Ultrafine-grained and Heterostructured Materials (UFGH XII): On-Demand Oral Presentations
Sponsored by: TMS: Shaping and Forming Committee
Program Organizers: Penghui Cao, University of California, Irvine; Xiaoxu Huang, Chongqing University; Enrique Lavernia, University of California, Irvine; Xiaozhou Liao, University of Sydney; Lee Semiatin, MRL Materials Resources LLC; Nobuhiro Tsuji, Kyoto University; Caizhi Zhou, University of South Carolina; Yuntian Zhu, City University of Hong Kong

Monday 8:00 AM
March 14, 2022
Room: Nanostructured Materials
Location: On-Demand Room


2022 Institute of Metals Lecture/Robert Franklin Mehl Award: Schwarz Crystal Structures in Extremely Fine-grained Metals: Ke Lu1; 1Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences
    Metals usually exist in form of polycrystalline solids, in which the networks of disordered grain boundaries tend to get eliminated through grain coarsening upon heating or straining, or to transform into metastable amorphous states when the grains are small enough. This is why nano-grained metals have a much reduced stability relative to their coarse-grained counterparts. Through experiments and MD simulations, we recently discovered a new metastable state for extremely fine-grained metals (typically below 10 nm), namely Schwarz crystal structure with 3D minimal interfaces constrained by twin boundaries. The polycrystalline structure is stable against grain coarsening even close to the equilibrium melting point and exhibits a hardness in vicinity of the theoretical value. In this presentation, I will introduce the formation process, structure characteristics, and some properties of the Schwarz crystal structures in a number of pure metals and alloys.

Micro-macro Synergy Effects in Harmonic Structure Steels Design: Kei Ameyama1; 1Ritsumeikan University
    Harmonic Structure (HS) design was applied to low carbon, medium carbon and stainless steels. The HS is a heterogeneous microstructure with a spatial distribution of ultra-fine grains (UFG) and coarse grains (CG), that is, the CG areas (‘Core’) embedded in the matrix of three-dimensionally continuously connected network of UFG areas (‘Shell’). The HS materials demonstrate unique deformation behaviors which were attributed to the micro-macro synergy effects. A low carbon HS steel demonstrated an avalanche discontinuous yielding, and an SUS316L HS stainless steel showed preferential recrystallization in the Shell by thermo-mechanical treatments. These unique behaviors are because of the stress concentration to the UFG-Shell network structure, which also accelerates the strain hardening of HS materials to increase the strength as well as ductility. Additionally, a medium carbon steel was found that carbon content heterogeneity during HS fabrication process resulted in an unusual microstructure with a superior strength / tensile toughness balance.

Unravelling the Strengthening Effects of Microstructural Heterogeneities: Ting Zhu1; Yin Zhang1; 1Georgia Institute of Technology
    Recent studies of gradient nanotwinned copper, 3D-printed multi-component alloys, metallic nanocomposites, and several other heterogeneous materials raise a fundamental question on the role of microstructural heterogeneities, both topographic and chemical, in controlling the mechanical behavior of these novel material systems. Here we combine the mechanics modeling and experimental characterization to unravel the extra-strengthening effects of microstructural gradients and resultant plastic strain gradients and back stresses, as opposed to their homogeneous counterparts. Quantitative comparison is made between modeling and experimental results, highlighting the less appreciated notions of different types of back-stress hardening or different stages of extra-hardening that can operate in the same material system with complex microstructural heterogeneities. New insights are obtained for promoting the strength-ductility synergy in the design of high-performance heterogeneous materials.

Ultra-strong Low Carbon Nano-steel Produced by Heterostructure and Interstitial Mediated Warm Rolling: Hao Zhou1; 1Nanjing University of Science and Technology
    Ultra-strong materials can significantly help with improving the energy efficiency of transportation vehicles by reducing their weight. Grain refinement by severe plastic deformation is so far the most effective approach to produce bulk strong nanostructured metals, but its scaling-up for industrial production has been a challenge. Here we report an ultra-strong (2.15 GPa) low-carbon nano-steel processed by heterostructure and interstitial mediated warm rolling. The nano-steel consists of thin (~17.8 nm) lamellae, which was enabled by two novel mechanisms: i) improving deformation compatibility of dual-phase heterostructure by adjusting warm-rolling temperature, and ii) segregating carbon atoms to lamellar boundaries to stabilize the nano-lamellae. Defying our intuition, warm-rolling produced finer lamellae than cold rolling, which demonstrates the potential and importance of tuning deformation compatibility of interstitial-containing heterostructure for nanocrystallization. This novel approach is applicable to most low-carbon, low alloy steels for producing ultra-high strength materials in industrial scale.

