Gradient twinned 304 stainless steels for high strength and high ductility

Abstract

Gradient materials often have attractive mechanical properties that outperform uniform microstructure counterparts. It remains a difficult task to investigate and compare the performance of various gradient microstructures due to the difficulty of fabrication, the wide range of length scales involved, and their respective volume percentage variations. We have investigated four types of gradient microstructures in 304 stainless steels that utilize submicrotwins, nanotwins, nanocrystals, ultrafine-and coarse-grains as building blocks. Tensile tests reveal that the gradient microstructure consisting of submicrotwins and nanotwins has a persistent and stable work hardening rate and yields an impressive combination of high strength and high ductility, leading to a toughness that is nearly 50% higher than the coarse-grained counterpart. Ex-and in-situ transmission electron microscopy indicates that nanoscale and submicroscale twins help to suppress and limit martensitic phase transformation via the confinement of martensite within the twin lamellar. Twinning and detwinning remain active during tensile deformation and contribute to the work hardening behavior. We discuss the advantageous properties of using submicrotwins as the main load carrier and nanotwins as the strengthening layers over those coarse and nanocrystalline grains. Our work uncovers a new gradient design strategy to help metals and alloys achieve high strength and high ductility.

Heterogeneous materials: a new class of materials with unprecedented mechanical properties

“…With the above two criteria for high back-stress, we can make comments on the effectiveness of various heterogeneous structures. For the gradient structure [27][28][29][30][31][32], there will be two dynamically migrating interfaces during the tensile tests [28,29], which allows dislocation density accumulation over the whole sample volume. However, the low interface density also limits its capability of back-stress work hardening.…”

“…Another major obstacle to practical structural applications of nanostructured metals is the challenge in scaling up for industrial production at low cost [26]. ferent microstructures, including the gradient structure [27][28][29][30][31][32], heterogeneous lamella structure [33], bimodal structure [10,25,34,35], harmonic structure [36][37][38], laminate structure [39,40], dual-phase steel [41][42][43], nanodomained structure [44], nanotwinned grains [45,46], etc. [47].…”

Perspective on hetero-deformation induced (HDI) hardening and back stress

“…HS materials have very diverse microstructures [1], including heterogeneous lamella structure [2], gradient structure [3][4][5][6][7][8], laminate structure [9,10], dual phase CONTACT Yuntian Zhu ytzhu@ncsu.edu; Xiaolei Wu xlwu@imech.ac.cn structure [11][12][13], harmonic structure [14][15][16], bi-modal structure [17][18][19][20], metal matrix composites [21][22][23][24][25][26], etc. These apparently very diverse structures have a common feature: all of them consist of both soft domains and hard domains with dramatically different flow stresses (or strength) [1].…”

Nanotwinned and hierarchical nanotwinned metals: a review of experimental, computational and theoretical efforts

“…13 Although nanocrystallization failed to surmount the common sense that strength and ductility are mutually exclusive, many other efforts have been made to explore strategies for simultaneously improving the strength and ductility of NC metals by designing the tunable nanostructures of materials in recent years. 14 Several approaches have been proposed, such as bimodal or multimodal grain-size distribution, [15][16][17][18] second-phase particle hardening, [19][20][21] dual-phase nanostructuring, 22 fabricating high dislocation density in deformation-induced martensitic transformation system, 23 introducing NT structures into nanosized grains 14,24,25 and the construction of gradient/hierarchical structures via surface mechanical grinding treatment (SMGT), 26,27 surface mechanical attrition treatment (SMAT), 28,29 pre-torsion 30 and uniaxial tension. 31 Inspired by the fact that strain localization can be suppressed in NC metal films adhesion with a ductile substrate under tension, 32,33 the delocalized effect induced fracture toughening is realized in multilayered steel sheets.…”