Mechanical Behavior of Nanostructured Materials: Mechanical Behavior of Bulk Nanostructured Materials II
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Mechanical Behavior of Materials Committee, TMS: Nanomechanical Materials Behavior Committee
Program Organizers: Xinghang Zhang, Purdue University; Yuntian Zhu, North Carolina State University; Joseph Poon, University of Virginia; Suryanarayana Challapalli, University of Central Florida; Enrique Lavernia, University of California, Irvine; Haiyan Wang, Texas A&M University

Tuesday 8:30 AM
February 28, 2017
Room: 30D
Location: San Diego Convention Ctr

Funding support provided by: AJA International; Hysitron Inc.

Session Chair: Enrique Lavernia, University of California; Xiaoxu Huang, Technical University of Denmark ; Kaiyuan Yu, China University of Petroleum

8:30 AM  Invited
Mechanical Behaviors of Gradient Nanostructured Materials: Ke Lu1; 1Institute of Metal Research, Chinese Academy of Sciences
    Gradient nanostructured materials are structurally characterized by a graded spatial variation of microstructural length scale from nanometers to the macro-scale. Such a unique architecture enables fundamentally different mechanical responses in the gradient nanostructured materials compared with their homogeneous nanostructured counterparts. In this talk, experimental studies over the past years on mechanical properties and performance of gradient nanostructured materials will be summarized and reviewed, including tensile strength and ductility, work-hardening, fatigue behavior, strain-induced surface roughening, as well as friction and wear properties. Plastic deformation mechanism of the gradient nanostructures will be analyzed in comparison with the homogeneous nanostructures. Future perspectives and major challenges on this novel nanostructured material will be addressed as well.

8:55 AM  
Microstructure and Mechanical Behavior of ECAP and HPT Processed Austenitic and Ferritic-martenstic Steels: Haiming Wen1; Rinat Islamgaliev2; Marina Nikitina2; 1Idaho State University; 2Ufa State Aviation Technical University
    Austenitic and ferritic-martenstic steels have important applications in current and advanced nuclear reactors, however, their irradiation tolerance and mechanical properties need to be improved. Bulk nanostructured metals possess dramatically higher strength than their conventional coarse-grained counterparts, and have significantly enhanced irradiation tolerance due to significant volume fraction of grain boundaries that serve as sinks or recombination centers for radiation-induced defects. In this study, austenitic and ferritic-martenstic steels were processed by equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). The microstructure and mechanical behavior (tensile and creep behavior) of the ECAP and HPT processed steels were carefully studied, and their thermal stability was also investigated. Advanced microstructural characterization techniques such as high-resolution transmission electron microscopy, atom probe tomography and precession electron diffraction were utilized to investigate grain sizes/morphologies, dislocations, grain boundary characteristics, solute segregation at grain boundaries, pre-existing precipitates, and phase boundaries. Neutron irradiation has been planned to study irradiation behavior.

9:15 AM  Invited
Mechanical Properties and Microstructure Stability in Fe-Cr base Alloys for Nuclear Energy Applications: Ronald Scattergood1; Carl Koch1; 1NC State University
    Fe-14Cr base bcc alloys are used in structural applications that include nuclear reactor components. Nanostructured alloys that can contribute to the improved strengthening and related mechanical properties at elevated temperatures, as well as mitigating the formation of He bubble void swelling, are paramount to these applications. The strengthening mechanisms can be due to nanocrystalline grain size (Hall Petch) or nanoscale oxide particles (Orowan). Grain size stabilization can be due to thermodynamic stabilization or Zener pinning. Void swelling can be reduced by He bubble trapping in grain boundaries or by particles. The results of investigations for Fe-14CrX alloys are discussed where X is a nanocrystalline stabilizing element chosen based on thermodynamic stabilization model predictions. Zr, Hf, and Sc were used individually for thermodynamic stabilization, and Y-oxide was added for Zener pinning. Hardness data on the base and stabilized alloys was used to extract the contributions of Hall Petch vs. Orowan strengthening.

9:40 AM  
Hierarchical Structure and Strengthening Mechanisms in Pearlitic Steel Wire: Xiaodan Zhang1; Niels Hansen1; Xiaoxu Huang1; Andrew Godfrey2; 1Technical University of Denmark; 2Tsinghua University
    Microstructure evolution and strengthening mechanisms have been analyzed in a cold-drawn pearlitic steel wire (the strongest engineering materials in the world) with a nanostructure down to 10 nm and a flow stress up to 5.4 GPa. The interlamellar spacing and the cementite lamellae thickness are reduced during drawing in accordance with the change in wire diameter up to a strain of 2.5. At a higher strain enhanced thinning of cementite lamellae points to decomposition and carbon enrichment of the ferrite lamellae. Dislocations are stored as individual dislocations and in low angle boundaries. No saturation in the dislocation density is observed and it increases to 5E16 m-2 at a strain of 5.4. A high dislocation density at the ferrite/cementite(ferrite) interface is also observed. Boundary strengthening, dislocation strengthening and solid solution hardening are suggested and good agreement is found between the calculated flow stresses and experimental values.

