性能变化的内在原因和规律,对获得针对性的性能改进方案以及探索更优性能的高熵陶瓷体系显得十分重要。
图 2 (TiZrHfNbTa)C 陶瓷的明场像(a, d, g),相应晶粒的 SAD 花样(b, e, h)和高分辨图像(c, f, i)
通过几何相位分析法(GPA)可以获得结构应变场大小等信息,准确度可达 0.003 nm[39]。通过对[12̅ 13̅ ]晶带轴获得的(TiZrHfNbTa)B2 高分辨图像(图 3a)和[011]晶带轴获得的(TiZrHfNbTa)C 高分辨图像(图 3e)进行 GPA 分析分别得到结构应变场图像, 如图 3(b-d)和图 3(f-h)所示, 从中可以看到整个区域被红色(正应变)和绿色(负应变)覆盖,对比应变大小的标尺,说明(TiZrHfNbTa)B2 和(TiZrHfNbTa)C 都存在纳米尺度的结构畸变。图 3(b-d)中的正应变和负应变交叉点部分集中在右侧区域,而图 3(f-h)中的应变交叉点分布相对均匀一些。由此可见,多元过渡金属固溶形成的高熵陶瓷存在一定的结构畸变,且会存在应变不均的情况。内应力的大小是否会因为固溶元素的不同而有所区别,是否会影响位错的滑移机制,从而影响材料的力学性能,都是亟待解决的科学问题。
图 3 (TiZrHfNbTa)B2(a)和(TiZrHfNbTa)C(e)的高分辨图像,右上角插图是相应的傅里叶变换的衍射花
2.2 原子尺度的结构信息
由于高熵陶瓷中固溶的各种金属原子的半径存在差异,在形成高熵相时,原子离散度对于不同固溶元素存在差别,因此金属元素原子离散度可以通过 ACTEM 中的 HAADF 原子像进行观察,然后通过 StatSTEM 软件获得原子坐标进行量化处理[40]。高熵硼化物(TiZrHfNbTa)B2 和碳化物(TiZrHfNbTa)C 陶瓷的 HADDF 原子相如图 4(a, d)所示。通过StatSTEM 中 的 高 斯 拟 合 获 得 相 应 原 子 坐 标 , 由 此 计 算 得 出 (TiZrHfNbTa)B2 和(TiZrHfNbTa)C 的原子离散度分别是 d1=0.00101 nm, d2=0.00056 nm 和 d1=0.00109 nm,d2=0.00204 nm。如图 4(b,c,e,f)所示,其中,d1 和 d2 分别为 x 方向和 y 方向的原子间距,将晶格间距由小到大按照由绿色到橘色标注,颜色变化差异大小直观地表现原子离散度的分布情况。由此可见,虽有不同原子半径的过渡金属元素固溶,但离散度较小,说明这几种元
素很好地固溶在晶格当中。原子离散度反应的是金属阳离子的原子结构信息,在一定程度上反映了晶格应力的大小;而高分辨成像包含了阴阳离子整体效应的结构信息,结合原子离散度和 GPA 分析可以很清晰地得到局部区域的原子分布及其结构应变信息。
图 4 (TiZrHfNbTa)B2(a)和 (TiZrHfNbTa)C(d)的 HAADF 图像及其相应的原子离散分布图(b, c)和(e,f)
通过 HAADF 原子像及其 EDS 面分布可以清晰地看到各个过渡元素在局部区域的分布,然而 B 和 C 的元素信号容易被样品吸收,很难通过 EDS 获得确切的分布信息,如图 5 所示。通过过渡元素的 EDS 面分布表征,可以发现在高熵(TiZrHfNbTa)B2 和 (TiZrHfNbTa)C 陶瓷中存在纳米尺度浓度分布不均匀的现象,而且这种浓度波动没有规律性。类似现象在其它高熵陶瓷的研究中也有所发现 [41-42]。这种浓度波动或者类似的原子分布不均匀的现象,将减小按照均匀分布计算的混合熵数值。如果在高熵陶瓷中普遍存在这种现象,从热力学角度来看,则会减弱熵稳定的陶瓷体系的“熵稳定”效果。
图 5 (TiZrHfNbTa)B2(a)和 (TiZrHfNbTa)C(b)的原子级 HADDF 图像及其 EDS 面分布
3 结论
研究制备了高熵(TiZrHfNbTa)B2 和 (TiZrHfNbTa)C 陶瓷,并利用 TEM 和 ACTEM 对其进行了纳米和原子尺度的结构表征。研究结果发现,高熵陶瓷保持了单一晶体结构的完整性和微观尺度的固溶元素分布均匀性,没有结构缺陷或固溶元素的长程有序分布。但多元过渡元素的固溶会造成一定大小的晶格应力和原子离散,并在晶格中的分布存在纳米尺度的浓度不均匀。本工作获得了高熵硼化物和高熵碳化物的固溶结构和微结构特征,据此可以开展相关理论计算以明确固溶结构稳定性的热力学机制,并通过相关物理性能的表征(包括硬度、强度、韧性和抗辐照损伤行为),探讨高熵结构和性能之间的内在关系。
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