Copper and Brass Laminate and Lamella Materials: Probing the Fundamental Deformation Mechanisms of Heterostructured Materials: Xiaolong Ma1; Xiaotian Fang2; Chongxiang Huang3; Yuntian Zhu4; 1Pacific Northwest National Laboratory; 2Ames Laboratory; 3Sichuan University; 4City University of Hong kong
    Heterostructured materials are an emerging class of materials with unprecedented mechanical properties. In this talk, heterostructured copper and brass laminate and lamella materials are picked as model systems to probe the fundamental deformation mechanisms of heterostructured materials. A particular emphasis is placed on the role of heterostructured interfaces during deformation and how their parameters affect the dislocation accumulation, hardening behavior and, eventually, the strength-ductility. Results show such interfaces strongly influence the geometrically necessary dislocation (GND) evolution and give rise to the so-called hetero-deformation-induced (HDI) hardening. Important implications for optimization of heterostructure design to achieve superior mechanical properties will be discussed.

The Impact of Hydrogen on the Deformation Behavior of Nanostructured Iron and Nickel: Marlene Kapp1; Jürgen Eckert1; Reinhard Pippan1; Oliver Renk1; 1Erich-Schmid-Institute of Materials Science
    Hydrogen is becoming one of the corner stones for the energy revolution. However, it is known for over a century, that hydrogen tends to embrittle even ductile metals such as iron or nickel. Because of its enormous diffusivity even at ambient temperatures, making a direct detection or interaction with defects difficult, the reasons for the embrittlement and altered deformation behavior remain controversially discussed. Even less is known for the highest strength levels. To this end we study here the effect of hydrogen on the deformation behavior of nanostructured metals. Nickel and iron with grain sizes between 100 and 200 nm prepared by high-pressure torsion serve as model materials. Altering the charging parameters, different hydrogen levels up to 0.2 at.% can be introduced. The effects on the deformation behavior at different temperatures and strain rates, being the center of discussion, are studied at small and large plastic strains after hydrogen charging.

Defect and Substructure Evolution in Nanocrystalline Cu/Ni Composites under Continuous Shear Deformation and Thermal Annealing: Nanjun Chen1; Shenyang Hu1; Arun Devaraj1; Wahyu Setyawan1; Peter Sushko1; Suveen Mathaudhu2; 1Pacific Northwest National Laboratory; 2University of California, Riverside
    Modeling material response under continuous shear deformation is of broad interest for understanding significant changes in microstructure and texture and achieving extraordinary material properties via solid phase process. In this work, large scale molecular dynamics (MD) simulations are used to explore defect and substructure evolution in nanocrystalline Cu/Ni composites under continuous shear deformation. To perform continuous shear, we introduce an on-the-fly periodic boundary condition adjustment to accumulate shear strain to reach any value. The simulations to a large shear strain (~200%) demonstrate the capability of proposed MD simulation scheme. The results reveal mechanisms of deformation and mass transport by atom cluster rotation, dislocation sliding, and nanograin sliding. The evolution of defects also indicates dramatic effects of temperature and loading path on defect accumulation and annihilation. The output of this work will fill the knowledge gap in understanding of deformation mechanisms, defect and substructure evolution under continuous shear at atomic level.

Structural Characterization and Thermal Evolution of Severe Plastic Deformation Processed Materials, by Advanced Synchrotron and Neutron Methods: Klaus-Dieter Liss1; Xiaojing Liu1; Jae-Kyung Han2; Yusuke Onuki3; Malte Blankenburg4; Megumi Kawasaki2; 1Guangdong Technion - Israel Institute of Technology (GTIIT); 2Oregon State University; 3Ibaraki University; 4Deutsches Elektronen Synchrotron (DESY)
    Synchrotron and neutron diffraction bear the advantage of complementary high resolution and bulk penetration to study heterogeneous and anisotropic materials as produced by high-pressure torsion and subjecting to other extreme environments. The investigations reveal integrated and local crystallographic properties, as stress, texture, disorder and phase composition. We present the phase and microstructural evolution of various metals upon heating. In particular, advanced synchrotron high-energy X-ray experiments using a 3˟6 µm2 beam spot size reveal microstructural changes, such as recovery, recrystallization and grain growth. An overview of the methods and selected examples are given on a variety of representative metals and alloys.

