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退火处理对Co-Cr-Fe-Ni-Ti多主元合金显微组织及硬度的影响
论文作者:童鞋论文网  论文来源:www.txlunwenw.com  发布时间:2019/9/20 8:44:10  

摘要:多主元高熵合金由于其优异的性能,已成为金属材料研究的重要领域之一。但是现在对高熵合金的研究大多数侧重于五元或五元以上等原子比的铸态合金,对相关的五元系(如Co-Cr-Fe-Ni-Ti体系)元素含量变化及高温长时间退火后的组织性能变化研究甚少。本文将合金中的三种元素固定为20 at.%,改变另外两种元素的含量,通过用真空电弧炉熔炼制备一系列的Co-Cr-Fe-Ni-Ti五元合金,利用扫描电镜、能谱仪和X射线衍射仪、维氏显微硬度计等仪器研究了铸态以及800°C和1000°C退火30天后合金的显微组织及显微硬度。

研究表明,(1)CoxCrFeNi2-xTi铸态合金在Co含量5~15 at.%时出现共晶组织。σ相和BCC相存在于铸态和800°C退火态合金中,而经1000°C退火后转变为Cr13Fe35Ni3Ti7相。随着Co含量的增加,D024相逐渐转变为C14相。(2)随着Ti含量的增加,CoCrFeNi2-xTix合金的相变过程由FCC相到D024相到析出Cr13Fe35Ni3Ti7相,最终D024相全部转变为C14相。800°C退火30天后对其相组成影响不大,而1000°C退火后将发生很大变化,含15~20 at.%Ti的合金中将析出Cr13Fe35Ni3Ti7相。随着Ti含量的进一步增加,其转变成σ相。(3)Co2.5-xCrxFeNiTi0.5合金在铸态和800°C退火态中随着Cr含量的增多从FCC相中析出σ相,在1000°C退火态下FCC相转变成σ和D024相。Cr含量较高时合金在退火后析出了BCC相。(4)Co2-xCrxFeNiTi合金在铸态和800°C退火态中存在共晶组织。铸态和800°C退火态中析出σ相,1000°C退火态中σ相消失,出现Cr13Fe35Ni3Ti7相,Ti含量较高时合金中都析出了BCC相。退火温度越高,合金中越容易形成金属间化合物,且晶粒越粗大。

研究发现,Co-Cr-Fe-Ni-Ti合金的硬度分别随着Co或Ti元素含量的增加而增大。随着退火温度的增高,合金的硬度总体上小幅降低,且合金元素含量的不同使其硬度受退火温度的影响程度不同。当合金中出现BCC相时,合金的硬度有所降低。

此外,实验中的铸态合金相组成及其变化规律与混合熵及价电子浓度的模型参数的预测结果基本符合。但是实验测定结果与Factsage数据库中的热力学计算结果差异较大。所以,并不能完全依靠现有的热力学数据库计算结果来预测合金的相稳定性。

Multi-component high-entropy alloys havebecome one of the important fields in metal materials research due to theirexcellent properties. However, most of them focus on the study of as-castalloys with equal atomic percent of five or more elements. And few studies havebeen done on the changes of elemental content in related quinary systems, suchas Co-Cr-Fe-Ni-Ti system, and the changes of structure and properties afterhigh temperature and long time annealing. In the present work, three elementsof the alloy were fixed at 20 at.%, and the contents of the other two elementswere changed, and a series of Co-Cr-Fe-Ni-Ti system alloys were prepared bymelting in a vacuum arc furnace. Microstructure and microhardness of as-castand annealed alloys at 800 °C and 1000 °C for 30 days were studied by ScanningElectron Microscopy (SEM) equipped with an energy dispersive spectrometer(EDS), X-ray diffraction (XRD) and Vickers microhardness tester.

The results show that (1) the eutecticstructure occurs in as-cast CoxCrFeNi2-xTi alloy with 5-15 at.% Co. The σ andBCC phases exist in the as-cast and 800 °C annealed alloys, which turn toCr13Fe35Ni3Ti7 phase after annealing at 1000 °C for 30 days. With the increaseof Co content, the D024 phase gradually transforms into the C14 phase. (2) Withthe increase of Ti content, the phase transformation process of theCoCrFeNi2-xTix alloy changes from FCC to D024 to Cr13Fe35Ni3Ti7, and finallythe D024 phase turns to the C14 phase. Annealed at 800 °C for 30 days haslimited effect on its phase composition, while annealing at 1000 °C will greatchange it. The Cr13Fe35Ni3Ti7 phase precipitates in the alloy with 15~20 at.%Ti. With the further increase of Ti content, it transforms into the σ phase.(3) When Ti content reaches 30 at.%, the BCC phase precipitates in the alloyand increases gradually. (4) Co2.5-xCrxFeNiTi0.5 alloy precipitates the σ phasefrom the FCC phase in as-cast and annealed state at 800 °C with the increase ofCr content, and converts FCC phase into σ and D024 phases at annealed state at1000 °C. BCC phase was precipitated after annealing at high Cr content.Eutectic structure exists in Co2-xCrxFeNiTi alloy as-cast and annealed at 800°C, and disappears after annealing at 1000 °C. σ phase precipitated in as-castand annealed at 800 °C, disappeared in annealed at 1000 °C, and Cr13Fe35Ni3Ti7phase appeared. BCC phase precipitated in alloys with high Ti content. Thehigher the annealing temperature is, the easier the intermetallic compounds areformed and the larger the grain size is.

It is found that the hardness ofCo-Cr-Fe-Ni-Ti alloy increases with the increase of Co or Ti content,respectively. With the increase of annealing temperature, the hardness of thealloy generally decreases, and the effect of annealing temperature on thehardness of the alloy varies with the content of alloying elements. When theBCC phase appears in the alloys, the hardness of the alloy will decrease.

In addition, the phase composition and itsvariation of as-cast alloys in the experiment are basically consistent with thepredicted results of the model parameters of mixing entropy and valenceelectron concentration. However, the experimental results are quite differentfrom the thermodynamic calculation results in Factsage database. Therefore, thephase stability of the alloy can not be predicted by the calculation results ofthe existing thermodynamic database.

关键词:Co-Cr-Fe-Ni-Ti;高熵合金;相组成;显微硬度

Co-Cr-Fe-Ni-Ti; High entropy alloys; PhaseComposition; Microhardne

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