Microstructure and Mechanical Properties of Refractory Metal Alloys Formed by Field Assisted Sintering Technique (FAST)

Restricted (Penn State Only)
Author:
Browning, Paul Nathan
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 12, 2016
Committee Members:
  • Jogender Singh, Dissertation Advisor
  • Anil Kulkarni, Committee Chair
  • Todd Palmer, Committee Member
  • Ivica Smid, Committee Member
  • Ivica Smid, Outside Member
Keywords:
  • Refractory Metals
  • Field Assisted Sintering
  • Spark Plasma Sintering
  • Mechanical Properties
Abstract:
Refractory metals and their alloys are of critical importance for a large number of applications ranging from nuclear fission and fusion reactors to the defense and aerospace industries. Of the refractory metal alloys, tungsten, tantalum, and molybdenum alloys are dominant in both commercial and defense applications. Unfortunately, due to their high melting temperatures and relatively poor sinterability, formation of high-density components of these alloys is difficult, requiring either the use of time-consuming and expensive techniques such as hot isostatic pressing or post-processing such as hot extrusion and annealing to produce components acceptable for structural use. Furthermore, as components of these compounds are generally by their very nature used at elevated operating temperatures, they frequently experience failure due to recrystallization, grain growth, and creep. Field Assisted Sintering Technique (FAST), also commonly referred to as spark plasma sintering (SPS) or pulsed electric current sintering (PECS) is a promising method for formation of dense compacts of traditionally difficult to sinter materials. FAST uses a combination of elevated temperatures, pressures, and the application of an electric field to rapidly sinter components over the course of several minutes to hours. FAST is an excellent technique for rapid production of refractory alloys, as it produces near net-shaped components featuring equiaxed grain structures. As FASTs uses rapid heating and cooling rates, it is also capable of producing dense components of much finer grain sizes in comparison to traditional sintering techniques. This dissertation explores the use of FAST and alloying additions to produce refractory metal alloys featuring improved mechanical properties over current commercially available materials. Work begins by exploring the room and ultra-high temperature mechanical properties of a series of tungsten alloys (Chapter 3) attempting to make use of Ta and TiC alloying additions. W-TiC alloys are found to be promising for high-strength applications where ductility is less of an issue, and display substantial resistance to grain growth at temperatures up to1927 ˚C (3500 ˚F). This strengthening effect was found to be lost upon testing at 2204 ˚C (4000 ˚F). In contrast, a tested W-10vol%Ta alloy was found to display high strength and ductility at temperatures of 1649 ˚C (3000 ˚F) and higher, making this an excellent candidate material for ultrahigh temperature applications. To gain increased understanding of the microstructural effects that were resulting in observed mechanical behaviors, a detailed study of W-10vol%Ta and W-10vol%Ta-5vol%TiC alloys including optical and SEM imaging with EDS mapping, TEM imaging, X-ray diffraction, and hardness measurements was performed to examine elemental diffusion and microstructural changes in these two alloys. W-Ta cross diffusion was discovered to occur over a relatively large area on order of 10s of microns, producing a Ta interface region featuring increased hardness than that expected for pure tantalum. Interestingly, interstitial elements such as C and O were found to preferentially migrate to the center of Ta particles, producing regions displaying extremely high hardness in comparison to the surrounding ductile matrices, possibly accounting for the apparent strengthening seen in the W-10vol%Ta samples at both room and elevated temperatures. The loss of strength in W-TiC alloys at 2204 ˚C was disappointing given their excellent grain growth resistance up to 1927 ˚C, therefore work was done investigating the centering and mechanical behavior of W with other nanoparticle additives. This resulted in identification of oxidation of nanoparticle additions in W due to oxygen present in the matrix phase, which is in agreement with previous oxidation seen in W-TiC alloys. A Zener-type model allowed for successful description of grain growth inhibition of W during sintering with carbide particle additions, and a simple model combining the Hall-Petch and second-phase precipitate strengthening mechanisms was capable of prediction alloy hardness values with relative accuracy, alloying for tailoring of W hardness with specific nanoparticle additions to desired values. Tungsten alloys were found to be highly brittle at room temperature regardless of alloying changes, therefore work was also performed exploring room and ultra-high temperature Ta alloys (Chapter 4). This resulted in identification of Ta-10vol%W as an excellent alloy for use over a wide range of temperatures including up to temperatures in excess of 2204 ˚C. It was further demonstrated that this alloy displayed superior mechanical properties to Ta-10vol%W produced by other commercial methods at elevated temperatures which was attributed to the improved microstructure obtained by FAST sintering in comparison to Ta-10vol%W alloys obtained by typical arc casting, forging, and heat treatment methods. Finally, as TiC nanoparticle addition was found to produce extremely strong W alloys up to 1927 ˚C, it was of interest whether use of this alloy was possible with molybdenum, a material used at lower temperatures than tungsten for which the loss of strength seen at 2204 ˚C would not be a concern. Sintering studies were therefore performed determining optimal sintering conditions for production of the molybdenum alloy TZM (titanium zirconium molybdenum) alloys with and without TiC addition (Chapter 5). After determining these conditions, commercial-scale 4”x4” plates were sintered at optimized conditions, producing similar densities to that seen in smaller samples demonstrating potential scalability of FAST production of these alloys for commercial-scale sintering. Mechanical testing of these alloys identified the presence of high degrees of ductility in pure TZM samples and TZM-5vol%TiC samples, although this ductility was lost at 10 vol% TiC addition.