Ultrahigh-strength and Ductile High-entropy Alloys with Coherent Nano-lamellar Architectures: Zengbao Jiao1; 1The Hong Kong Polytechnic University
    Nano-lamellar materials with ultrahigh strengths and unusual physical properties are of technological importance for structural applications. However, these materials generally suffer from low tensile ductility, which severely limits their practical utility. In this talk, we show that markedly enhanced tensile ductility can be achieved in coherent nano-lamellar high-entropy alloys, which exhibit an unprecedented combination of over 2 GPa yield strength and 16% uniform tensile ductility. The ultrahigh strength originates mainly from the lamellar boundary strengthening, whereas the large ductility correlates to a progressive work-hardening mechanism regulated by the unique nano-lamellar architecture. The coherent lamellar boundaries facilitate the dislocation transmission, which eliminates the stress concentrations at the boundaries. Meanwhile, deformation-induced hierarchical stacking-fault networks and associated high-density Lomer-Cottrell locks enhance the work hardening response, leading to unusually large tensile ductilities. The coherent nano-lamellar strategy can potentially be applied to other alloys and open new avenues for designing ultrastrong-yet-ductile materials for technological applications.

Achieving Large Super-elasticity through Changing Relative Easiness of Deformation Modes in Ti-Nb-Mo Alloy by Ultra-grain Refinement: Bingjie Zhang1; Mingda Huang1; Chong Yan2; Wenqi Mao3; Wu Gong3; Ruixiao Zheng4; Yu Bai5; Dong Wang1; Qiaoyan Sun1; Yunzhi Wang6; Nobuhiro Tsuji2; 1Xi’an Jiaotong University; 2Kyoto University; 3JAEA J-PARC Center; 4Beihang University; 5Dalian University of Technology; 6Ohio State University
    Large super-elasticity approaching its theoretically expected value was achieved in Ti-13.3Nb-4.6Mo alloy having an ultrafine-grained beta-phase. In-situ synchrotron radiation X-ray diffraction analysis revealed that the dominant yielding mechanism changed from dislocation slip to martensitic transformation by decreasing the beta-grain size down to sub-micrometer. Different grain size dependence of the critical stress to initiate dislocation slip and martensitic transformation, which was reflected by the transition of yielding behavior, was considered to be the main reason for the large super-elasticity in the ultrafine-grained specimen.

Microstructure, Texture and Mechanical Properties of cp-Ti Processed by Rotational Constrained Bending: Milos Janecek1; Tomįš Krajňįk1; Peter Minįrik1; Jozef Veselż1; Petr Cejpek1; Jeno Gubicza2; Dalibor Preisler1; Georgy Raab3; Arseniy Raab3; 1Charles University; 2Eötvös Lorįnd University; 3Ufa State Aviation Technical University
    A novel technique of severe plastic deformation referred to as rotational constrained bending (RCB) was used for the processing of commercially pure titanium. For lower strains imposed by RCB (lower number of passes) a gradient-type microstructure was formed. With increasing number of RCB passes this microstructure heterogeneity was continuously smeared out and a fully refined, homogenous microstructure with an average grain size below 300 nm was observed after 10 passes. Besides the significant microstructure refinement, a substantial increase in the dislocation density and gradual transformation of the extruded texture was observed with an increasing number of passes. In contrast to the grain refinement, which proceeded up to the 10 passes, both the proof stress and tensile strength increased only up to 4 passes. The contribution of the grain size, the dislocation density and the texture, to the room temperature proof stress is discussed in detail.

Origin of Extra Strengthening in Gradient Nanotwinned Metals: Yin Zhang1; Zhao Cheng2; Linfeng Bu3; Hengan Wu3; Lei Lu2; Ting Zhu1; 1Georgia Institute of Technology; 2Institute of Metal Research, Chinese Academy of Sciences; 3University of Science and Technology of China
    Materials containing heterogeneous nanostructures hold great promise for achieving superior mechanical properties. However, the strengthening effect due to plastically inhomogeneous deformation in heterogeneous nanostructures has not been clearly understood. Here we investigate a prototypical nanostructured material of gradient nanotwinned (GNT) Cu to unravel the mechanistic origin of extra strengthening associated with gradient twin structures. We characterize the mechanical heterogeneity in gradient nanotwins in terms of back and effective stresses, as compared with those from homogeneous nanotwinned components. The results reveal a new type of back stress that predominantly controls the extra strengthening of GNT Cu. Such extra back stress results from the plastic strain gradient and associated bundles of concentrated dislocations that stem from the structural gradient in GNT Cu. This work establishes mechanistic linkages between the extra strength and gradient structure in GNT Cu and thus exemplifies a general approach to unraveling the strengthening mechanism of heterogeneous nanostructured materials.

Strengthening and Improving Fracture Toughness of Tungsten-copper Nanocomposites: Klemens Schmuck1; Markus Alfreider1; Daniel Kiener1; 1Montanuniversität Leoben
    Tungsten-copper composites are frequently considered for high performance applications. Compared to pure tungsten, tungsten-copper composites exhibit improved damage and fracture tolerance, but reduced material strength, mainly attributed to the ductile copper phase. Therefore, improving mechanical properties of the copper phase, i.e. by alloying, is desirable. Elemental powders were used as precursor to fabricate tungsten-copper-zinc composites with constant tungsten content of 80 wt.%, which lead to a substitution of the pure copper phase with a mechanically stronger brass-alloy. The copper/zinc ratio was varied up to 30 wt.% zinc, α-brass region, which exhibits high twinning tendency. Additionally, the powders were consolidated and grains were refined by conducting HPT, which allows to further increase composite’s strength, while sustaining ductility down to a certain grain size. In-situ experiments on FIB fabricated micro-cantilevers in a SEM were performed to measure crack growth and determined fracture toughness with respect to the zinc content and grain sizes.

