Mechanical Behavior at the Nanoscale VI: Size Effects
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Computational Materials Science and Engineering Committee, TMS: Mechanical Behavior of Materials Committee, TMS: Nanomechanical Materials Behavior Committee
Program Organizers: Matthew Daly, University of Illinois-Chicago; Douglas Stauffer, Bruker Nano Surfaces & Metrology; Wei Gao, University of Texas at San Antonio; Changhong Cao, McGill University; Mohsen Asle Zaeem, Colorado School of Mines

Monday 2:00 PM
February 28, 2022
Room: 262B
Location: Anaheim Convention Center

Session Chair: Changhong Cao, McGill University; Kevin Turner, University of Pennsylvania

2:00 PM  Invited
In Situ Atomic-scale Observation of Surface-diffusion-controlled Softening in Metallic Nanocrystals: Scott Mao1; 1University of Pittsburgh
    In nanoscale metals, it is generally believed that surface dislocation nucleation is the dominant mechanism. Thus far, the role of surface diffusion in the process of yielding and plastic flow in metallic nanocrystals remains largely unclear. Here,in situ nanomechanical testing under high-resolution transmission electron microscope was conducted on silver and platinum nanocrystals, we found that the yield strength-size relationship of the silver nanocrystals changed from a normal to an inverse Hall-Petch relation with the reduction of size scales. Surface diffusion-assisted dislocation nucleation and plastic flow resulted in an abnormal decrease in yielding and flow stress in Ag nanowires, whereas Pt nanowires followed the classical “Hall-Petch” routine, which is controlled by pure displacive deformation. This work provides novel insights into the atomic-scale mechanisms of coupled diffusive and displacive deformation in small-sized metals.

2:30 PM  
Dynamic Recrystallization-induced Strengthening in Amorphous-nanocrystalline Silver-nickel Nanolaminates: Ryan Pringle1; Frederic Sansoz1; 1The University of Vermont
    Silver has the highest electrical conductivity, but also is one of the softest metals under mechanical deformation. Therefore, developing means for strengthening silver without reducing its conductivity is critically important for electrode applications. This talk presents an experimental and atomistic simulation study of a nanoscale strengthening mechanism by intercalating nanocrystalline Ag films with thin amorphous Ni layers that characterizes the structure of nanolayered materials synthesized by sputtering. It is found that the addition of Ni nanolayers plays a significant strengthening role, as the layer thickness increases, while dramatically improving the stability of nanosized grains in the Ag films. Furthermore, the normal Hall-Petch breakdown observed in nanocrystalline Ag at a mean grain size of 15 nm disappears in the presence of Ni layers. We show that this phenomenon results from dynamic recrystallization as the Ni nanolayer become less amorphous during deformation, differing from conventional strengthening mechanisms for nanoscale amorphous-crystalline interfaces.

2:50 PM  
Achieving Micron-scale Plasticity and Theoretical Strength in Silicon: Ming Chen1; Laszlo Peth2; Alla Sologubenko3; Johann Michler2; Ralph Spolenak4; Jeffrey Wheeler5; 1Paul Scherrer Institute; 2Empa; 3ScopeM/ETH Zurich; 4ETH Zurich; 5FemtoTools AG, Furtbachstrasse 4, CH-8107 Buchs/ZH, Switzerland
    As the backbone material of the information age, silicon is extensively used as a functional semiconductor and structural material in microelectronics and microsystems. At ambient temperature, the brittleness of Si limits its mechanical application in devices. Here, we demonstrate that Si processed by modern lithography procedures exhibits an ultrahigh elastic strain limit, near ideal strength (shear strength ~4 GPa) and plastic deformation at the micron-scale, one order of magnitude larger than samples made using focused ion beams, due to superior surface quality. Further, the micron-scale plasticity of Si allows the investigation of the intrinsic size effects and dislocation behavior in diamond-structured materials. This reveals a transition in deformation mechanisms from full to partial dislocations upon increasing specimen size at ambient temperature. Deformation mechanism transitions at ambient and elevated temperatures are discussed as a function of surface quality.

3:10 PM  
Grain Size Dependent Deformation Mechanisms in Complex Concentrated Oxides (Co,Cu,Mg,Ni,Zn)O: Xin Wang1; Justin Cortez1; Alexander Dupuy1; Julie Schoenung1; William Bowman1; 1University of California Irvine
    As a new class of ceramic materials, complex concentrated oxides (CCOs, often called high-entropy oxides, HEOs) offer entropy-enhanced stability and compositionally tunable properties, which are demanded for various functional and structural applications. In the present study, we explore the effect of grain size on deformation mechanisms in CCOs, an important issue for materials development that remains unresolved. Fully dense bulk samples of polycrystalline single-phase CCO (Co,Cu,Mg,Ni,Zn)O with grain sizes ranging from the nanometer to the micron scale were prepared from solid-state synthesized powders and consolidated using appropriate sintering conditions. The as-sintered CCO samples were mechanically deformed, and the post-deformation microstructures were characterized using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). We find that the deformation response is significantly different between nano-grain CCO and micron-grain CCO samples, due to changes in the deformation mechanisms, which are observed in the microstructure.

