Ultrafine-grained and Heterostructured Materials (UFGH XI): Microstructure & Property
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Caizhi Zhou, University of South Carolina; Megumi Kawasaki, Oregon State University; Enrique Lavernia, University of California, Irvine; Terry Lowe, Colorado School of Mines; Suveen Mathaudhu, Colorado School of Mines; Ruslan Valiev, UFA State Aviation Technical University; Yuntian Zhu, City University of Hong Kong
Monday 8:00 AM
February 24, 2020
Room: Marina Ballroom D
Location: Marriott Marquis Hotel
Session Chair: Qin Yu, University of California, Berkeley; Paulo Branicio, University of Southern California; Xinghang Zhang, Purdue University; Terry Lowe, Colorado School of Mines
8:00 AM Invited
Mechanical Behavior of Structurally Gradient Alloys: Jie Ding1; Qiang Li1; Zhongxia Shang1; Xinghang Zhang1; 1Purdue University
Certain structurally gradient materials have shown a combination of high strength and work hardening capability. The fundamental mechanisms behind such a unique mechanical behavior remain less well understood. Here we processed NiCrMo based alloys by surface mechanical grinding treatment to achieve a gradient microstructure consisting of surface nanolaminated layer, deformation twinned layer and severely deformed layers. In situ micropillar compression tests performed inside a scanning electron microscope reveal different mechanical behaviors of each layer. This study suggests that the increase of intergranular back stress may have contributed to the high strain hardening behavior of the gradient material.
8:20 AM Invited
Deformation and Failure of Gradient Metallic Nanoglasses: Paulo Branicio1; 1University of Southern California
The deformation and failure of gradient nanoglasses are investigated with tensile loading large-scale molecular dynamics simulations. Cu64Zr36 metallic glasses are used as parent alloys for the construction of functionally graded nanoglasses with grain sizes from 3 to 15 nm. Tensile loading simulations are used to characterize the mechanical behavior of the heterogeneous metallic glass and clarify the plastic deformation mechanisms. Results indicate a strong localization of the plastic deformation beyond the yield point for loading parallel to the gradient direction in soft regions with high concentration of reduced grain sizes. Loading perpendicular to the gradient direction indicates overall significant delocalization of plasticity throughout the sample. Results indicate a synergy of the gradient design and a superior combination of strength and ductility for gradient nanoglasses with a hard core (large grain size) compare with gradient nanoglasses with soft core (small grain sizes), in agreement with reports for gradient nanocrystalline systems.
8:40 AM
Properties of Ultrafine Grain Titanium Fabricated by Multimode Deformation Processing: Benjamin Ewing1; Benjamin Davis1; Lane Bailey1; Jeffrey Slater1; Mathew Hayne2; Skyler Davis2; Melina Endsley2; Terry Lowe2; Tamás Ungár3; 1Fort Wayne Metals; 2Colorado School of Mines; 3Eötvös University
Ultrafine grain titanium can provide exceptional strength for industrial applications, plus added biocompatibility for medical applications. The tensile and fatigue properties of conventional Grade 4 are compared with ultrafine grain Grade 4 Ti produced by multimode deformation, including Equal Channel Angular Pressing-Conform. Critical microstructural features, including activated slip systems, dislocation densities, dislocation configurations, grain size distributions, grain shape anisotropy, and crystallographic textures are shown that explain the origins of mechanical behavior. The attributes of ultrafine Grade 4 titanium are compared with other medical alloys. The exceptional performance and metallurgical simplicity of ultrafine grain titanium make it attractive as a substitute for conventional alpha-beta titanium alloys.
9:00 AM Cancelled
Deformation Compatibility between Nanotwinned and Recrystallized Grains Enhances Resistance to Interface Cracking in Cyclic Loaded Stainless Steel: Qian Li1; Fen Kai Yan1; Nairong Tao1; Ke Lu1; 1Institute of Metal Research, Chinese Academy of Sciences
Cracks often initiate at phase boundaries in conventional second phase reinforced alloys during cyclic loading, which limits their fatigue properties. We prepared a nanotwin strengthened 316L stainless steel consisting of nanotwinned and recrystallized grains by using plastic deformation and subsequent partial recrystallization annealing. Fatigue tests revealed that interfaces separating hard nanotwinned grains from soft recrystallized ones exhibited an excellent resistance to crack initiation. This is ascribed to the elastic homogeneity and cyclic deformation compatibility between nanotwinned and recrystallized grains. At small cumulative cyclic strains, nanotwinned grains deform compatibly with the recrystallized grains without noticeable strain localization at their interfaces. Nanotwins can accommodate cyclic plastic strains by interaction of dislocations with twin boundaries, especially through the motion of the well-ordered threading dislocations inside the twin lamellae. Therefore, improved fatigue property with high fatigue limit and fatigue ratio is achieved in the nanotwin strengthened stainless steel.
