Gd及退火溫度對CoCrNi顯微結構及機械性質之影響

Abstract

通過電弧熔煉製造了一系列 (CoCrNi)100–xGdx (x = 0/0.1/0.3/0.5/1.0) 中熵合金 (MEA)。研究了稀土元素Gd添加劑對 CoCrNi MEA 微觀結構演變和力學性能的影響。使用XRD進行相鑑定，SEM和EPMA進行微觀結構觀察及元素。拉伸性能通過 MTS 測量，硬度透過維氏硬度和奈米壓痕測量。隨著 Gd 濃度的增加，發現從 FCC 轉變為FCC + HCP 的雙相結構。與降伏強度為 400 MPa、斷裂強度為 880 MPa 的單相 CoCrNi MEA 相比，CoCrNi99.5Gd0.5 MEA 的降伏強度為 751 MPa，極限抗拉強度為 1216 MPa，而延展性仍保持較高值約 55%。機械性質的提升主要來自固溶、析出、晶粒細化。 為了瞭解Gd添加對CoCrNi MEAs退火微結構的演變，(CoCrNi)100–xGdx (x = 0/0.3/0.5/1.0) MEAs在未進行退火處理及不同溫度700、800和900 ℃下進行退火做比較。XRD結果顯示在CoCrNi MEAs在不同溫度下皆呈現單一FCC相，此外在CoCrNi99Gd1 MEAs在不同溫度下皆呈現雙相FCC+HCP，有趣的是，在 700 ℃時，FCC 基材在CoCrNi99.5Gd0.5 中呈現出兩個不同的區域。一種是再結晶晶粒，另一種是富含大量差排的未再結晶晶粒。在相同退火溫度700 ℃下，隨著Gd含量的增加，未再結晶區域在0到0.3 at.%會先上升隨後0.3到1.0 at.%呈現下降的趨勢。在相同成分CoCrNi99Gd1下，隨著退火溫度上升，未再結晶區域減少。熱壓後造成具有部分再結晶結構可以有效提升機械性質CoCrNi99.5Gd0.5 MEAs在退火700 ℃後表現出超過1 GPa的降伏強度同時仍有25%的延性。綜觀上述實驗結果，可以藉由調控Gd元素添加量以及適當的退火溫度來提升CoCrNi MEAs的機械性質。

A series of (CoCrNi)100–xGdx (x = 0/0.1/0.3/0.5/1.0) medium entropy alloys (MEAs) was fabricated by arc melting. The effects of rare-earth element, Gd, additions on the evolution of microstructures and mechanical properties of CoCrNi MEAs were investigated. The phase and microstructure were examined using X-ray diffraction, scanning electron microscopy and electron probe microanalyzer. The tensile properties were measured by MTS and the hardness was measured by both Vickers indenter and nanoindentation. As the Gd concentration increased, it was found that the phase transformed from FCC to HCP precipitates in FCC matrix. Compared to the single phase CoCrNi MEA with yield strength of 400 MPa and fracture strength of 880 MPa, CoCrNi99.5Gd0.5 MEA exhibited a higher yield strength of 751 MPa and ultimate tensile strength of 1216 MPa, while the ductility still maintained a high value of ~55%. The enhanced mechanical properties were ascribed to solid solution, precipitation and grain refinement strengthening mechanisms. To investigate the hot-working process and annealing treatment, the series of (CoCrNi)100–xGdx (x = 0/0.3/0.5/1.0) MEAs was annealed at different temperature 700, 800, and 900 ℃. The XRD results displayed the single FCC in CoCrNi and FCC + HCP in CoCrNi99Gd1, respectively. The crystal structure of CoCrNi99Gd1 remained as a dual phase at different annealing temperatures. Interestingly, for annealing at 700 ℃, the FCC matrix showed two different regions in CoCrNi99.5Gd0.5. One was recrystallized grain, and the other was non-recrystallized grain which contained numerous dislocations. For annealing at 700 ºC, the non-recrystallized region increased from 0 to 0.3 at.% of Gd additions and then decreased from 0.3 to 1.0 at.% of Gd additions. For CoCrNi99Gd1, the non-recrystallized region decreased with the increasing annealing temperature. The partial-recrystallization structure was considered as an effective way for improving mechanical properties. CoCrNi99.5Gd0.5 MEAs annealed at 700 ℃ exhibited an ultrahigh yield strength which exceeded 1 GPa with 25% fracture elongation. Hence, appropriate amount of Gd additions and annealing temperatures could enhance the mechanical properties of CoCrNi MEAs effectively.