高熵合金独特的高熵效应、晶格畸变效应和迟滞扩散效应，使其具有高强度、高硬度、耐磨损以及高温稳定性等优异性能[1~3]。以钛(Ti)和锆(Zr)为主要组元的低密度Ti-Zr基高熵合金，加入V、Nb、Ta等元素产生的晶格畸变使其具有显著的固溶强化效应，从而实现了比强度与室温塑性之间的平衡。具有轻质、优异的力学性能和良好变形性能的Ti-Zr基高熵合金有极大的应用潜力[4~6]，在其中添加增强相制备高熵合金基复合材料可使其力学性能进一步提高[7~13]。欧阳维等[14]采用机械球磨和放电等离子烧结工艺制备TiC/FeCrCoMnNi复合材料，研究了TiC含量和烧结温度对其组织和性能的影响。结果表明，添加适量的TiC和在适当的温度烧结可使增强相均匀分布，生成TiC和Cr23C6双碳化物实现协同强化，使材料的硬度和强度显著提高。W具有高熔点、高强度和较高的化学稳定性，可用作复合材料的增强相[15~17]。Chen等[18]发现，FeNiMnAlW高熵合金体系中均匀分布的W颗粒与FCC基体和有序B2相形成的良好协同作用，使其具有优异的力学性能。对于W颗粒/锆基非晶复合材料[19]，随着W含量的提高其屈服强度虽有所降低但是极限强度和塑性提高，其原因是两相弹性模量的差异产生了剪切带增殖和W颗粒的塑性变形。
采用烧结工艺制备的复合材料中颗粒间的结合不紧密和孔隙率较高，限制了其性能的提高。热等静压工艺(HIP)的高温和高压有利于粉末颗粒间扩散结合和孔隙闭合，可制备出高性能致密材料[20~23]。与真空熔炼和传统无压烧结(PLS)相比，使用惰性气体(如氩气)传压介质的HIP能在颗粒表面施加高达100~200 MPa的均匀静水压力，使材料的密度接近理论值(相对密度通常≥ 99.5%)并能消除孔隙和微裂纹。HIP的工艺参数(包括温度、粉粒度等)对粉末冶金高温合金IN740H的显微组织和性能有显著的影响[24]。同时，与单向加压易产生显微结构各向异性的热压烧结(HP)相比，HIP尤其适用于制备均匀致密的复杂形状构件 [25]。本文采用HIP工艺制备W/TiZrVAl高熵合金复合材料，调控W与TiZrVAl高熵合金的摩尔比，系统研究W含量对其微观组织和力学性能的影响和强塑性的增强机制。
1 实验方法
1.1 W/TiZrVAl高熵合金复合材料的制备
W/TiZrVAl高熵合金复合材料的增强相是纯度高于99.99%形状不规则的钨(W)粉，颗粒尺寸为150~250 μm，基体是Ti7Zr1.5VAl0.5高熵合金(记为HEA)粉。这种高熵合金粉是用等离子旋转电极法(Plasma rotating electrode process, PREP)制备的，用230目筛网筛分得到粒径小于65 μm的粉末。
将摩尔比分别为1∶1、1.4∶1、1.6∶1和2∶1(分别记为WHC1、WHC2、WHC3和WHC4)的W和HEA以及球料比(质量比)为5∶1的钢球放入用氩气保护的三维混料机的混料罐中，混料5 h使W颗粒和HEA粉混合均匀。
将混好的复合粉装入不锈钢包套中并进行真空处理以降低粉中O、N及水蒸气等杂质，然后将其在温度为920 ℃、压力为120 MPa的条件下HIP烧结2 h。为了比较，在相同的HIP工艺条件下用纯HEA粉制备块体试样。烧结完成后去除不锈钢包套，用电火花线切割、快速切割锯制备出用于组织结构分析和性能表征的试样。
1.2 结构和性能表征
用MiniFlex600-C型X射线衍射仪测定复合材料的XRD谱，辐射光源为Cu Kα，电压为40 kV，电流为15 mA，扫描速率为10 (°)/min，扫描范围为20°~120°。用ThermoFisher Apreo2C场发射扫描电子显微镜(SEM)及其配备的Oxford Explore 30型能谱仪(EDS)观察试样的微观组织、断口形貌和元素分布。
在WANCE ETM205D微机控制的电子万能试验机上进行室温准静态压缩，试样的直径为4 mm 长度为8 mm，应变速率为5 × 10-4 /s。用Archimedes排水法测量试样的密度(
式中
各项测试均重复3次取其结果的平均值。
2 实验结果
2.1 HEA粉和W颗粒的形貌
图1
图2
图2
不同比例的W颗粒与HEA粉混料5 h后的SEM照片
Fig.2
SEM images of W particle and HEA powder with different ratio after 5 h blending (a) WHC1, (b) WHC2, (c) WHC3, (d) WHC4
2.2 W/TiZrVAl高熵合金复合材料的微观结构
图3给出了W/TiZrVAl高熵合金复合材料的SEM照片和面扫描图，图中的灰色区域为HEA基体，亮白色区域为W增强相。可以观察到，W颗粒在基体中发生了一定程度的团聚。W含量较低时团聚不显著；随着W颗粒含量的提高，团聚显著且分布均匀性降低。图4给出了WHC1复合材料中增强相与基体界面的二次电子像及其对应线扫描结果。可以看出，W颗粒与基体之间界面结合良好。在界面区域发生了W、Zr和Ti等元素的互扩散，可能与Zr-W以及Ti-W之间负的混合焓有关[26,27]。HIP工艺的高温高压促进了W与Zr、Ti元素的互扩散，有利于形成稳定的界面结合。同时，在界面区域未生成脆性化合物。良好的界面结合有利于提高W颗粒与基体之间的结合强度和载荷的传递，使材料在变形过程中保持较高的强度。
