Probing the Size and Rate-dependent Deformation Mechanisms in fcc Metals: Pure Elements to Multicomponent Alloys

Y. Xiao

(2019) 146-146

Over the last century, alloyed metals have been the primary structural material enabling modern industries, such as automobile, aerospace, marine, oil, and nuclear. In response to accelerating demands for component safety, efficiency, and resistance to harsh environments, new developments in alloy design continue to advance the state of the art. This has conventionally been achieved by additions of a small amount of alloying elements to a primary component element or varying fractions in binary alloys. Recently, a different paradigm has been introduced, which involves the combination of multiple principal elements in high concentrations. These are usually called multicomponent or high-entropy alloys, where the compositional complexity is dramatically increased to achieve properties superlative to conventional alloys. Simultaneously, recent advances in mechanical characterization and testing methods with resolution in the nano-/microscale have allowed understanding of deformation behavior at intrinsic materials’ length scales, which enable multi-scale behavior of alloys to be determined. Hence, a good understanding of the deformation mechanisms at small scale of the novel complex alloys compared with conventional alloys is critical to the overall structure-property relations from the point of view for better alloy designs. Of these small-scale mechanical techniques, microcompression of pillars fabricated using focused ion beams (FIB) is perhaps the most prominent. FIB-machining is currently the most popular technique for producing micro-scale test geometries for investigating the properties of materials at small length scales. However, there are concerns associated with these ion- prepared samples, including irradiation damage from FIB ions and Ga segregation at grain boundaries resulting in liquid metal embrittlement. Therefore, the influence of ion species on the deformation behavior of aluminum micropillars produced by Ga and Xe FIBs using in situ strain rate jump (SRJ) tests was investigated in this work. The different ion species have little effect on the yield strength of single crystalline aluminum pillars. However, in polycrystalline aluminum, FIB-machining using Ga ions appears to reduce the mechanical strength proportionally to the Ga implantation dose and the grain boundary concentration. However, the measured apparent activation volumes remain largely consistent among all testing methods, suggesting that the dominant deformation mechanism remains unchanged despite the presenceof Ga at the boundaries. In broad terms, these results indicate that the influence of Ga ions from FIB-machining on deformation mechanisms is rather small. With this established, the size-dependent deformation mechanisms of a family of metallic materials, including pure element (Ni), equiatomic binary (Co-Ni), ternary (Cr-Co-Ni), quaternary (Cr-Fe-Co-Ni) and quinary (Mn-Cr-Fe-Co-Ni) metals was investigated using in situ SRJ microcompression tests and advanced electron microscopy. Two questions have been answered throughout this part: What is the influence of the combined effects of external size and intrinsic microstructural characteristics (e.g. compisitional complexity) on the size- dependent strength? And is there a difference in deformation mechanisms between complex multicomponent and conventional alloys? Here, I show that the size-dependent strength of metallic material does not simply depend on the complexity of structure/composition. Furthermore, the mechanical strength and size effect exponent is strongly related to the intrinsic Peierls’ stress. The activation volumes are found to scale exponentially with pillar diameter, which agrees well with the phenomenological model indicating the single arm sources strongly contribute to the size dependence in the pillars. Moreover, the high compositional complexity in the complex multicomponent alloys does not introduce any new intrinsic deformation mechanisms compared to conventional pure fcc element and binary alloy. This thesis, using the in situ micropillar compression techn que, provides a fundamental understanding of the deformation behavior in conventional pure elements, binary alloys, and complex multicomponent alloys. The results of this thesis will provide useful input for alloy design in novel metallic materials.