14 at.pt) [7,8] (U-6Nb), is of particular interest because it significantly increases the corrosion resistance and ductility relative to pure uranium [7] when quenched at sufficient rates to maintain the niobium in solution. With niobium concentrations between roughly 4 and 7 wt pct (~ 10-16 at. pct) [3,9] and quench rates greater than 20K/s [7 542 542], a metastable martensitic phase with a monoclinic crystal structure at room tempera- ture [2,3,10–12] referred to as a¢¢ [13] is formed. The resulting room temperature microstructure is comprised of the 12 twin-related equivalent variants of the martensitic phase transformation. [9,14] The monoclinic room temperature crystal structure coupled with a heavily twinned martensitic microstruc- ture introduces de-twinning as a deformation mode in a¢¢ uranium niobium alloys near 6 wt pct niobium. [15,16] The relative ease of twin boundary motion is responsible for the enhanced ductility of the alloy and also enables the shape memory effect (SME). Jackson et al. first reported SME in a U-Nb alloy in 1978, [11] but Vander- meer and co-workers made the first concerted study of SME in uranium niobium alloys. [17–19] The advent of neutron (high penetrability in uranium) diffraction facilities containing in situ testing equipment precipi- tated further study of the micro-mechanics of the deformation of a¢¢ uranium niobium alloys. [15,16,20] Consistent with other shape memory alloys (e.g., References 21–23), U-6Nb exhibits a sigmoidal flow curve in both tension and compression [15,18,24] where the first yield and subsequent low-stress plateau (to ~ 0.04 strain) are associated with de-twinning of the as-quenched microstructure. Deformation in this regime is recoverable through subsequent thermal cycling of the deformed material into the high-temperature c-phase. [18] In the regime beyond shape memory (~ 0.05 to 0.08 strain), deformation in tension occurs through DONALD W. BROWN, BJORN CLAUSEN, SVEN VOGEL, and ELOISA ZEPEDA-ALARCON are with the Los Alamos National Laboratory, PO Box 1663 MS H805, Los Alamos, NM 87545. Contact e-mail: dbrown@lanl.gov KESTER D. CLARKE is with the Colorado School of Mines, 1500 Illinois St., Golden, CO 80401. CATHERINE N. TUPPER is with the Northwestern University, Materials Science and Engineering Department, 2220 Campus Drive, Cook 2036, Evanston, IL 60208. Manuscript submitted 25 June 2020; accepted 14 February 2020. Article published online March 28, 2021 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 52A, JUNE 2021—2195