UC Berkeley

A comprehensive study of the microstructural evolution of pure rhenium during ex-situ compression, in-situ and ex-situ tension has been conducted using standard TEM and EBSD metallographic analysis. Little information is available on how the microstructure of pure rhenium effects its more notable properties. Some of the unique properties that make rhenium notable include extreme strength, work hardening rate and ductility, all of which are maintained through an extreme temperature range. The insights gained through this improved understanding of the rhenium microstructure, along with previous simulation work, was used as a model around which a rhenium replacement alloy was developed. Due to the extreme cost of pure Re, ruthenium, tantalum, tungsten, and molybdenum alloys were chosen as the proper system for replacement.

Under compression, slip and dislocation plasticity in rhenium was predominantly active in the low strain regime, with {112 ̅1}〈112 ̅6〉 twinning occurring at all strain amounts and dominating after yield. This is contrary to deformation twins typically being active during the initial low strain regime of twinning dominated metals. It was observed that twins could bypass grain boundaries, as previously reported. In addition, a mechanism of twins “jogging” along the c-axis past twin boundaries was observed using TEM. This mechanism allowed for multiple twin variants to be active within individual grains. Twin “jogging” helps to explain the excellent ductility in rhenium accommodating the lack of dislocation plasticity. Postmortem TEM imaging showed dislocation density steadily increased around the twins, this is likely a result of twin boundaries impeding dislocation slip, confirming observations seen in highly work-hardened tension samples. Additionally, dislocation populated twin boundaries have shown {112 ̅1}〈112 ̅6〉 type twins resist growing in size when surrounded by dislocations, tending instead to form new twins as strain increases.

The use of EBSD has shown that under tension the twin area fraction plateaued after one third of the failure strain was achieved. After twin saturation, all changes to the deformed microstructure was seen as increased interior grain misorientations. Through Schmidt factor analysis, EBSD had shown that prismatic and pyramidal slip activated appreciably only at strain values above half of the failure strain. Conventional TEM investigation has shown that type b ⃗=[112 ̅0] screw dislocations operated on the basal planes in loosely aligned slip bands, no dislocations were seen operating on the prism planes within the TEM. At failure stresses, type b ⃗=[112 ̅0] basal screw dislocations were again observed in dislocation slip bands. type b ⃗=[112 ̅3] pyramidal screw dislocations which formed tangled dislocation nets interfering with type glide were observed. HAADF STEM imaging was used to view the morphology of tension induced {112 ̅1}〈112 ̅6〉 twins. These were more representative of classical twin structures involving well defined twin boundaries surrounding a region of crystal with a single new orientation. This did not represent the {112 ̅1}〈112 ̅6〉 type twins seen in compression which consisted of twin aggregates. This compression-tension asymmetry is likely due to the twin favorability of the microstructure forcing the creation of many more twins during compression. Twin transmission with {112 ̅1}〈112 ̅6〉 changing twin plane between parent and matrix orientations was observed. TEM observations confirmed the observations made using purely EBSD maps with Schmidt factor analysis.

In-situ tensile straining was performed on rhenium both at room temperature and at 920°C. The samples had a texture which suppressed possible {112 ̅1}〈112 ̅6〉 twinning, while exaggerating basal or prismatic dislocation activity. Two main samples were successfully tested, with both confirming the observation that type 〈112 ̅0〉 basal dislocations were the dominant mechanism during tensile straining. Samples were all tested both at room temperature and elevated temperature in order to find any differences in behavior based on temperature. Dislocation motion was characterized as “jerky”, as detailed by Clouet in Ti. The only appreciable effect of raising the temperature to 920°C was slightly increased dislocation density. Dislocation motion was unaffected by this slight temperature rise.

The effects of alloying ruthenium with tungsten, tantalum, and molybdenum were investigated, using powder x-ray diffraction to determine crystal structures. A novel indentation stress-strain method was developed as a method for mechanical property screening, using indents on pure Re and Ru as a basis for comparison. Three chemically complex alloys low-Ru content alloys with HCP structures were characterized using XRD. The Ru 45 - Ta 10 - W 20 - Mo 25 (compositions in wt.%) alloy was shown to be near the solubility limit of the HCP phase, proving to be the composition with the lowest Ru content and having completely HCP single phase after annealing at 1850 °C.

Through initial indentation tests, Ru was seen to be extremely hard and brittle with extensive cracking around the indentation tip. Both Berkovich and spherical indentation was performed on all alloys, with indents in pure Re and Ru used as a basis for comparison Tungsten was shown to provide a consistent improvement to Ru alloy behavior throughout all three alloying systems. In general, low level additions of tantalum were seen to improve the mechanical properties significantly, however significant additions of tantalum generally lead to the formation of additional phases and reduced mechanical response. Molybdenum increased the solubility of all solute elements within the HCP alloys, further improving the mechanical properties of any alloy by reducing Ru content. EBSD scans were performed on all of the alloy systems with compositions near the solubility limit. Indents within the Ru-Ta-W system had the unique {112 ̅1}〈112 ̅6〉 twins seen near the indent tip. The Ru-Mo-W system showed {112 ̅2}〈112 ̅3〉 type twins underneath the indents. Finally, the Ru-Ta-W-Mo system showed {101 ̅2}〈101 ̅1〉 twins beneath the indent surface. All twin variants that were observed when indenting Re were also observed in the various designed chemically complex alloys. The observation of these twins is extremely promising for the future of Re-replacement alloy development.