Hetero-deformation Induced (HDI) Stress in Heterostructured Materials: What We Know and Need to Know: Yuntian Zhu1; 1City University of Hong Kong
    Abstract: Recently, heterostructures are found to produce unprecedented strength and ductility that are considered impossible based on the materials science in our textbooks. Such superior mechanical properties are enabled by a new scientific principle: hetero-deformation induced (HDI) stress. Our recent experimental observations have verified that the HDI stress is produced by the piling up of geometrically necessary dislocations (GNDs) against the hetero-zone boundaries. However, there are still many fundamental issues that need to be probed so as to understand how GNDs interact with the zone boundaries. In this talk, I’ll briefly review the recent progresses on this topic and what we need to furtehr study experimentally using in-situ TEM and other techniques.

High-strength Gradient-microstructure Steels by Additive Manufacturing: Wenqi Liu1; Zaiqing Que2; Roy Björkstrand1; Mika Salmi1; Jouni Partanen1; Junhe Lian1; 1Aalto University; 2VTT Technical Research Centre of Finland Ltd
    This study proves the possibility of producing the heterogeneously gradient structure by additive manufacturing (AM), which benefits to simultaneously improved strength and ductility. The laser powder bed fusion technique with rotation scanning strategy is employed for high-strength steel in this study. With the same process parameters and specimen dimensions, a block stacked along the transverse direction (TD) performs a multiscale gradient structure instead of the typical AM melting pool structure in a building direction (BD) stacked block. The hierarchical electron backscatter diffraction is developed to investigate the hierarchical microstructure heterogeneity regarding phase, grain, and sub-grain morphology. The in-situ tension coupling micro-computed tomography is employed to observe porosity evolution. A hypothesis on the intrinsic thermal field during production is proposed to correlate process, microstructure, and plasticity. Consequently, gradient structures in TD samples significantly contribute to its high strength of 1200 MPa with the ductility of averagely 34% longer than BD ones.

High Load Sliding, Deformation Microstructures, Strength, and Hardening for Gradient Bulk Nanostructures: Darcy Hughes1; 1Sandia National Laboratories (ret.)
    Plowing of wedge shaped micro asperities under large sliding loads, from 0.4 to 3.7 of the yield stress at 293 or 77 K, is utilized to induce extremely high hardness 1.7 to 2.8 GPa and strain levels in the subsurface regions of Cu. A steep gradient in the size scale with increasing depth of the deformation microstructures is measured with transmission electron microscopy including an unprecedented size of 5 nm and high dislocation density near the surface. Subsurface stress estimates use the measured structural parameters in a linear addition of the Hall-Petch formulation for the inverse spacing of geometrically necessary boundaries, 1/DavGNB, plus Taylor dislocation hardening, ρav . A further unification of these parameters is made since the density ρav is directly proportional to 1/DavGNB. Quantitative comparisons and energy balances between the friction and deformation energies corroborate the high hardness and strain levels that may be applied to hard components.

Severely Deformed Stainless Steel Reveals an Anomaly in Thermal Expansion Behavior: Oliver Renk1; Robert Enzinger2; Christoph Gammer1; Daniel Scheiber3; Wolfgang Sprengel2; Reinhard Pippan1; Jürgen Eckert1; Lorenz Romaner4; Andrei Ruban5; 1Erich Schmid Institute; 2Graz University of Technology; 3Materials Center Leoben; 4Montanuniversität Leoben; 5KTH Royal Institute of Technology
    The large densities of lattice defects in severely deformed metals can alter the thermal expansion behavior. Upon annihilation, the excess volume associated with them generally reduces the global thermal expansion compared to the defect-scarce reference sample. Here we report on the striking observation that a severely deformed austenitic steel expands more than its defect-scare reference. We will show that this yet unknown thermal expansion behavior is induced by the exceptionally high density of stacking faults (1.8 nm average spacing), inducing a fcc to hcp transition at the atomic scale. Due to the peculiar magnetic structure of the austenitic steel, longitudinal spin fluctuations induce a pronouncedly different expansion behavior of the fcc and hcp phase, provoking a strong thermal expansion normal to the fault plane. Other severely deformed samples with similar stacking fault density but where fcc and hcp phase expand similarly do not show the anomalous expansion, supporting this conclusion.