图3
图3
高熵合金复合材料的SEM照片和EDS面扫描图
Fig.3
SEM images and corresponding EDS mapping of high-entropy alloy based composites (a) WHC1, (b) WHC2, (c) WHC3, (d) WHC4
图4
图4
WHC1复合材料的SEM照片和EDS线扫描图
Fig.4
SEM image (a) and EDS line scan (b) of WHC1 composite
图5给出了高熵合金复合材料的XRD谱，可见其均具有体心立方(BCC)结构。与HEA基体相比，复合材料由BCC结构的W相和BCC结构的高熵合金相组成。
图5
2.3 W/TiZrVAl高熵合金复合材料的密度
表1列出了复合材料的
表1 W/HEA复合材料的理论密度和实际密度
Table 1
| Composites | |||
|---|---|---|---|
| WHC1 | 11.61 | 11.57 | 0.99 |
| WHC2 | 12.83 | 12.68 | 0.99 |
| WHC3 | 13.30 | 12.72 | 0.96 |
| WHC4 | 14.07 | 13.08 | 0.93 |
2.4 W/TiZrVAl高熵合金复合材料的压缩性能和断口形貌
图6给出了高熵合金复合材料的室温压缩工程应力-应变曲线。可以看出，所有试样的屈服强度都比较高，但是屈服强度和塑性都随着W含量的提高逐渐降低(表2)。根据Chawla所述的复合材料混合定律[28]，理论屈服强度应该随着W含量的提高而提高，但是实验结果却表明呈降低的趋势。除了摩尔比为2∶1的试样，其余试样的实际屈服强度均高于根据混合定律计算出的理论值。WHC3复合材料的塑性比HEA基体的稍低。W含量为2∶1的试样，其塑性显著降低(约7%)。W∶HEA的比例从1∶1提高到2∶1，复合材料的屈服强度由1146 MPa降低到892 MPa，断裂应变由36%降低到16%。力学性能降低的原因是，W含量的提高使材料的微观结构发生变化。对于W含量较低的试样，作为强化相的W颗粒通过载荷传递与位错的相互作用使基体的强度提高[29,30]。但是，随着W含量的进一步提高，材料中的孔隙随之增多。孔隙的聚集和连通成为应力集中源，也使材料的有效承载面积减小。外加载荷使孔隙结构失稳扩展并相互连接，从而使材料在较低的载荷下过早断裂，表现为屈服强度和塑性的降低。
图6
图6
W/HEA复合材料的室温压缩工程应力-应变曲线
Fig.6
Room-temperature compressive stress-strain curves of the W/HEA based composites
表 2 W/HEA复合材料的室温压缩力学性能
Table 2
| Composites | Theoretical yield stress / MPa | Yield stress/ MPa | Fracture strain / % |
|---|---|---|---|
| WHC1 | 855.8 | 1146 | 36 |
| WHC2 | 878.6 | 965 | 31 |
| WHC3 | 886.8 | 955 | 22 |
| WHC4 | 901.6 | 892 | 16 |
| HEA | - | 940 | 23 |
图7
图7
HEA和WHC1复合材料在室温压缩断裂后的宏观和微观形貌
Fig.7
Macro- and micro-morphologies of fracture of HEA (a, c) and WHC1 composites (b, d) after compression at room temperature
3 讨论
3.1 W含量对复合材料致密度的影响
3.2 W颗粒团聚的形成及其对性能的影响
图8
3.3 孔隙和界面脱粘对强塑性的影响
4 结论
(1) 采用热等静压(HIP)可制备W颗粒增强Ti7Zr1.5VAl0.5高熵合金基复合材料，W颗粒与基体之间的结合界面良好。
(2) W与HEA含量比为1∶1的W/TiZrVAl高熵合金基复合材料综合力学性能最优。随着W含量的提高，材料的断裂机制由以韧性断裂为主逐渐向以脆性断裂为主转变。
(3) W含量的变化通过双重竞争机制影响这种复合材料的强韧性匹配。在W含量较低的材料中，W颗粒通过载荷传递和位错相互作用发挥强化作用，有利于使其强度提高。较高的W含量使材料中孔隙的数量增多和致密度降低，孔隙聚集形成的微裂纹网络导致强度和塑性降低。
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