| Summary: | 碩士 === 逢甲大學 === 材料科學與工程學系 === 102 === In this study, the effects of Cr on phase transformation and properties of AlCoCrxFeNi (x = 0.3, 0.5, 0.7, 1.0, in molar ratio) high entropy alloys were investigated. The crystal structure, microstructure, hardness, compressibility, and electrochemical corrosion properties of these alloys were studied. The alloys were performed solution treatment at 1300℃ for 24 hours, and then aged at 600~1200℃ for 24, 48, 72, and 144 hours. The microstructure and hardness of these alloys are analyzed.
The as-cast AlCoCr0.3FeNi alloy contains dual phases which are ordered BCC phase and FCC phase. When Cr content increases to x = 1.0, the FCC phase disappears and the microstructure displays a spinodal decomposition structure which formed by ordered BCC phase and BCC phase. It indicates Cr is a BCC former in AlCoCrxFeNi alloys. The hardness of as-cast alloys increases from HV415 to HV498 with the increase of Cr content.
Among these alloys, the AlCoCr0.3FeNi, AlCoCr0.5FeNi and AlCoCr0.7FeNi alloys exhibit higher fracture strains due to the formation of FCC phase in matrix. However, the highest yield strength (1394 MPa) and compressive strength (1841 MPa) presented by AlCoCrFeNi alloy are due to the existence of spinodal decomposition structure. The corrosion resistances of AlCoCrxFeNi alloys decrease slightly as Cr content increasing. In addition, as Cr content x = 0.5, the alloy exhibits the worst pitting resistance due to galvanic corrosion.
After 1300℃-24 hrs. solution treatment, the AlCoCrxFeNi (x = 0.3, 0.5, 0.7, 1.0) alloys show a single ordered BCC phase. For all alloys, the best age hardening occurs at 600℃ because of the precipitation of numerous nanoparticles. The AlCoCr0.3FeNi and AlCoCr0.5FeNi alloys aged at 700~1200℃ after 144 hours, age softening occurs since the FCC phase precipitates and then coarsens with temperature increasing.
The hardness of AlCoCr0.7FeNi alloy aged at 700~800℃ after 144 hours is close to the hardness of this alloy after solution treatment. However, age softening occurs at 900~1200℃ after 144 hours due to the coarsening effect of FCC phase with temperature increasing.
Age hardening is observed for AlCoCrFeNi alloy aged at 800~900℃ after 144 hours which is caused by the precipitation of σ phase. The hardness of this alloy aged at 1000℃ after 144 hours is the lowest since the σ phase transforms into BCC phase. As comparison with alloy aged 1000℃ for 144 hours, the hardness of alloy aged at 1100~1200℃ after 144 hours increase because the FCC phase is dissolved into matrix.
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