Room temperature deformation in the Fe7Mo6 μ-Phase

Abstract

The role of topologically close packed (TCP) phases in deformation of superalloys and steels is still not fully resolved. In particular, the intrinsic deformation mechanisms of these phases are largely unknown including the active slip systems in most of these complex crystal structures. Here, we present a first detailed investigation of the mechanical properties of the Fe7Mo6 µ-phase at room temperature using microcompression and nanoindentation with statistical EBSD-assisted slip trace analysis and TEM imaging. Slip occurs predominantly on the basal and prismatic planes, resulting also in decohesion on prismatic planes with high defect density. The correlation of the deformation structures and measured hardness reveals pronounced hardening where interaction of slip planes occurs and prevalent deformation at pre-existing defects.

Finding and Characterising Active Slip Systems: A Short Review and Tutorial with Automation Tools

“…This first angle—that made with the surface—is subsequently discarded in the interest of speed in the following analysis, due to its measurement requiring a three-dimensional analysis technique such as atomic force microscopy (AFM) [ 33 , 34 ]. An increase in non-unique solutions results (by 72% for example in the µ-phase [ 16 ]), although the exact value will depend on crystal symmetry, as different potential slip planes may intersect the surface at the same angle. However, this type of analysis comes with the benefit that the indents can be rapidly analysed using SEM imaging and interpreted based on the local crystal orientation.…”

“…Nanoindentation loads or depths should thus be chosen to be “just right”; namely, producing enough deformation such that clear slip lines are obtained without producing overlapping deformation bands on the surface, or significant deformation underneath the indent to preclude analysis by TEM. As an example, for the µ-phase in the Fe-Mo system [ 16 ], with an average hardness of 11.7 GPa, this corresponded to indents made to 100 mN and 200 mN, with an indentation depth in the range of 800–1000 nm. In most cases, even nanoindentation systems limited to smaller loads, e.g., 50 mN, can be used successfully to introduce deformation suitable for slip line analysis.…”

“…Indeed, this kind of analysis is also performed on metallic crystals (e.g., [ 37 , 38 , 39 , 40 ]). The analysis of the surface traces is well-described in the literature, e.g., [ 16 ], but is repeated in brief and illustrated here so as to simplify matters for the reader ( Figure 3 ). The goal is to determine whether an observed surface line could belong to the intersection of a slip plane in the indented crystal with the surface of the sample.…”

“…Here, we will focus on single crystalline BCC [ 8 ] and intermetallic crystals [ 15 , 16 ] as examples, but the methods, analyses and code can be applied to any material that forms sufficiently clear traces of plasticity on its surface and in which these traces are governed by the crystal structure.…”

“…With respect to the intermetallics taken as an example here [ 15 , 16 ], there are not only a great number that remain to be studied, but many different applications in which their mechanical properties need to be known beyond a simple stiffness or hardness. They are used as phases for alloy reinforcement [ 1 ], they form unintentionally in modern materials with high alloying contents, such as the superalloys [ 17 , 18 ], and for the optimistic researcher they may still be hiding a new class of bulk alloys amongst their numbers that combines safety as a structural material with great resistance to harsh environmental conditions.…”