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Abstract: Ultra-fine grained (UFG) W–TiC compacts fabricated by powder metallurgical methods utilizing mechanical alloying (MA) are very promising for use in irradiation environments. However, the assurance of room-temperature ductility and enhancement in surface resistances to low-energy hydrogen irradiation are unsettled issues. As an approach to solution to these, microstructural modification by hot plastic working has been applied to UFG W–TiC processed by MA in a purified Ar or H2 atmosphere and hot isostatic pressing (HIP). Hot plastically worked compacts have been subjected to 3-point bend tests at room temperature and TEM microstructural examinations. It is found that the microstructural modification allows us to convert UFG W–1.1%TiC to compacts exhibiting a very high fracture strength and appreciable ductility at room temperature. The compacts of W–1.1%TiC/Ar (MA atmosphere: Ar) and W–1.1%TiC/H2 (MA atmosphere: H2) exhibit re-crystallized structures with approximately 0.5 and 1.5μm in grain size, respectively. It is shown that the enhancement of fracture resistance by microstructural modifications is attributed to significant strengthening of weak grain boundaries in the re-crystallized state. As a result the modified compacts exhibit superior surface resistance to low-energy deuteron irradiation. [Copyright &y& Elsevier]

Abstract: Nano-sized Ar bubbles give negative influence on the fracture resistance and occurrence of superplasticity in ultra-fine grained (UFG) W–TiC compacts. In order to enhance deformability in UFG, Ar-contained W–TiC compacts, effects of TiC addition on the high-temperature deformation behavior were examined. W–TiC compacts with TiC additions of 0, 0.25, 0.5, 0.8 and 1.1wt% were fabricated by mechanical alloying in a purified Ar atmosphere and hot isostatic pressing. Tensile tests were conducted at 1673–1973K (0.45–0.54 T m, T m: melting point of W) at initial strain rates from 5×10−5 to 5×10−3 s−1. It is found that as TiC addition increases, the elongation to fracture significantly increases, e.g., from 3 to 7% for W–0 and 0.25TiC/Ar to above 160% for W–1.1TiC/Ar when tested at 1873 and 1973K at 5×10−4 s−1. The flow stress takes a peak at 0.25%TiC and decreases to a nearly constant level at 0.5–1.1%TiC. The ranges of the strain rate sensitivity of flow stress, m, and the activation energy for deformation, Q, with TiC additions are 0.17–0.30 and 310–600kJ/mol, respectively. The observed effects of the TiC additions on the tensile properties are discussed. [Copyright &y& Elsevier]

Abstract: In order to develop an ultra-fine grained vanadium alloy dispersed with nano-sized dispersoids having superior ductility and high strength, powder metallurgical methods utilizing mechanical alloying (MA) and hot isostatic pressing (HIP) processes were applied to vanadium with 1.7mass% yttrium addition. The developed alloy was annealed at 1273–1573K for 3.6ks and subjected to TEM observations and tensile tests at room temperature. The alloy has average grain sizes from 330 to 750nm, and average particle sizes of Y2O3 and YN from 7 to 23nm, depending on annealing temperature. It exhibits superior ductility and high strength even in the as-HIPed state (no plastic working). Eighty percent cold rolling slightly improves the tensile properties. These results indicate that the microstructure of the alloy is almost free from weak portions associated with gaseous interstitial impurities that cause significant loss of ductility. [Copyright &y& Elsevier]

One key issue for small specimen test techniques is to clarify specimen size effects on test results. In consideration of size effects on determining the ductile-to-brittle transition temperature (DBTT) in Charpy impact testing, a method to evaluate the plastic constraint loss for differently sized Charpy V-notch (CVN) specimens is proposed and applied to a ferritic–martensitic steel, 2WFK, developed by JNC. In the method, a constraint factor, α, that is an index of the plastic constraint is defined as α=σ*/σy*. Here, σ* is the critical cleavage fracture stress which is a material constant and σy* is the uniaxial yield stress at the DBTT at the strain rate generated in the Charpy impact test. The procedures for evaluating each of σ* and σy* are described and a result of σ* and σy*, thus the value of α, is presented for different types of miniaturized and full-sized CVN specimens of 2WFK. [Copyright &y& Elsevier]

There were considerable differences in ductility and recrystallization temperatures among fine-grained, carbide-dispersed tungsten alloys developed so far by mechanical alloying, followed by sintering and hot working. The differences are attributable mainly to three microstructural factors giving detrimental effects on the ductility and recrystallization temperature; (1) precipitation of the brittle W2C phase, (2) heterogeneity in grain size and particle distributions and (3) loss of carbon which is a constituent of transition metal carbides. Therefore, a process to eliminate these factors is presented. The improved process was applied to fabricate tungsten alloys, and microstructural observation and three-point bending tests were performed on the alloys. It is demonstrated that the developed alloys have microstructures almost free from the three factors; the developed alloys exhibited no ductility before fracture in the as-HIPed state, but showed appreciable ductility in the as-forged state, indicating importance of plastic working to improve the ductility of the alloys. [Copyright &y& Elsevier]