Some of ultrafine-grained (UFG) metals including UFG twinning induced plasticity (TWIP) steels have been found to overcome the paradox of strength and ductility in metals benefiting from their unique deformation modes. Here, this study provides insights into the atomistic process of deformation twin nucleation at Σ3{111} twin boundaries, the dominant type of grain boundary in this UFG high manganese TWIP steel. In response to the applied tensile stresses, grain boundary sliding takes place which changes the structure of coherent Σ3{111} twin boundary from atomistically smooth to partly defective. High resolution transmission electron microscopy demonstrates that the formation of disconnection on Σ3{111} twin boundaries is associated with the motion of Shockley partial dislocations on the boundaries. The twin boundary disconnections act as preferential nucleation sites for deformation twin that is a characteristic difference from the coarse-grained counterpart, and is likely correlated with the lethargy of grain interior dislocation activities, frequently seen in UFG metals. The deformation twin nucleation behavior will be discussed based on in-situ TEM deformation experiments and nanoscale strain distribution analyses results.
Atomic-scale observation of nucleation- and growth-controlled deformation twinning in body-centered cubic nanocrystals
Twinning is an essential mode of plastic deformation for achieving superior strength and ductility in metallic nanostructures. It has been generally believed that twinning-induced plasticity in body-centered cubic (BCC) metals is controlled by twin nucleation, but facilitated by rapid twin growth once the nucleation energy barrier is overcome. By performing in situ atomic-scale transmission electron microscopy straining experiments and atomistic simulations, we find that deformation twinning in BCC Ta nanocrystals larger than 15 nm in diameter proceeds by reluctant twin growth, resulting from slow advancement of twinning partials along the boundaries of finite-sized twin structures. In contrast, reluctant twin growth can be obviated by reducing the nanocrystal diameter to below 15 nm. As a result, the nucleated twin structure penetrates quickly through the cross section of nanocrystals, enabling fast twin growth via facile migration of twin boundaries leading to large uniform plastic deformation. The present work reveals a size-dependent transition in the nucleation- and growth-controlled twinning mechanism in BCC metals, and provides insights for exploiting twinning-induced plasticity and breaking strength-ductility limits in nanostructured BCC metals.
more » « less- NSF-PAR ID:
- 10486156
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature Communications
- Volume:
- 15
- Issue:
- 1
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The abrupt occurrence of twinning when Mg is deformed leads to a highly anisotropic response, making it too unreliable for structural use and too unpredictable for observation. Here, we describe an in-situ transmission electron microscopy experiment on Mg crystals with strategically designed geometries for visualization of a long-proposed but unverified twinning mechanism. Combining with atomistic simulations and topological analysis, we conclude that twin nucleation occurs through a pure-shuffle mechanism that requires prismatic-basal transformations. Also, we verified a crystal geometry dependent twin growth mechanism, that is the early-stage growth associated with instability of plasticity flow, which can be dominated either by slower movement of prismatic-basal boundary steps, or by faster glide-shuffle along the twinning plane. The fundamental understanding of twinning provides a pathway to understand deformation from a scientific standpoint and the microstructure design principles to engineer metals with enhanced behavior from a technological standpoint.more » « less
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Deformation twinning is a prevalent plastic deformation mode in hexagonal close-packed (HCP) materials, such as magnesium, titanium, and zirconium, and their alloys. Experimental observations indicate that these twins occur heterogeneously across the polycrystalline microstructure during deformation. Morphological and crystallographic distribution of twins in a deformed microstructure, or the so-called twinning microstructure, significantly controls material deformation behavior, ductility, formability, and failure response. Understanding the development of the twinning microstructure at the grain scale can benefit design efforts to optimize microstructures of HCP materials for specific high-performance structural applications. This article reviews recent research efforts that aim to relate the polycrystalline microstructure with the development of its twinning microstructure through knowledge of local stress fields, specifically local stresses produced by twins and at twin/grain–boundary intersections on the formation and thickening of twins, twin transmission across grain boundaries, twin–twin junction formation, and secondary twinning.more » « less
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null (Ed.)The deformation behavior of the extruded magnesium alloys Mg2Nd and Mg2Yb was investigated at room temperature. By using in situ energy-dispersive synchrotron X-ray diffraction compression and tensile tests, accompanied by Elasto-Plastic Self-Consistent (EPSC) modeling, the differences in the active deformation systems were analyzed. Both alloying elements change and weaken the extrusion texture and form precipitates during extrusion and subsequent heat treatments relative to common Mg alloys. By varying the extrusion parameters and subsequent heat treatment, the strengths and ductility can be adjusted over a wide range while still maintaining a strength differential effect (SDE) of close to zero. Remarkably, the compressive and tensile yield strengths are similar and there is no mechanical anisotropy when comparing tensile and compressive deformation, which is desirable for industrial applications. Uncommon for Mg alloys, Mg2Nd shows a low tensile twinning activity during compression tests. We show that heat treatments promote the nucleation and growth of precipitates and increase the yield strengths isotopically up to 200 MPa. The anisotropy of the yield strength is reduced to a minimum and elongations to failure of about 0.2 are still achieved. At lower strengths, elongations to failure of up to 0.41 are reached. In the Mg2Yb alloy, adjusting the extrusion parameters enhances the rare-earth texture and reduces the grain size. Excessive deformation twinning is, however, observed, but despite this the SDE is still minimized.more » « less
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Deformation twinning is a prevalent mode of plastic deformation in hexagonal close packed (HCP) magnesium. Twin domains are associated with significant lattice reorientation and localized shear. The theoretical misorientation angle for the most common 1012 tensile twin in magnesium is 86.3°. Through electron backscatter diffraction characterization of twinning microstructure, we show that the twin boundary misorientation at the twin tips is approximately 85°, and it is close to the theoretical value only along the central part of the twin. The variations in twin/matrix misorientation along the twin boundary control the twin thickening process by affecting the nucleation, glide of twinning partials, and migration of twinning facets. To understand this observation, we employ a 3D crystal plasticity model with explicit twinning. The model successfully captures the experimentally observed misorientation variation, and it reveals that the twin boundary misorientation variations are governed by the local plasticity that accommodates the characteristic twin shear.more » « less
