Hardness across the Multi-Scales of Structure and Loading Rate: Structural Aspects I
Sponsored by: MS&T Organization
Program Organizers: Ronald Armstrong, University of Maryland; David Bahr, Purdue University; Naresh Thadhani, Georgia Institute of Technology; Stephen Walley, Physics and Chemistry of Solids Cavendish Laboratory

Tuesday 8:00 AM
October 18, 2011
Room: C210
Location: Greater Columbus Convention Center

Session Chair: David Bahr, Washington State University; William Gerberich, University of Minnesota

8:00 AM  Invited
Exploiting Interactions between Indentation Size and Structure Size Effects: Theory, Experiment and Some Practical Implications: Xiaodong Hou1; Nigel Jennett1; 1National Physical Lab, UK
    Enhanced yield (or flow) stress of materials linked to indentation size or structure size (e.g. some critical dimension such as grain size or pillar diameter) is well known. In most cases, strength enhancement is proportional to an inverse square root of dimension; the most well-known being (arguably) the Hall-Petch (H-P) effect. In this study, we investigate the combination of indentation size effect (ISE) and H-P effect, by conducting indentations in a range of Cu poly-crystal grain sizes. We show that the critical dimension becomes the average mean free path for dislocation movement, consistent with a slip-distance-like theory of plastic deformation. We directly show that a small grain size does not guarantee a constant hardness and a significant ISE increase occurs even when the contact dimension is an order of magnitude greater than the grain size. Our results point the way to generalised design rules for yield stress in asperity contact.

8:20 AM  Invited
On the Origin of High Hardness in Iron Alloys: Donald Lesuer1; Chol Syn1; Oleg Sherby2; Jeffrey Wadsworth3; 1Lawrence Livermore National Laboratory; 2Stanford University; 3Battelle
    Iron alloys are known to exhibit very high hardness depending largely on composition and processing history. For many of these materials the high hardness has been attributed to a supersaturated solid solution. However recent work on Fe-C alloys processed via severe plastic deformation or quenching has shown the importance of ultra-fine particles. For materials subjected to ball milling or high strain rate deformation, nano-indentation hardness measurements have identified local high hardness regions that can be associated with the creation of particles. For quenched Fe-C alloys, the formation of lath martensite has been predicted to result from an FCC->HCP->BCC transformation sequence. The final structure involves very fine particle-like iron-carbon clusters. These embryonic particles are proposed to be the source of high strength in lath martensite. The influence of particles on hardness in other Fe alloys, including complex Fe-based solid solutions and Fe alloys subjected to shock loading, will be discussed.

8:40 AM  Invited
Probing the Plastic Deformation Mechanisms of Ultrafine Grained and Nanocrystalline Metals Using Instrumented Indentation: Qiuming Wei1; 1University of North Carolina at Charlotte
    Novel phenomena have been uncovered for ultrafine grained (UFG) and nanocrystalline (NC) metals. The strain rate sensitivity (SRS) of fcc metals increases with decreased grain size, and that of bcc metals decreases. It has further been found that SRS and the derived activation volume can be used as a finger print for the plastic deformation mechanisms. Such behaviors impart strong influence on other properties such as tendency to plastic instability. Intrumented indentation has played an active role in probing the mechanical properties of UFG/NC metals. In certain respect, it has become an indispensable tool. In the case of specimens of very limited dimensions, it may be the only experimental technique available to obtain the needed experimental data. In this presentaion, we review the application of instrumented indentation to probing plastic deformaiton of UFG/NC metals. Comparisons are made against conventional methods based on literature results and the author’s own work.

9:00 AM  Invited
Size–Dependence of Rate Controlling Deformation Mechanism in Nanotwinned Copper: Lei Lu1; 1Institute of Metal Research, CAS
    Nanotwinned metals exhibits an unusual combination of ultrahigh strength, considerable ductility and enhanced rate sensitivity. Here we provide studies of the effects of twin thickness on the plastic deformation of ultrafine grained Cu samples by means of instrumented indentation, tensile jump as well as stress relaxation tests. It is demonstrated that decreasing the twin thickness results in a significant increase in strength, hardness and rate sensitivity of flow stress. The variation of activation volume as a function of twin lamellar thickness can be captured by a Hall-Petch-type relation. This structure size-dependence is interpreted to arise from a transition of the rate-controlling mechanism from the intra-twin to twin boundary-mediated processes with decreasing twin thickness. Furthermore, we find that the exhaustion rate of mobile dislocations reduces with decreasing twin thickness. Such a twin size dependence is attributed to the increased strain hardening rate associated with a high density of coherent twin boundaries.

9:20 AM  
Effects of Differently Oriented Twin Boundaries on Mechanical Properties in Nanotwinned Ag Films: Daniel Bufford1; Haiyan Wang1; Xinghang Zhang1; 1Texas A&M University
    Epitaxial nanotwinned Ag (111) and Ag (110) films were deposited by magnetron sputtering. The twin boundary spacing is 9 nm in Ag (111) and 42 nm in Ag (110), even though the two were deposited under identical conditions. Twin boundaries in the Ag (111) films are oriented normal to the growth direction, and at an angle in the Ag (110) films. The twin boundaries greatly enhance the indentation hardness of both films; there is a factor of two difference in hardness between the (111) and (110) Ag films. The mechanisms for twin formation and the role of twin boundary orientation on mechanical properties will be discussed.

