Divergent evolution of slip banding in CrCoNi alloys

Abstract

Metallic materials under high stress often exhibit deformation localization, manifesting as slip banding. Over seven decades ago, Frank and Read introduced the well-known model of dislocation multiplication at a source, explaining slip band formation. Here, we reveal two distinct types of slip bands (confined and extended) in compressed CrCoNi alloys through multi-scale testing and modeling from microscopic to atomic scales. The confined slip band, characterized by a thin glide zone, arises from the conventional process of repetitive full dislocation emissions at Frank-Read source. Contrary to the classical model, the extended band stems from slip-induced deactivation of dislocation sources, followed by consequent generation of new sources on adjacent planes, leading to rapid band thickening. Our findings provide insights into atomic-scale collective dislocation motion and microscopic deformation instability in advanced structural materials.

Fig. 1. Confined slip band (C-SB) and its microscopic and atomic structures.

a Schmid factors for the slip systems of partial dislocations gliding on the ABC and ABD planes, including their changes with compression-induced rotation. Dashed and solid lines denote trailing and leading partial dislocations, respectively. b SEM image of [110]-oriented micropillar after plastic compression, exhibiting a stepped surface and deformation strips. c BF-STEM images of compressed micropillar reveal sharp surface steps, signifying highly concentrated deformation within the confined slip bands. d Magnified TEM images elucidate the structural features of C-SB from submicron to atomic scales. The slip band, aligned along the primary slip plane (ABC), retains a nearly pristine structure with sparse deformation defects. A dense array of stacking faults is evident along the secondary slip plane (ABD).

Fig. 2. Extended slip band (E-SB) and the microscopic and atomic structures.

a Schmid factors for partial dislocations gliding on the four non-parallel planes and their variations with compression-induced rotation. Solid and dashed lines denote leading and trailing partial dislocations, respectively. b SEM image of [100]-oriented micropillar after plastic compression, showing widened deformation strips. c BF-STEM images exhibit ribbon-like extended slip bands along the ABD plane. The red color highlights these E-SBs. d Magnified BF-STEM image illustrates three representative regimes of the deformed pillar. Regime i represents the boundary of the band, regime ii for the band interior, and regime iii for the interface between two bands. e–g Atomic resolution HAADF-STEM elucidates the defects in the three regimes. The E-SB comprises a series of nano-twins, interspaced by thin hcp lamellae and stacking faults.

Fig. 3. Local plastic strain progression and microstructural evolution.

a, b Evolution of local plastic strain and structure with increasing strain for pillars forming confined slip band and extended slip band, respectively. The structure is depicted as follows: green color represents the fcc structure, yellow denotes TB, blue indicates SF, and red represents the hcp phase. c Statistical distributions of local strain for pillars compressed at 25% strain. d The corresponding atomic fraction of TB, SF, hcp phase. The category of ‘all hcp atoms’ summarizes all three types of planar defects. c, d The atomic strain and structure type are analyzed for the entire deformed system, which consists of 14 million atoms.

Fig. 4. Dislocation mechanism underpinning confined slip band formation.

a Schematic illustration of dislocation multiplication at an activated Frank-Read source, where glide occurs through a pair of partial dislocations (a full dislocation mechanism), owing to the higher Schmid factor of the trailing partial dislocation compared to the leading one. b, c Atomistic modeling and schematic representation show the C-SB formation. An activated source on the ABC plane emits dislocations repetitively, forming a confined slip band accompanied by sharp surface steps. The continuous slip on the ABC plane displaces the SFs located on ABD planes, resulting in the stepped SFs at the band. In the deforming pillar (c), only defective microstructures are shown.

Fig. 5. Dislocation mechanisms driving extended slip band formation.

a Schematic illustration of Frank-Read source deactivation. Expansion of a leading partial dislocation loop generates stacking fault in its traversed area, eventually leaving a new segment of the leading partial dislocation at the source. The formed SF pinning leading partial, consequently deactivating the source. b Schematics of dynamic dislocation sources creation. The new dislocation source, formed at intersections of stacking fault and twin boundary, moves its location with the twin boundary. c, d Atomistic modeling shows the E-SB formation. The dynamically relocating dislocation sources enable dislocation emission across successive layers, thus thickening the slip band. d Only the defective atoms are shown. e High-resolution HAADF-STEM image shows the concentrated SF-TB junctions and steps along the twin boundary. The dense twinning sources collectively drive TB migration and promote rapid twin growth at the microscale.