10:00 AM  
Back-stress Strengthening and Strain Hardening in Heterogeneous Materials: Muxin Yang1; Fuping Yuan1; Xiaolei Wu1; Yuntian Zhu2; 1Institute of Mechanics, Chinese Academy of Sciences; 2North Carolina State University
    Heterogeneous materials have a superior combination of strength and ductility that is not accessible to their homogeneous counterparts. A previously not-well-understood mechanism, back-stress strengthening and back-stress strain hardening, is found responsible for such a dramatic improvement of mechanical properties in heterogeneous materials. Back stress is long-range stress caused by the pileup of geometrically necessary dislocations. We have developed a simple equation and procedure to calculate the back stress basing on its formation physics from the tensile unloading-reloading hysteresis loop. Heterogeneous structures have a mechanical incompatibility due to the grain size difference in different structural domains. This induces strain gradient, which needs to be accommodated by geometrically necessary dislocations. Back stress not only raises the yield strength but also significantly enhance strain hardening to increase the ductility. Utilizing back-stress hardening to design metals and alloys for superior mechanical properties represents a new design paradigm that may produce disruptive mechanical properties.

10:20 AM Break

10:40 AM  Invited
Correlation between Nanostructuring and Precipitation in Age-hardened Aluminum Alloys: Kaka Ma1; Tao Hu2; Ryan Cohn3; Troy Topping4; Enrique Lavernia5; Julie Schoenung5; 1Colorado State University ; 2University of California San Diego; 3University of California Davis; 4California State University Sacramento; 5University of California Irvine
    The strategy of creating nanostructures has been applied in a variety of metals and alloys to achieve unprecedented mechanical properties. Primary approaches include: (1) reducing the grain size to the nanoscale for Hall-Petch strengthening, and (2) using phase transformation to introduce nanoscale precipitates for dislocation shearing or Orowan strengthening. When precipitation occurs in alloys with nanoscale or ultrafine matrix grains (i.e., with a high volume of grain boundaries), the complexity in microstructural evolution leads to significant challenges in predicting and understanding the corresponding mechanical behavior. Recent studies have revealed that grain boundary strengthening and precipitation strengthening cannot be added in a simple linear manner for ultrafine grained age-hardened aluminum alloys. Using Al-Mg-Zn-(Cu) alloys as a model system, this talk is focused on describing the observed effects of nano/ultrafine grained microstructure on the precipitation behavior and the resultant mechanical behavior.

11:00 AM  
In Situ Synchrotron X-ray Studies on the Deformation Mechanism of Carbon-steel/Copper Nanocomposites: Kaiyuan Yu1; Yadong Ru1; Yang Ren2; Lishan Cui1; 1China University of Petroleum-Beijing; 2APS, Argonne National Laboratory, USA
    Nanostructured steels are expected to acquire a combination of high strength and ductility and exhibit unusual phase transformation and plastic deformation behavior. Nanolamellar high-carbon-steel/Cu composites are fabricated by hot pressing, rolling and wire drawing. Cu suppresses the growth of austenite at elevated temperatures, and subsequent quenching and partitioning treatments result in a large amount of nanosized retained austenite phases, which in return enhance the plasticity of the materials to a maximum elongation of 60%. Unusual double yield plateaus related to stress-induced martensitic transformation are observed. In situ synchrotron X-ray studies suggest: (1) stress-induced martensitic transformation occurs in a manner of Luders band; (2) much more retained austenite is transformed during the stress-induced regime than the strain-induced regime; (3) the deformation mechanism of the composites is a result of the competition between transformation and slip of the retained austenite phases.

11:20 AM  
Study of Dynamic Recovery in Nanocrystalline Metals Using In-situ X-ray Diffraction and MD Simulations: zhen Sun1; Steven Van Petegem1; Christian Brandl2; Maxime Dupraz1; Karsten Durst3; Wolfgang Blum4; 1Paul Scherrer Institut; 2Karlsruhe Institut of Technology; 3Technische Universität Darmstadt; 4University Erlangen-Nürnberg
    Transient testing has proven to be a suitable tool to gather information on rate limiting deformation mechanisms. In this work, we combine stress reduction tests with in situ synchrotron x-ray powder diffraction and apply this for electrode posited nanocrystallline Ni. The transient response is captured in terms of the evolution of the macroscopic strain rate and diffraction peak broadening. It is shown that the constant flow stress reached during uniaxial deformation reflects a quasi-stationary balance between dislocation slip and recovery grain boundaries. In order to to gain further knowledge on this interplay molecular dynamics simulations (MD) have been performed. Virtual stress reduction followed by a creep period have been simulated for various magnitudes of stress drops. These simulations confirm the interpretation of the behavior of the peak broadening during in situ diffraction experiments: after a stress drop strain is predominantly produced by grain boundary accommodation mechanisms.

11:40 AM  
Gradient Nanostructure and Mechanical Behavior of Ultrasonic Shot Peened Ti-6Al-4V: Fei Yin1; Hannah Han2; Qingyou Han1; 1Purdue University; 2West Lafayette High and Junior School
     A gradient nanostructured surface layer was developed on a Ti-6Al-4V sample by using ultrasonic shot peening. The microstructure, nanohardness and Young’s modulus of the gradient nanostructured surface layer was characterized and measured by field emission Scanning Electron Microscope (FESEM) and nanoindenter, respectively. The average grain size at the topmost surface layer of the ultrasonic shot peened Ti-6Al-4V is around 153.68nm. Relationship between the mechanical performance and microstructure of the gradient nanostructured Ti-6Al-4V was evaluated and discussed. Research results indicated that the average nanohardness and Young’s Modulus of the nanostructured Ti-6Al-4V has been increased by 42% and 15.6%, respectively compared to the initial coarse grained Ti-6Al-4V. The increased nanohardness and Young’s Modulus at the surface layer is attributed to the grain refinement after ultrasonic shot peening because grain boundaries will block dislocation movement when materials undergo plastic deformation. This study suggested that ultrasonic shot peening can be used as an effective method for surface nanocrystallization and mechanical strengthen of the Ti-6Al-4V.