| Summary: | 碩士 === 國立清華大學 === 材料科學工程學系 === 103 === Conventional sintered carbides (CSCs) are composed of non-refractory metal(s), Co, Ni, or Fe, and carbide(s), while the invented refractory metal fused carbides (RMFCs) in this laboratory are composed of refractory metals and their alloys (as binders) and carbides (as strengtheners) that are mixed in liquid phase and solidified, instead of sintered process. By introducing the configurational entropy in the carbides in RMFCs, the new composites have improved ambient and elevated temperature properties such as 100% relative density and good mechanical properties at super high temperatures and also have merits such as rapid speed and cost effective for manufacturing.
This study takes tungsten as the binder. Seven strengtheners utilized are TiC, ZrC, HfC, VC, NbC, TaC, and WC that are transition metal carbides of high hardness and of high strength. Seven one binder-one strengenther composites are firstly made to investigate their properties. The next are W-xTiC, where x is the mole ratios, to observe the effect of x on mechanical properties and microstructure of the fused composites (W fused carbides, WFCs). The observation results show that good mechanical properties occur for x at the interval of 30 at% ≤ x ≤ 60 at%. The third step is to increase the number of strengtheners that show better properties in the first step, and the results are composites with mixed strengtheners of TiC, NbC, TaC, and WC that have superior mechanical properties. With the assistance of XRD to investigate crystal strcture; SEM to observe microstructure; EDS and EPMA to analyze composition; and ESCA to analyze bond structure; as well as tests such as ambient temperature hardness and toughness, elevated temperature hardness, and both ambient and elevated temperature abrasion tests, the properties of tungsten fused carbides (WFCs) in this study are researched.
WFCs have the typical melted-solidified dendrite-interdendrite microstructure that has almost both a BCC solid solution of W and an FCC solid solution carbide, which is a one multimetal carbide due to the configurational entropy mixing effect. Ambient temperature hardness of WFCs that contain 40 at% W ranges from 1100 Hv to 2200 HV, while toughness at room temperature ranges from 6 MPa m1/2 to 13 MPa m1/2. Both hardness and toughness of WFCs can be adjusted by changing the x value.
The abrasion test with 6 kgf loading and ground with Al2O3 belt show a very high value of abrasion resistance of 156 m/mm3. At a proper toughness the abrasion resistance of WFCs has positive relation to their hardness. Tested speimens show 1000oC-hardness of 800 HV to 1400 HV, while they show 1100 oC-hardness of 700 HV to 1200 HV. Small depth of 0.5 mm in turning show two WFCs have tool life slightly larger than commercial WC-Co, while large depths of 2.5 mm and 4 mm show the same two WFCs have much larger tool life than the same WC-Co.
In summary, in a vast application field WFCs have great developing potential not only in high power and high temperature tool, wear resistant, and drill materials in bulky, welding, or coating form, but possibly in high temperature parts in aerospace engines, thermal insulators, sprayers and even in military applications such as high temperature parts of artilery or rocket bomb, armer, and heavy duty gun barrel.
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