Microstructure And Mechanical Properties Of Refractory Metal Alloys And Ultra High Temperature Ceramics Formed By Field Assisted Sintering Technique (FAST)
Open Access
- Author:
- Jacobs, James Anthony
- Graduate Program:
- Mechanical Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 24, 2019
- Committee Members:
- Jogender Singh, Thesis Advisor/Co-Advisor
Md Amanul Haque, Thesis Advisor/Co-Advisor
Anil Kamalakant Kulkarni, Committee Member - Keywords:
- Tungsten
Tantalum
UHTC
Field Assisted Sintering Technology
Grain Growth Inhibitor
Mechanical Properties - Abstract:
- Tungsten (W) refractory alloys are of high importance in the development of high temperature application products seen in furnace elements, the aerospace industry, and many other areas. Traditional manufacturing processes produce tungsten alloys with undesirable mechanical properties due to a large grain microstructure. Commercial sintering techniques yield low density products due to the poor sinterability of tungsten alloys. Field Assisted Sintering Technology (FAST) is used in this work to produce tungsten alloys with high density, and uniform microstructures. Limited research has been done with hafnium carbide (HfC) as grain growth inhibitors. W, W-1vol%HfC, W-2vol%HfC, and W-5vol%HfC were sintered at 2100-2150C, 35 MPa, for 25 min. Microstructure of each composition was characterized and reported. Volume additions of hafnium carbide at two percent or more shows a decrease in grain size of over 67% while increasing the hardness by over 19% when compared with a pure tungsten composition. A good interface was observed between the W matrix and HfC particles as confirmed by high resolution transmission electron microscopy (HRTEM). The introduction of hafnium carbide in small volume fraction also greatly increases the flexural strength of the tungsten matrix during room temperature bend testing. Tungsten-Tantalum (W-Ta) alloys are of high importance in the development of high temperature application products used in extreme environments such as nuclear fusion elements and defense related applications. Powder metallurgy has been used over traditional melting and casting techniques in order to improve mechanical properties and microstructure. Tungsten, tantalum, and hafnium carbide (HfC) were ball milled in compositions of W, W-10vol%Ta, Ta-10vol%W, and Ta-10vol%W-1.5vol%HfC and were sintered at 2100-2200C, 35 MPa, for 25 minutes. Mechanical properties and microstructure were characterized and reported. X-ray diffraction (XRD), optical micrographs, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) showed that W-Ta samples displayed two phase regions of pure tantalum and W-Ta. The addition of hafnium carbide to the Ta-10vol%W composition did not affect elastic modulus but greatly reduced the hardness of the material. The hardness decreases from 451 to 336 VHN, or a 25.5% decrease in hardness with the addition of HfC. Small volume fraction additions of hafnium carbide and titanium carbide to the tungsten and tantalum matrices greatly increase the ultimate flexural strength of these materials. Ultrahigh temperature ceramic (UHTC) systems with a composition of hafnium carbide and tantalum carbide have been theorized to exhibit the highest melting temperatures among all materials. Hafnium carbide and tantalum carbide are of high importance in the development of thermal shock systems for extremely high temperature environments due to the high hardness, melting temperature, and oxidation resistance properties of these carbides. Limited research has been done with hafnium carbide and tantalum carbide (HfC-TaC) systems. To address this shortcoming, HfC, HfC-TaC, HfC-TaC-Ta, HfC-TaC-TiC, and TaC compositions were sintered using FAST with high density and microstructure. Blended compositions of HfC-TaC sintered achieved solid state solution. These compositions of powders were also joined with a carbon disc to display the feasibility of joining the HfC-TaC to carbon systems for thermal protection applications. Microstructure of each composition was characterized by various techniques including optical, SEM, X-ray diffraction and HRTEM. HfC-TaC compositions sintered displayed densities above 98.5% and Vickers Hardness between 1448VHN and 2222VHN. Tungsten, tantalum, and UHTC materials are used for high temperature applications and are highly used in the aerospace industry. Because of this, some leading edge fabrication and design is modeled within the appendix.