3:30 PM Break

3:50 PM  Invited
Tuning the Fracture Toughness of Polymer-infiltrated Nanoparticle Films via Nanoconfinement: Kevin Turner1; Yiwei Qiang1; Daeyeon Lee1; 1University of Pennsylvania
    Physical nanoconfinement of polymers in nanocomposites with high filler content (>50 wt%) can dramatically alter the mechanical properties of the polymers in the composite. Here, we investigate the fracture behavior of polymer-infiltrated nanoparticle films (PINFs) prepared by capillary-driven infiltration of polymers into nanoparticle (NP) packings. PINFs have applications as multifunctional coatings and membranes and also provide a unique platform to investigate the role of nanoconfinement on fracture. PINFs with different molecular weight (MW) polymers and varying NP size, and hence different degrees of confinement, were prepared. The fracture toughness of these materials was experimentally measured to investigate the role of nanoconfinement on the failure behavior of PINFs. The results show increasing toughness of the PINFs with increasing MW as expected, but also show that confinement plays a large role in fracture behavior. The full results and the contributions of nanoscale polymer confinement on fracture properties will be discussed.

4:20 PM  
Mechanical Behavior of Nanotwinned Al Alloys at Elevated Temperatures: Xinghang Zhang1; Qiang Li2; Dongyue Xie3; Jian Wang3; 1Purdue University; 2Ames Lab; 3University of Nebraska, Lincoln
    Al alloys often have inherently low mechanical strength and limited operation tempera-ture. Most Al alloys have mechanical strength of 200-500 MPa, and soften rapidly at low homol-ogous temperature. We have used far-from-equilibrium approach to fabricate various nanot-winned Al solid solution alloys. Some of the nanotwinned Al alloys have flow stresses exceeding 1.5 GPa, but the mechanical strength degrade quickly when tested at 300oC. By using a solute synergy approach, we can effectively improve the thermal stability of nanotwinned Al alloys to 400oC. In situ micropillar compression studies reveal an outstanding high-temperature flow stress, greater than 1 GPa at 300 C. DFT calculations and microscopy studies reveal the funda-mental mechanisms of high temperature stability and exceptional mechanical strength in these nanotwinned Al alloys.

4:40 PM  
Precipitation Hardening in BCC Multilayer Thin Films: Yailuth Loaiza Lopera1; David Bahr1; 1Purdue University
    Multilayered Nb-Cr/Cu thin films were deposited by magnetron sputtering contain approximately 20 layer of 25 nm average thickness per layer, with a nominal Cu concentration of 5% in the Cr layer. The films were then annealed at 573K for 1 to 8 hours, and indentation at room temperature post-annealing demonstrated a typical precipitation hardening behavior. A peak hardness of 10GPa, 43% higher than the as-deposited film, and a modulus of 180 GPa was observed at 1.5 hours annealing. Combinations of SEM, TEM, and XRD were used to characterize grain sizes and phases. The observed behavior and characterization results suggest that both solid solution and precipitation hardening are responsible for the changes in mechanical properties. Elevated temperature indentation post-annealing demonstrated the strengthening from Cu precipitation in the hard Cr layer remains effect at moderately elevated temperatures.

5:00 PM  
The Effects of Grain Boundaries on the Micromechanical Properties of Transparent Nanocrystalline Spinel: Jessica Maita1; Sarshad Rommel1; Jacob Davis2; James Wollmershauser3; Edward Gorzkowski3; Boris Feigelson3; Mark Aindow1; Seok-Woo Lee1; 1University of Connecticut; 2University of Massachusetts Amherst; 3U.S. Naval Research Laboratory
    Transparent materials are used extensively due to their ability to transmit light and provide physical protection from chemical and mechanical interactions. The state-of-the-art transparent nanocrystalline MgAl₂O₄ with grain sizes ranging 3.7-80 nm, the smallest currently reported, has been fabricated using high-pressure sintering which can effectively suppress grain growth. Transmission electron microscopy revealed the presence of nanoscale angular pores for grain sizes greater than 10.5 nm but full densification for smaller ones. Nanoindentation, micropillar compression, and micro-cantilever testing were performed to elucidate the effects of grain size on plasticity and fracture properties. We found that both the maximum hardness in nanoindentation and maximum fracture strength in micropillar compression occurs at 10.5 nm grain size, below which the grain-boundary-assisted mechanism promotes plastic deformation under nanoindentation and fracture under uniaxial compression. Our results show that the grain-boundary-assisted process plays an important role in various deformation and fracture modes.