Glenn D. Hibbard

An initial analysis of mechanisms leading to late stage abnormal grain growth in nanocrystalline Ni

Abstract A late stage of abnormal grain growth, having unusual planar abnormal growth interfaces, is observed during the isothermal annealing of electrodeposited nanocrystalline Ni and Ni–Fe alloys. This communication presents observations of grain embedding and the presence of a wetting, sulfur-rich second phase at the abnormal growth interface and suggests a growth mechanism.

Thermal stability of nanostructured electrodeposits

As a result of their unique, but well-behaved structure-property relationships, porosity free nanostructured electrodeposits in the form of thin and thick coatings, free-standing sheet, foil or wire, and complex shapes, are rapidly finding applications in many different areas. Because of the large driving force for grain growth in these materials, however, their thermal stability may be a critical issue for some applications. This paper reviews previous grain growth studies of nanostructured electrodeposits. Thermal stability has been evaluated using several different experimental approaches. Calorimetric studies of nanocrystalline nickel based electrodeposits have shown a general trend of increasing thermal stability by alloying with either P or Fe. There is, however, little agreement between the various studies in terms of suggested growth mechanisms. Indeed, because of the large range of annealed structures obtained from different annealing treatments, several distinct growth mechanisms have been suggested. Recent grain growth studies of nanostructured Ni electrodeposits, covering a much broader range of annealing conditions than used before, have shown that the multiple types of previously reported annealed structures are in fact the product of a multi-staged growth process.

Compression testing of periodic cellular sandwich cores

Periodic cellular metal (PCM) hybrid sandwich cores with 95 pet open porosity have been constructed from perforated 3003 aluminum alloy (AA3003) sheets using perforation-stretching methods. Two compressive collapse mechanisms (i.e., plastic hinging and plastic buckling) were studied using two limiting test conditions: first, where the PCM nodes were restricted only by interfacial friction (i.e., free compression) and compressive forces were resisted primarily through strut bending and plastic hinging mechanisms; and, second, where the PCM nodes were laterally confined (i.e., confined compression) and compressive forces were resisted primarily through strutbuckling mechanisms. The contribution of each collapse mechanism to the overall truss core performance was studied. The strut bending during free compression was tracked by a nodal displacement mapping (NDM) technique, while the progression of confined compression strut buckling was correlated to the truss core stress-strain profile. The present data can be used to illustrate the different strengths between strut bending (free-PCM) and strut buckling (confined-PCM) collapse mechanisms.

Periodic Cellular Metal/Polyurethane Foam Hybrid Materials

A new type of hybrid material was designed and created by reinforcing periodic cellular metals (PCMs) with rigid polyurethane foams. Two different PCM geometries and two types of rigid polyurethane foam were used to create four different hybrid materials. These novel materials may find useful application as cores in sandwich structures. The two-phase foam/PCM hybrids offered comparable stiffness and up to 59% higher strength as well as up to 86% higher resilience than the PCMs alone. Furthermore, the two-phase foam hybrids were up to 46% stronger and up to 63% stiffer than the one-phase foam hybrids.

Grain Growth in Nanocrystalline Nickel

In this study, the effect of grain size distribution on the thermal stability of electrodeposited nanocrystalline nickel was investigated by pre-annealing material such that a limited amount of abnormal grain growth was introduced. This work was done in an effort to understand the previously reported, unexpected effect, of increasing thermal stability with decreasing grain size seen in some nanocrystalline systems. Pre-annealing produced a range of grain size distributions in materials with relatively unchanged crystallographic texture and total solute content. Subsequent thermal analysis of the pre-annealed samples by differential scanning calorimetry showed that the activation energy of further grain growth was unchanged from the as-deposited nanocrystalline nickel.

Effect of grain size on the optimal architecture of electrodeposited metal/polymer microtrusses

Nanocrystalline microtruss materials are novel cellular hybrids of metal and polymer produced by electrodepositing thin coatings of nanocrystalline metal over rapid prototyped polymer preforms. This study develops an optimisation method for the architectural design of electrodeposited metal/polymer composite microtrusses used as cores in sandwich beams. For an optimally designed structure employing conventional polycrystalline nickel, a direct substitution of nanocrystalline nickel will improve structural performance; however, it is likely that the structure will also become significantly sub-optimal. Achieving optimal design with nanocrystalline nickel entails large geometric changes from the conventional polycrystalline case. The same applies if the polymer preform is removed after electrodeposition. The strong connection between optimal architecture and grain size was therefore examined for the limiting cases of polymer-filled and hollow microtrusses. It was found that grain size reduction was more important than polymer preform removal such that grain size effects dominate over the majority of microtruss design space.

Thermal stability of nanocrystalline Ni- and Co-based pulsed current electrodeposits: correlation of calorimetric measurements and microstructure development upon annealing

The exceptional properties associated with nanocrystalline materials are, to a large extent, a result of their high inter-crystalline volume fraction. However, the intrinsic instability of the nanostructured state may compromise the gain in properties by the occurrence of grain growth during exposure at elevated temperatures. Thermal stability is, therefore, a fundamental materials issue for nanocrystalline materials. This article describes what can be deduced from calorimetric measurements in the context of what is known about the microstructural evolution upon annealing of nanocrystalline Ni- and Co-based pulsed current electrodeposits. Special emphasis is put on interpreting the shape of the curves obtained by a differential scanning calorimetry (DSC). The temperature ranges for relaxation, segregation, precipitation, as well as abnormal and normal grain growth can be predicted. Also, by evaluating the shift in peak temperature with heating rate (Kissinger plot), the activation energies for grain growth can be obtained for the different materials.

Enhanced thermal stability of a cobalt-boron carbide nanocomposite by ion-implantation

Abstract A first investigation of the thermal stability in a wear resistant cobalt-boron carbide (Co – B4C) nanocomposite has been performed by the combination of calorimetry and transmission electron microscopy. The calorimetric measurements show that the thermal stability of Co – B4C is not influenced by the presence of the 10 vol.% μm-sized boron carbide particles. However, grain growth is shifted to significantly higher temperatures during in-situ annealing (in the transmission electron microscope), and abnormal grain growth is not observed to be as extensive as in conventional nanocrystalline Co. This effect is mainly attributed to the observed implantation of Ga atoms during transmission electron microscope specimen thinning by focused ion beam. Grain boundary segregation mechanisms are discussed as possible reasons for the retarded grain growth.

Scale-dependent failure of stereolithographic polymer microtrusses in three-point bending

Microtrusses fabricated by a combination of stereolithography and nanocrystalline electrodeposition are composites at both a global architectural scale and at an individual strut scale. This fabrication route is especially attractive in producing high performance lightweight materials because the key stages of forming optimally efficient templates and depositing ultra high strength material are fully decoupled. Using novel 3-D printing techniques, this study analyzes microtrusses built bottom-up from photopolymerizable polymer where the complex structure can be constructed of unit cells a few millimeters in length. While the predicted failure mechanisms matched the experimental observations, deterministic energetic size effects were needed to account for the critical loads. For strut radii <5 × the 3-D printer layer thickness, stochastic size effects became important.

Optimizing the Compressive Strength of Strain-Hardenable Stretch-Formed Microtruss Architectures

The mechanical performance of stretch-formed microtrusses is determined by both the internal strut architecture and the accumulated plastic strain during fabrication. The current study addresses the question of optimization, by taking into consideration the interdependency between fabrication path, material properties and architecture. Low carbon steel (AISI1006) and aluminum (AA3003) material systems were investigated experimentally, with good agreement between measured values and the analytical model. The compressive performance of the microtrusses was then optimized on a minimum weight basis under design constraints such as fixed starting sheet thickness and final microtruss height by satisfying the Karush–Kuhn–Tucker condition. The optimization results were summarized as carpet plots in order to meaningfully visualize the interdependency between architecture, microstructural state, and mechanical performance, enabling material and processing path selection.