9:40 AM Break

10:00 AM  Invited
Microhardness and Nanohardess Characterization of Advanced Multiphase Steels: Xiaoxu Huang1; Masafumi Azuma1; Xiaodan Zhang1; Grethe Winther1; Niels Hansen1; 1Ris° National Laboratory for Sustainable Energy, Technical University of Denmark
    In fine grained dual phase steels and cold drawn pearlitic steel wires, the structural scales are in the range of Ám to a few nm. Properties and local property gradients in the individual phases and near the interfaces determined by microhardness and nanohardness indentation are important in order to understand their strengthening mechanisms, mechanical response and failure mechanisms. In parallel, the microstructural evolution during loading must be analyzed in order to elucidate relationships between the mechanical behavior and the microstructure. The hardness difference between the ferrite and martensite in a dual phase steel is related to the initiation of small cracks in martensite that leads to the void formation and eventually failure. The local hardness gradient measured in a cold drawn steel wire is analyzed with respect to the degree of structural heterogeneities and related to the processing conditions.

10:20 AM  Invited
Understanding and Controlling Nanostructure Formation and Development in Nano Steels : Daniel Branagan1; Alla Sergueeva1; Jason Walleser1; Brian Meacham1; 1The Nanosteel Co
    Microstructural changes occurring on the nanoscale level are often challenging and complex to understand, however, once controlled, can lead to compelling properties and material performance. In this talk, the focus will be on solid state transformations which can occur in glass forming steel alloys. Two enabling routes to achieve nanoscale structures will be focused on involving either glass devitrification transformations to form devitrified nanocomposite microstructures or spinodal decomposition to form spinodal glass matrix microconstituent (SGMM) structures. The linkage of structural changes to physical properties will be highlighted with a focus on hardness, strength, and wear resistance changes related to the development of specific nanoscale structures.

10:40 AM  Invited
Effect of Solute Atoms on Nanoindentation Behavior in Fine-Grained Magnesium Alloys: Hidetoshi Somekawa1; Christopher Schuh1; 1MIT
    Fine-grained magnesium and its alloys have been found to exhibit grain boundary sliding even at room temperature, which is promising for the development of toughness and ductility. Here we report a nanoindentation study on such materials, including measurements of hardness and strain rate sensitivity at scales larger than the grain size, as well as incipient plasticity and its rate sensitivity at scales below the grain size. Pure magnesium and four magnesium binary alloys (Y, Zn, Al and Li) are explored, and the effects of solid solution elements on the indentation response are also discussed. In addition, high rate testing is conducted by instrumented impact indentation, further developing the rate sensitivity trends. The trends in rate sensitivity can be linked to alloying effects, such as on the stacking fault energy.

11:00 AM  Invited
Capturing the Hardness of Nanomaterials through Gradient Plasticity: Katerina Aifantis1; 1Aristotle University
    In order to interpret the inverse Hall-Petch dependence of the yield stress on the grain size that is observed for nanostructured materials below a critical grain size, various creep, rule of mictures or dislocation-based models have been proposed. Treating the grain boundary as a separate phase within a gradient plasticity model elucidates implicititly to the latter two frameworks, while it explicitly captures experimental data for both the normal and ab-normal H-P behavior of nanomaterials, predicting the critical grain size at which the transition occurs.

11:20 AM  Student
Spherical Nanoindentation and Kinking Non-Linear Elasticity of LiTaO3 Single Crystals: Babak Anasori1; Kurt Sickafus2; Igor Usov2; Michel Barsoum1; 1Drexel University; 2Los Alamos National Laboratory
    Herein, spherical nanoindentation (NI) – with various radii - was used to investigate the room temperature deformation behavior of LiTaO3 C-plane single crystals loaded along [0001]. When the NI load-displacement curves are converted to NI stress-strain curves, deformation starts elastically, with a Young’s modulus of 205±5 GPa and continues to plastic deformation at ≈ 6 GPa. Repeated loading into the same location results in large, reproducible, fully and spontaneously reversible nested hysteresis loops attributed to the formation of incipient kink bands, IKBs. The latter are co-axial fully reversible dislocation loops that spontaneously shrink when the load is removed. The IKBs most probably nucleate within the (10-12) twins that form near the surface. The changes in the reversible loops’ shape and areas can be related to the width of the twins that form. The latter were proportional to the nanoindenter tip radii and confirmed by scanning electron microscopy.

11:40 AM  
Spherical Nanoindentation Stress-Strain Curves, Kinking Nonlinear Elastic Solids and Low Dimensionality Solids: Michel Barsoum1; 1Drexel University
    Recently we designated solids with high c/a ratios – that renders non-basal slip prohibitively expensive – to be kinking nonlinear elastic, KNE. The signature of KNE solids is the formation of fully and spontaneously reversible, hysteretic, stress-strain loops, on repeat loadings. This full reversibility is believed to be due to the formation of incipient kink bands, that are comprised of two, nearly parallel, dislocation walls of opposite polarity that are attracted to each other; when the load is removed they annihilate. The energy dissipated per cycle is usually substantial and increases with increasing stress. In this paper we show that repeated spherical nanoindentations results - when converted to nanoindentation stress strain curves - on the same location is a technique that is ideally suited to characterize KNE solids, that tend to be layered and thus of lower dimensionality. Examples on Ti3SiC2, mica, graphite, sapphire and LiNbO3, among others are presented.