Kink Band Structure in BCC HEAs and In-situ Observation of Its Evolution During Plastic Deformation

Recently, body-centered cubic (bcc) refractory multicomponent alloys (MEA) composed of Group IV, V, and VI elements have demonstrated significant potential as structural materials under extreme temperature environments [1-4] because of their high melting points, and ability to retain excellent compression strength under high temperatures [3]. However, their practical applications remain limited due to their high ductile to brittle transition temperatures, low tensile ductility and damage tolerance ability at room temperature [2, 5]. Kink bands, a deformation mechanism that reorients crystals into favorable slip directions, has emerged as a promising approach to enhance plasticity and subsequently damage tolerance in these alloys [6]. Specifically, this mechanism has been attributed to the remarkably high toughness of a recently discovered Nb45Ta25Ti15Hf15 (NTTH) bcc HEA. Experimental observation indicates that these bands are formed through activation of multiple slip systems (Fig. 1). To investigate the formation and evolution of kink bands in bcc multicomponent alloys, we performed multiscale characterization using in-situ EBSD as well as in-situ and high-resolution TEM analysis of kink band structures and evolution with increasing strain, focusing particularly on the exceptionally tough NTTH alloy. Our experimental procedure is described in Fig. 2, where we use in situ EBSD tensile testing coordinated in-situ TEM deformation from selected regions of the kink bands. The in-situ EBSD test interrupted at 1-2% plastic strain, from which a FIB lift-out at a newly formed kink band boundary is then used to conduct an in situ tensile test in TEM with the same loading direction as in the in-situ EBSD tensile test. Through this method, we can compare the in-situ EBSD results in micro scale with the in-situ TEM results in nanoscale side by side and describe the complete mechanism of how kink bands grow and how they interact with dislocations [7]. (a) Virtual bright field image of a 4D-STEM scan over a kink band boundary on an FIB lift-out sample. (b) and (c) are in-plane and out-of-plane orientation maps of the region shown in (a), respectively. Sample is on [110] zone axis. The kink band rotates crystal by 7.6 degrees with respect to [110] axis, which kink band descriptors would indicate they are formed by the higher order <111>{112} slip system. Scale bars shown in figures are 50 nm. (a) Schematic of our experimental procedure. First, an in-situ tensile test is conducted inside an SEM and the EBSD scan at several plastic strain states is recorded. Then, the test is interrupted at 1-2% plastic strain and a FIB lift-out sample is made from a newly formed kink band boundary. The FIB lift-out sample is then used to conduct an in-situ tensile tests inside a TEM to observe the evolution of plasticity within the kink boundary while the in situ EBSD tensile test continues until the sample necks. (b) and (c) are IPF-X figures from an in situ EBSD test at 0% and 0.5% plastic strain, respectively. X is in the horizontal direction, which is also the loading direction. A kink band has been clearly observed at 0.5% plastic strain. (d) Misorientation angle change of the kink band shown in (c) during the entire in-situ EBSD tensile test. The misorientation angle linearly increases as the bulk plastic strain increases.