2014 TMS RF Mehl Medal Symposium on Frontiers in Nanostructured Materials and Their Applications: Nanometals II-Processing and Strengthening Mechanisms
Sponsored by: TMS Electronic, Magnetic, and Photonic Materials Division, TMS: Thin Films and Interfaces Committee
Program Organizers: Nuggehalli Ravindra, New Jersey Institute of Technology; Ramki Kalyanaraman, University of Tennessee; Haiyan Wang, Texas A & M University; Yuntian Zhu, North Carolina State University; Justin Schwartz, North Carolina State University; Amit Goyal, Oak Ridge National Laboratories

Wednesday 8:30 AM
February 19, 2014
Room: Ballroom E
Location: San Diego Marriott Marquis & Marina

Session Chair: Evan Ma, Johns Hopkins University; Suveen Mathahaudhu, US Army Research Office

8:30 AM  Invited
Superior Strength in Bulk Nanostructured Metallic Materials Produced by SPD Processing: Ruslan Valiev1; Nariman Enikeev1; Sergei Firstov2; 1Ufa State Aviation Technical University; 2Frantsevich Institute for Problems of Materials Science
    Recent studies demonstrated that the processing of metallic alloys by severe plastic deformation (SPD) can result in not only strong grain refinement but also different phase transformations dealing with second phase dissolution, formation of grain boundary segregations and precipitations. These nanostructural features of SPD-processed alloys produce considerable influence on their mechanical properties. The report presents experimental data and modeling results demonstrating “positive” slope of the Hall-Petch relation when passing from micro- to nanostructured state in a number of metallic materials subjected to SPD. The observed extra-strength is related to the changes in strengthening mechanisms of ultrafine-grained materials as a result of changes in grain boundaries structure. The nature of superior strength is associated with the difficulty of generation of dislocations from grain boundaries with segregations. This new approach is used for achieving the enhanced strength in several commercial Al and Ti alloys as well as steels subjected to SPD processing.

8:50 AM  
Dynamic Strain Aging in Ultrafine Grained Titanium: Felipe Lopes1; Sergio Monteiro1; Daniel Fernandes1; Carlos Elias1; Chia-Hui Lu2; Ruslan Valiev3; Marc Meyers2; 1IME; 2UC San Diego; 3Ufa State Aviation Technical University
    Titanium and its alloys are very important for the industries and medicine, because of the properties, mainly corrosion resistance, lightweight, strength and biocompatibility. This new titanium was processed by ECAP-Conform, with this new structure the strength was improved and some authors say that UFG Ti is stronger than its alloys commonly used, for example: Ti-6Al-4V, but it’s not true at all. UFG Ti is stronger in comparison with this alloy, just in some range of strain rate, exactly, when happens the dynamic strain aging (DSA), it's necessary to understand better the phenomenon. The objective of this paper was investigate into which range of strain rate and temperature this phenomenon happens by compression tests and indentify the deformation mechanism by TEM. The results show that happens in UFG Ti at 10-2.5 to 10-3.5 s-1 for room temperature and the deformation mechanism observed by TEM were grain-boundary rotation and perhaps twin bands.

9:10 AM  Invited
Generation of Bulk Nanocomposites and Supersaturated Solid Solutions by Severe Plastic Deformation: Andrea Bachmaier1; Anton Hohenwarter2; Reinhard Pippan3; 1Saarland University; 2University of Leoben; 3Austrian Academy of Sciences
    Severe plastic deformation (SPD) can be used to produce ultrafine grained materials from nearly all kind of bulk coarse grained metal materials. This work is devoted to the generation of nanocomposites and stable nanocrystallites by SPD. A High-Pressure Torsion (HPT) powder compaction technique permits the generation of nanocrystalline composite materials with enhanced thermal stability. During HPT of composites or powder mixtures, the formation of supersaturated solid solutions or even amorphization reactions can occur in alloys with a positive heat of mixing ∆H due to a kind of “mechanically alloying in bulk form”. Nanometer sized grains are reported to be a basic requirement to obtain supersaturated solid solutions. Hence, a novel two-step HPT powder consolidation and deformation process was developed, which has an increased effectiveness compared to conventional ones. During annealing, the supersaturated solid solutions decompose and form two-phase nanostructured composites with an enhanced hardness and an improved thermal stability.

