Orginal Article

# 过冷Co75B25合金的凝固

1 上海交通大学材料科学与工程学院金属基复合材料国家重点实验室 上海 200240
2 上海交通大学材料科学与工程学院上海市激光制造与材料改性重点实验室 上海 200240

# Solidification of Undercooled Co75B25 Alloy

LI Yun12, LIU Lianjie12, LI Xinming12, LI Jinfu12

1 State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
2 Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

Abstract

The method to deeply undercool alloy melts far below the liquidus temperature by eliminating heterogeneous nucleation sites inside is frequently used in studying non-equilibrium solidification behavior, preparing quasi-crystal, amorphous alloy and other metastable materials. Previous work on the solidification of Co-(18.5~20.7)%B (atomic fraction) alloys indicated that metastable Co23B6 phase instead of stable Co3B phase was formed as the primary phase from the melts undercooled by larger than 60 K. To know whether Co23B6 phase can still primarily form from the deeply undercooled melt of Co75B25, the nominal composition of Co3B phase, the Co75B25 alloy melt was undercooled to different degrees using the glass fluxing technique, and the solidification path was identified by analyzing the microstructures and cooling curves of the samples. There was nothing other than α-Co and Co2B phases to form during solidification, indicating that not only the peritectic reaction of L (liquid) and Co2B into Co3B, predicted by the Co-B phase diagram, but also the formation of Co3B as primary phase at large undercooling were inhibited. The peritectic reaction did not occur even though the solidification was designed to occur at a very small undercooling and a cooling rate decreased to 5 K/min.

Keywords： Co75B25 alloy ; undercooling ; non-equilibrium solidification ; phase selection

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LI Yun, LIU Lianjie, LI Xinming, LI Jinfu. Solidification of Undercooled Co75B25 Alloy[J]. Acta Metallurgica Sinica, 2018, 54(8): 1165-1170 https://doi.org/10.11900/0412.1961.2017.00504

## 1 实验方法

Co75B25合金由高纯度Co (99.99%)和B (99.999%)于高纯Ar保护下在电弧炉中熔炼配制而成,每个试样均反复重熔6次以确保成分均匀。

## 2 实验结果

Fig.1   XRD spectra of Co75B25 alloy solidified at typical undercoolings (ΔT)

Fig.2   Cooling curves of Co75B25 alloy solidified at typical undercoolings (The insets on top right are magnifications of the transformation from the first recalescence to the second one for ΔT=210 K and 284 K, respectively; TL is the liquidus temperature)

Fig.3   Microstructures of Co75B25 alloy solidified at typical undercoolings 7 K (a), 85 K (b), 210 K (c) and 284 K (d) (Insets are magnification of local microstructure for relevant undercoolings)

Fig.4   XRD spectrum (a) and microstructure (b) of Co75B25 alloy undercooled by 5 K and solidified at a cooling rate of 5 K/min

Fig.5   DSC curves of repeatedly heating (a, c and e) and cooling (b and d) of Co75B25 alloy (with all the heating rates of 20 K/min)

## 3 分析讨论

Co-B合金在富Co端的相图如图6[22]所示。α-Co和Co2B液相线延长线及二者构成的α-Co/Co2B共晶线也一并以虚线示于图中,其中α-Co/Co2B共晶温度点由Chizhevskii等[23]给出,在他们的工作中曾认为不存在包晶反应L+Co2B→Co3B。Co75B25合金在各种过冷度凝固后组织的XRD结果显示,凝固组织中既不含亚稳Co23B6相,也不含金属间化合物Co3B相,表明Co75B25合金的凝固依据α-Co/Co2B共晶相图进行,此时Co2B作为初生相首先自液相析出,对应冷却曲线上出现第一次再辉。随着初生相的逐渐生长,剩余液相中的含B量逐渐降低,直到一定温度后发生共晶转变,在冷却曲线上出现第二次再辉。中等过冷度下,初生Co2B相间距已较小,且剩余液相向共晶转变的速率还不太快,因此采取共晶Co2B相依附于初生Co2B相生长的方式进行共晶凝固,离异共晶的结果使得初生Co2B相间看不到网格状共晶组织。更大的过冷度下,因共晶生长速率很快,无法以离异共晶方式生长,初生Co2B相间α-Co/Co2B网格状共晶再次出现。

Fig.6   Partial phase diagram of Co-B system on the Co-rich side, with dash lines exhibiting the L→α-Co/Co2B eutectic reaction[22]

$Δx2=2tγ(C2-C1)D$(1)

## 4 结论

(1) Co75B25包晶合金在所实验的过冷度范围内(0~344 K)凝固时,凝固路径依据α-Co/Co2B共晶相图进行,即初生Co2B相首先从过冷熔体中析出,随后剩余液相形成α-Co/Co2B规则共晶或离异共晶,L+Co2B→Co3B包晶转变被完全抑制。

(2) 在实验所能获得的最大过冷度(344 K)下,无论是亚稳相Co23B6还是Co3B相都不能作为初生相优先从熔体中析出。

(3) 降低Co75B25合金小过冷度下凝固时的冷却速率至5 K/min,L+Co2B→Co3B包晶反应仍被完全抑制,表明凝固过程中Co3B相的析出极其困难。

The authors have declared that no competing interests exist.

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