Christopher Saldana

Stabilizing nanostructured materials by coherent nanotwins and their grain boundary triple junction drag

The role of nanotwin lamellae in enhancing thermal stability of nanostructured materials is examined. Nanostructured copper with varying densities of twins was generated by controlling the deformation strain rate during severe plastic deformation at cryogenic temperatures. While the nanostructured materials produced under cryogenic conditions are characteristically unstable even at room temperatures, their stability is markedly improved when a dense dispersion of nanotwins is introduced. Observations of the role of nanotwins in pinning grain and subgrain structures suggest an interfacial engineering approach to enhancing the stability of nanostructured alloys.

Applications of a New Handheld Reference Point Indentation Instrument Measuring Bone Material Strength

A novel, hand-held Reference Point Indentation (RPI) instrument, measures how well the bone of living patients and large animals resists indentation. The results presented here are reported in terms of Bone Material Strength, which is a normalized measure of how well the bone resists indentation, and is inversely related to the indentation distance into the bone. We present examples of the instruments use in: (1) laboratory experiments on bone, including experiments through a layer of soft tissue, (2) three human clinical trials, two ongoing in Barcelona and at the Mayo Clinic, and one completed in Portland, OR, and (3) two ongoing horse clinical trials, one at Purdue University and another at Alamo Pintado Stables in California. The instrument is capable of measuring consistent values when testing through soft tissue such as skin and periosteum, and does so handheld, an improvement over previous Reference Point Indentation instruments. Measurements conducted on horses showed reproducible results when testing the horse through tissue or on bare bone. In the human clinical trials, reasonable and consistent values were obtained, suggesting the Osteoprobe® is capable of measuring Bone Material Strength in vivo, but larger studies are needed to determine the efficacy of the instruments use in medical diagnosis.

Unusual Applications of Machining: Controlled Nanostructuring of Materials and Surfaces

A class of deformation processing applications based on the severe plastic deformation (SPD) inherent to chip formation in machining is described. The SPD can be controlled, in situ, to access a range of strains, strain rates, and temperatures. These parameters are tuned to engineer nanoscale microstructures (e.g., nanocrystalline, nanotwinned, and bimodal) by in situ control of the deformation rate. By constraining the chip formation, bulk forms (e.g., foil, sheet, and rod) with nanocrystalline and ultrafine grained microstructures are produced. Scaling down of the chip formation in the presence of a superimposed modulation enables production of nanostructured particulate with controlled particle shapes, including fiber, equiaxed, and platelet types. The SPD conditions also determine the deformation history of the machined surface, enabling microstructural engineering of surfaces. Application of the machining-based SPD to obtain deformation-microstructure maps is illustrated for a model material system-99.999% pure copper. Seemingly diverse, these unusual applications of machining are united by their common origins in the SPD phenomena prevailing in the deformation zone. Implications for large-scale manufacturing of nanostructured materials and optimization of SPD microstructures are briefly discussed.

Vacancies, twins, and the thermal stability of ultrafine-grained copper

Ultrafine-grained metals have impressive strength but lack the thermal stability necessary for most applications. