Mechanical Engineering
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Item The Hardness and Strength Properties of WC-Co Composites(MDPI, 2011-07-14) Armstrong, Ronald W.The industrially-important WC-Co composite materials provide a useful, albeit complicated materials system for understanding the combined influences on hardness and strength properties of the constituent WC particle strengths, the particle sizes, their contiguities, and of Co binder hardness and mean free paths, and in total, the volume fraction of constituents. A connection is made here between the composite material properties, especially including the material fracture toughness, and the several materials-type considerations of: (1) related hardness stress-strain behaviors; (2) dislocation (viscoplastic) thermal activation characterizations; (3) Hall-Petch type reciprocal square root of particle or grain size dependencies; and (4) indentation and conventional fracture mechanics results. Related behaviors of MgO and Al2O3 crystal and polycrystal materials are also described for the purpose of making comparisons.Item Symmetry Aspects of Dislocation-Effected Crystal Properties: Material Strength Levels and X-ray Topographic Imaging(MDPI, 2014-03-20) Armstrong, Ronald W.Several materials science type research topics are described in which advantageous use of crystal symmetry considerations has been helpful in ferreting the essential elements of dislocation behavior in determining material properties or for characterizing crystal/polycrystalline structural relationships; for example: (1) the mechanical strengthening produced by a symmetrical bicrystal grain boundary; (2) cleavage crack formation at the intersection within a crystal of symmetrical dislocation pile-ups; (3) symmetry aspects of anisotropic crystal indentation hardness measurements; (4) X-ray diffraction topography imaging of dislocation strains and subgrain boundary misorientations; and (5) point and space group aspects of twinning. Several applications are described in relation to the strengthening of grain boundaries in nanopolycrystals and of multiply-oriented crystal grains in polysilicon photovoltaic solar cell materials. A number of crystallographic aspects of the different topics are illustrated with a stereographic method of presentation.Item Crystal Dislocations(MDPI, 2016-01-06) Armstrong, Ronald W.Crystal dislocations were invisible until the mid-20th century although their presence had been inferred; the atomic and molecular scale dimensions had prevented earlier discovery. Now they are normally known to be just about everywhere, for example, in the softest molecularly-bonded crystals as well as within the hardest covalently-bonded diamonds. The advent of advanced techniques of atomic-scale probing has facilitated modern observations of dislocations in every crystal structure-type, particularly by X-ray diffraction topography and transmission electron microscopy. The present Special Issue provides a flavor of their ubiquitous presences, their characterizations and, especially, their influence on mechanical and electrical properties.Item Crystal Indentation Hardness(MDPI, 2017-01-12) Armstrong, Ronald W.; Walley, Stephen M.; Elban, Wayne L.There is expanded interest in the long-standing subject of the hardness properties of materials. A major part of such interest is due to the advent of nanoindentation hardness testing systems which have made available orders of magnitude increases in load and displacement measuring capabilities achieved in a continuously recorded test procedure. The new results have been smoothly merged with other advances in conventional hardness testing and with parallel developments in improved model descriptions of both elastic contact mechanics and dislocation mechanisms operative in the understanding of crystal plasticity and fracturing behaviors. No crystal is either too soft or too hard to prevent the determination of its elastic, plastic and cracking properties under a suitable probing indenter. A sampling of the wealth of measurements and reported analyses associated with the topic on a wide variety of materials are presented in the current Special Issue.Item Crystal Engineering for Mechanical Strength at Nano-Scale Dimensions(MDPI, 2017-10-18) Armstrong, Ronald W.The mechanical strengths of nano-scale individual crystal or nanopolycrystalline metals, and other dimensionally-related materials are increased by an order of magnitude or more as compared to those values measured at conventional crystal or polycrystal grain dimensions. An explanation for the result is attributed to the constraint provided at the surface of the crystals or, more importantly, at interfacial boundaries within or between crystals. The effect is most often described in terms either of two size dependencies: an inverse dependence on crystal size because of single dislocation behavior or, within a polycrystalline material, in terms of a reciprocal square root of grain size dependence, designated as a Hall-Petch relationship for the researchers first pointing to the effect for steel and who provided an enduring dislocation pile-up interpretation for the relationship. The current report provides an updated description of such strength properties for iron and steel materials, and describes applications of the relationship to a wider range of materials, including non-ferrous metals, nano-twinned, polyphase, and composite materials. At limiting small nm grain sizes, there is a generally minor strength reversal that is accompanied by an additional order-of-magnitude elevation of an increased strength dependence on deformation rate, thus giving an important emphasis to the strain rate sensitivity property of materials at nano-scale dimensions.Item Dislocation Mechanics Pile-Up and Thermal Activation Roles in Metal Plasticity and Fracturing(MDPI, 2019-01-31) Armstrong, Ronald W.Dislocation pile-up and thermal activation influences on the deformation and fracturing behaviors of polycrystalline metals are briefly reviewed, as examples of dislocation mechanics applications to understanding mechanical properties. To start, a reciprocal square root of grain size dependence was demonstrated for historical hardness measurements reported for cartridge brass, in line with a similar Hall-Petch grain size characterization of stress-strain measurements made on conventional grain size and nano-polycrystalline copper, nickel, and aluminum materials. Additional influences of loading rate (and temperature) were shown to be included in a dislocation model thermal activation basis, for calculated deformation shapes of impacted solid cylinders of copper and Armco iron materials. Connection was established for such grain size, temperature, and strain rate influences on the brittle fracturing transition exhibited by steel and other related metals. Lastly, for AISI 1040 steel material, a fracture mechanics based failure stress dependence on the inverse square root of crack size was shown to approach the yield stress at a very small crack size, also in line with a Hall-Petch dependence of the stress intensity on polycrystal grain size.Item Crystal Strengths at Micro- and Nano-Scale Dimensions(MDPI, 2020-02-05) Armstrong, Ronald W.; Elban, Wayne L.Higher strength levels, achieved for dimensionally-smaller micro- and nano-scale materials or material components, such as MEMS devices, are an important enabler of a broad range of present-day engineering devices and structures. Beyond such applications, there is an important effort to understand the dislocation mechanics basis for obtaining such improved strength properties. Four particular examples related to these issues are described in the present report: (1) a compilation of nano-indentation hardness measurements made on silicon crystals spanning nano- to micro-scale testing; (2) stress–strain measurements made on iron and steel materials at micro- to nano-crystal (grain size) dimensions; (3) assessment of small dislocation pile-ups relating to Griffith-type fracture stress vs. crack-size calculations for cleavage fracturing of α-iron; and (4) description of thermally-dependent strain rate sensitivities for grain size strengthening and weakening for macro- to micro- to nano-polycrystalline copper and nickel materials.Item High-Rate Crystal/Polycrystal Dislocation Dynamics(MDPI, 2022-05-16) Armstrong, Ronald W.The present report builds upon work recently published on crystal and polycrystal dislocation mechanics behaviors assessed, in part, in split-Hopkinson pressure bar (SHPB) and shock loading investigations. A connection between the flow stress dependencies on strain rate in the different tests had been established in the previous report, whereas additional results are assessed here for (1) relationship of the measurements to a nano-scale prismatic dislocation structure proposed to be generated at a propagating shock front and (2) further relationships between the modeled structure and corresponding thermal stress and strain rate sensitivity computations, including new evaluations of the engineering rate sensitivity parameter, m = [∆lnσ/∆ln(dε/dt)]T. A comparison is made of m values approaching 1.0 for simulated dislocation mechanics results computed for tantalum crystals. Other (lower) m value comparisons involve recently determined higher shock stress measurements made on copper material at higher temperatures.Item The Dislocation Mechanics of Crystal/Polycrystal Plasticity(MDPI, 2022-08-25) Armstrong, Ronald W.A brief history and update are given in four examples demonstrating that polycrystals are generally stronger than their individual component crystal grains because of obstructed dislocation pile-ups at grain boundaries. The example cases constitute diverse applications of a Hall–Petch dependence involving one or another aspects of the full polycrystal stress–strain behavior: (1) a Hall–Petch based description for a compilation of delayed yielding measurements compiled for steel; (2) computations for an H-P grain size dependent, tensile, plastic instability behavior of copper; (3) an H-P relationship for the true maximum stress for the limit of uniform straining of aluminum; and (4) the onset of a ductile-to-brittle transition in steel cleavage fracturing measurements that are connected to the material fracture toughness properties.Item Metal Crystal/Polycrystal Plasticity and Strengths(MDPI, 2022-12-01) Armstrong, Ronald W.A brief historical sketch is given of Taylor’s dislocation density-based model description, leading to the prediction of a parabolic, tensile, stress–strain curve for the plastic deformation of aluminum. The present focus is on additional results or analyses obtained on the subject for crystal/polycrystal strain hardening. Our current understanding of such material behavior is attributed to post-Taylor descriptions of sequential deformation stages in stress–strain measurements that are closely tied to specific dislocation interaction and reaction mechanisms. A schematic comparison is given for individual face-centered cubic (fcc), body-centered cubic (bcc), and hexagonal close-packed (hcp) crystal curves and to related strength properties determined for individual crystals and polycrystalline material. For the fcc case, an example sessile dislocation reaction is described based on a stereographic projection. Then, quantitative constitutive-relation-based assessments are presented for the tensile strain hardening leading to the plastic instability behaviors of copper and tantalum materials.