The mechanical size effect of nanostructured, dual-phase CrCoNi medium-entropy alloy (MEA) was investigated by combining in-situ micro-compression testing with post-mortem electron microscopy analysis. The alloy possesses a superior yield strength up to ∼4 GPa, primarily due to its hierarchical microstructure including column nanograins, preferred orientation, a high density of planar defects and the presence of the hexagonal close packed (HCP) phase. While the yield strength of the alloy has shown size-independency, the deformation behaviour was strongly dependent on the sample size. Specifically, with decreasing the pillar diameters, the dominant deformation mode changed from highly localized and catastrophic shear banding to apparently homogeneous deformation with appreciable plasticity. This transition is believed to be governed by the size-dependent critical stress required for a shear band traversing the pillar and mediated by the competition between shear-induced softening and subsequent hardening mechanisms. In addition, an unexpected phase transformation from HCP to face-centered cubic (FCC) was observed in the highly localized deformation zones, leading to strain softening that contributed to accommodating plasticity. These findings provide insights into the criticality of sample dimensions in influencing mechanical behaviors of nanostructured metallic materials used for nanoelectromechanical systems.

本文結合原位掃描電子顯微鏡微柱壓縮與透射電子顯微鏡技術, 研究了具有雙相多級納米結構的CrCoNi中熵合金變形行為的尺寸效應. 研究表明, 該合金的屈服強度高達~4 GPa, 這主要歸因於其多級微觀結構特徵, 包括柱狀納米尺寸晶粒、織構、高密度的層錯、孿晶界、晶界和相界. 在變形過程中, 該合金的屈服強度基本與微米尺度樣品的尺寸無關, 但其變形行為卻強烈依賴於樣品大小. 具體來說, 隨著微柱直徑減小, 材料主要的變形模式從突發的局部剪切帶變為具有明顯塑性的均勻變形. 這種轉變是由剪切帶穿過微柱所需的臨界應力與樣品尺寸緊密相關所決定的, 剪切誘導的軟化和隨後的硬化機制之間的競爭也起了重要作用. 此外, 變形引起了六方密排結構到面心立方結構的相變, 該相變導致的應變軟化對材料變形中的塑性有重要貢獻. 這些發現揭示了樣品尺寸對可用於微納機電系統的納米結構金屬材料的力學行為有著重要影響.