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Investigation of the Microstructure and Strengthening Mechanisms of Ti-6Cu-8Nb-xCr3C2 Alloy through Vacuum Sintering Process

Chang Shih-Hsien, Weng Chen-Yu, Huang Kuo-Tsung, Liang Cheng

Archives of Metallurgy and Materials

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2022

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Vol. 67, iss. 3

815--825

EN

This study mixes four different powders to produce Ti-6Cu-8Nb-xCr3C2 (x = 1, 3, and 5 mass%) alloys in three different proportions. The experimental results reveal that when 5 mass% Cr3C2 was added to the Ti-6Cu-8Nb alloys, the specimen possessed optimal mechanical properties after sintering at 1275°C for 1 h. The relative density reached 98.23%, hardness was enhanced to 67.8 HRA, and the transverse rupture strength (TRS) increased to 1821.2 MPa, respectively. The EBSD results show that the added Cr3C2 in situ decomposed into TiC and NbC during the sintering process, and the generated intermetallic compounds (Ti2Cu) were evenly dispersed in the Ti matrix. Furthermore, the reduced Cr atom acts as a β-phase stabilizing element and solid-solution in the Ti matrix. Consequently, the main strengthening mechanisms of the Ti-6Cu-8Nb-xCr3C2 alloys include dispersion strengthening, solid-solution strengthening, and precipitation hardening.

2

Study on the Sintered Characteristics and Properties of Co-30 mass% Cr Alloys under Various Vacuum Hot-Press Sintering Temperatures and Pressures

Chang Shih-Hsien, Chang Chih-Yao, Huang Kuo-Tsung

Archives of Metallurgy and Materials

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2020

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Vol. 65, iss. 1

393--402

EN

In this research, Co-30 mass% Cr alloys were fabricated by a vacuum hot-press sintering process. Different amounts of submicron cobalt and chromium (the mean grain size is 800 and 700 nm, respectively) powders were mixed by ball milling. Furthermore, this study imposed various hot-press sintering temperatures (1100, 1150, 1200 and 1250°C) and pressures (20, 35 and 50 MPa), while maintaining the sintering time at 1 h, respectively. The experimental results show that the optimum parameters of hot-press sintered Co-30 mass% Cr alloys are 1150°C at 35 MPa for 1 h. Mean while, the sintered density reaches 7.92 g·cm-3, the closed porosity decreases to 0.46%, and the hardness and transverse rupture strength (TRS) values increase to 77.2 HRA and 997.1 MPa, respectively. While the hot-press sintered Co-30 mass% Cr alloys at 1150°C and 20 MPa for 1 h, the electrical conductivity was slightly enhanced to 1.79 × 104 S·cm-1, and the phase transformation (FCC → HCP) of cobalt displayed a slight effect on sintering behaviors of Co-30 mass% Cr alloys. All these results confirm that the mechanical and electrical properties of Co-30 mass% Cr alloys are effectively improved by using the hot-press sintering technique.

3

Microstructure and Properties of Composite Produced by Vacuum Sintering of Vanadis 4 Extra Steel Powder with Tantalum Carbides and Following Heat Treatment

Huang Kuo-Tsung, Chang Shih-Hsien, Chuang Chan-Yu

Archives of Metallurgy and Materials

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2020

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Vol. 65, iss. 2

555--563

EN

In this study, different amounts of tantalum carbide (TaC) powders (5, 10 and 15 wt.%) are added to Vanadis 4 Extra steel powders. The composite powders are sintered at 1260, 1280, 1300, 1320, 1340 and 1360°C for 1 h, respectively. The experimental results showed that good mechanical properties (hardness 79.7 HRA, TRS 2246 MPa) were obtained by the addition of 10% TaC sintered at 1320°C for 1 h. Furthermore, the optimal sintered V4ES/TaC (Vanadis 4 Extra steel / TaC) composites after sub-zero treatment possess the highest hardness (80.9 HRA) and transverse rupture strength (TRS) values (2445 MPa), as well as a better polarization resistance (658.99 Ω·cm2). After sub-zero treatment, the VC carbides decompose and re-precipitate refined VC carbides within the grains (VC carbides are formed in steel powder); moreover, the TaC particles are still uniformly distributed around the grain boundaries, which results in dispersion strengthening and precipitation hardening. The results clearly reveal that sub-zero heat treatment effectively improves the microstructure and strengthens the V4ES/TaC composite.