9:30 AM  Invited
Crystallization of Metallic Glasses to Produce Nanostructured Materials: Ken Kelton1; 1Washington University
    Metallic glasses are increasingly finding new applications due to their good corrosion resistance, high yield strength, and other desirable physical properties. In some cases, the properties are further improved by partially crystallizing the glass to form a nanocrystal/amorphous composite. Nucleation and growth during crystallization are often complex, sometimes coupling to structural and chemical ordering in the amorphous phase and to other phase transitions. Knowledge of these processes is necessary if a tailored nanostructure is to be achieved. It is also essential to ensure the stability of the glass or nanocrystal/amorphous composite in the environment of the desired application. A few selected examples of glass and liquid crystallization will be presented to illustrate these points. Partially supported by the NSF (DMR-12-06707)

9:50 AM Break

10:10 AM  Invited
Industrially Useful Nanostructured Molybdenum Alloys with Unprecedented Tensile Ductility: Evan Ma1; 1Johns Hopkins University
    The high-temperature stability, creep resistance, thermal conductivity of refractory molybdenum alloys are highly desirable for a wide range of applications. But molybdenum alloys are also a well-known example of body-centered-cubic materials that suffer from low ductility and limited formability. In collaboration with J. Sun (XJTU) we solve this long-standing problem via a new nanostructuring route that optimizes the distribution of the grains, strengthening dispersions and solutes (G. Liu et al., Nature Materials, 2013). A simple and cost-effective molecular-level liquid-liquid mixing/doping technique is developed to achieve ultrafine submicron-sized grains with nanosized oxide particles uniformly distributed in the grain interior. The resulting hybrid Mo alloys boast an extraordinary tensile elongation as large as ~40% at room temperature. The new processing route is used for large-scale industrial productions of ductile Mo alloys that can be extensively shaped. Our findings can be used for engineering dispersion-strengthened architectured materials with simultaneously high strength and ductility.

10:30 AM  Invited
High-strength Low-alloyed Zinc Processed by High-pressure Torsion: Javier Gil Sevillano1; Tobias Zühlke1; Jon Iglesias Erausquin1; Jon Alkorta1; Heinz Werner Höppel2; Mathias Göken2; 1CEIT and TECNUN, University of Navarra; 2University of Erlangen-Nuremberg
     Coarse grain pure zinc and a low-alloyed zinc EN988 (0.16 % Cu, 0.08% Ti, in mass percent) have been processed by high-pressure torsion (HPT) up to very high plastic deformation (equivalent true strain: 400) at room temperature. Pure zinc undergoes a discontinuous dynamic recrystallization and reaches early a steady state of relatively coarse heterogeneous structure (≈15 μm) and low strength (130 MPa). By contrast, the zinc alloy undergoes a process of dynamic continuous recrystallization that induces a homogeneous sub-micron grain structure (≈150 nm) and a persistent strengthening up to a true equivalent strain of 300, reaching a maximum strength of about 700 MPa. The origin of such unusual strengthening is discussed on account of both the evolution of the crystallographic texture and grain size, and the influence of the small volume fraction of TiZn16 second phase present in the alloy during the HPT process.

10:50 AM  
Grain Size Effect on Deformation Physics of Nanostructured Materials: Yuntian Zhu1; Guangming Cheng1; Xiaozhou Liao2; Xiaolei Wu3; 1North Carolina State University; 2University of Sydney; 3Chinese Academy of Sciences
    Grain size has been found to have a significant effect on the deformation physics of nanomaterials and this consequently affects their mechanical behaviors. In this presentation, I’ll first present the grain size effect on the deformations physics of nano fcc, hcp and bcc metals. The extensive studies on deformation physics for the last two decades have been focused primarily on the fcc systems. There are still many issues in the bcc and hcp systems to be addressed by our materials community.

11:10 AM  Invited
Microstructural and Geometrical Size Scale Effects in Shape Memory Alloys: Raj Vaidyanathan1; 1UCF
    Results from microcompression experiments on single crystal micron-scaled pillars of NiTi of known orientations are extended to understand the effect of microstructural length scales and sample geometry on shape memory behavior. The microcompression experiments are supplemented by in situ neutron diffraction measurements at stress and temperature on bulk samples on engineering diffractometers at Los Alamos National Laboratory and Oak Ridge National Laboratory. The samples investigated include binary NiTi samples under isostress, isothermal and isostrain loading conditions and a ternary NiTiHf alloy with a coherent, nano-sized precipitate phase. Conclusions are drawn on the effect of microstructural length scales and sample geometry on the thermoelastic transformation and more specifically their implications on shape memory properties including: the gradient and the hysteresis during the phase transformation, the recoverable strain associated with the transformation and the evolutionary unrecoverable strain with thermomechanical cycling.

11:30 AM  Invited
Finding Strength in Our Faults: Extreme Strengthening of Mg-alloys via Nano-spaced Stacking Faults: Weiwei Jian1; Weizhong Xu1; Hao Yuan1; Ming-Hung Tsai1; Carl Koch1; Yuntian Zhu1; Suveen Mathaudhu; 1North Carolina State University
    Mg alloys are among the lightest alloys but their strengths are usually low. Here we report a new mechanism to make them ultrastrong and moderately ductile. Stacking faults with nanoscale spacing were introduced into a Mg-8.5Gd-2.3Y-1.8Ag-0.4Zr (wt. pct.) alloy by conventional hot rolling, which produced a yield strength of ~575 MPa, an ultimate strength of ~600 MPa, and a uniform elongation of ~ 5.2%. Low stacking fault energy played an essential role in producing a high density of stacking faults which impeded dislocation slip and promoted dislocation accumulation. These findings provide guidance for development of Mg alloys with superior mechanical properties.