9:20 AM Break
9:40 AM
Deformation Induced Microstructures in Ultrafine Grain Magnesium Fabricated by ECAP-C: Casey Davis1; Adam Griebel2; Jeremy Schaffer2; Terry Lowe1; Tamás Ungár3; 1Colorado School of Mines; 2Fort Wayne Metals; 3Eötvös University
Ultrafine grain magnesium alloys have the potential to provide a combination of properties making them suitable for absorbable medical material applications, including trauma fixation and stents. It is important to first understand how commercially viable methods of microstructure refinement can impart suitable microstructures. AZ31 was selected as a model system to which Equal Channel Angular Pressing-Conform (ECAP-C) was applied to study the range of microstructures that can be achieved. Relationships between ECAP-C processing parameters and critical microstructural features, including activated slip systems, dislocation densities, dislocation configurations, grain size distributions, grain shape anisotropy, and crystallographic textures are shown and analyzed. Experimentally measured textures are compared with the results of Taylor factor modeling. We found that ECAP-C temperature and strain path have the greatest effects on grain size refinement, grain size distribution, and dislocation configuration. Finally, we discuss how the structures induced by ECAP-C can be optimized to customize absorbability.
10:00 AM Invited
On the Fracture Toughness of Gradient Pure Nickel: Qin Yu1; Ruqing Cao2; Jie Pan2; Yan Lin2; Andrew Sweet1; Yi Li2; Robert Ritchie1; 1Lawrence Berkeley National Laboratory; 2Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences
Favorable combinations of strength and ductility can be achieved in gradient metallic materials. However, this does not necessarily guarantee that the fracture resistance will be similarly superior. We describe here a fracture-mechanics-based study to evaluate the fracture toughness of gradient structured (GS) nickel. Specifically, cracks are grown in grain-size gradient ranging from the coarse grains to nano-grains (CG→NG) and vice versa (NG→CG), with the measured crack-resistance R-curves, compared to corresponding behavior in uniform nano-grained (NG) and coarse-grained (CG) materials. The gradient structures display an improved combination of high strength and toughness compared to uniform grain-sized materials. The crack-initiation toughness is far higher for cracks grown in the direction of the CG→NG gradient than vice versa; a result which we ascribe primarily to excessive crack-tip blunting in the coarse-grained microstructure. Both gradient structures, however, exhibit marked rising R-curve behavior with exceptional crack-growth toughnesses exceeding 200 MPa.m˝.
10:20 AM Invited
Fracture Strain and Forging Limit Analysis of Polycrystalline Alloys at Elevated Temperatures: Yanfei Gao1; Wei Zhang1; 1University of Tennessee - Knoxville
A micromechanical model is developed to determine the failure strain of high temperature alloys, accounting for various governing deformation mechanisms at different length scales, including the nucleation of grain boundary cavities, their growth by competition of grain boundary diffusion and grain interior creep, viscous grain boundary sliding, and the emergence of microcracks by coalescence and their evolution to the ultimate failure. This methodology provides quantitative creep rupture lifetime prediction, which finds interesting applications in weldment failure and forging limit analysis. We will also present a mechanistic analysis of the ductility at elevated temperatures, which shows distinctive behavior than those strength-ductility-enhancement methodologies at room temperatures.
10:40 AM
Heterogeneities in Plastic Deformation Mechanisms in UFG Aluminum Studied by In-situ TEM Straining and Bulk Deformation: Witold Chrominski1; Malgorzata Lewandowska1; 1Warsaw University of Technology
Understating of the plastic deformation behaviour in UFG metals is necessary for the development of industrial routines for tooling of such metals. For this reason, detailed TEM study on deformation mechanisms behind macroscopic plastic deformation were utilized. The emphasis was laid on how local defects arrangements, including type of grain size and boundaries characteristics, impact dislocation glide. Firstly, heterogenous materials with similar double-texture components but different grain size and boundary distribution were studied. Then, samples with a relatively uniform microstructure were investigated. Once plastic deformation in nanoscale was described, these observations were transferred to a bulk deformation. A new set of samples taken from the same materials were studied before and after bulk compression test. Comparison of pre- and post-deformation microstructures allowed to deduce an effect of previously detected small-scale mechanisms on a more global behavior of such samples.
11:00 AM Cancelled
Fatigue Properties and Cyclic Behavior of Tensile Pre-deformed Nanotwinned Cu: Qingsong Pan1; Haofei Zhou2; Huajian Gao3; Lei Lu1; 1Institute of Metal Research, Chinese Academy of Sciences; 2Zhejiang University; 3Brown University
In service, many engineering components often undergo a very complex pre-loading history, associated with microstructural damages, which strongly influence subsequent cyclic behaviors, such as reduced fatigue life and so on. In this study, we investigate the fatigue behavior of nanotwinned Cu (NT-Cu) after different small levels of tensile pre-tension. Obvious cyclic asymmetry is detected in tensile pre-deformed NT-Cu, but still exhibiting cyclic stability and comparable fatigue life, relative to their as-deposited counterpart. Moreover, through variable-amplitude cyclic tests, a history-independent, stable cyclic response reported in as-deposited NT Cu, is still observed in pre-tensiled NT samples, where the stress amplitude is not obviously influenced by pre-strain or not. Microstructural observations revealed that the cyclic asymmetry is related to threading dislocations with extended tails lying on twin boundaries after pretension while their transforming to correlated necklace dislocations dominate the stable twin structure and the associated superior fatigue properties.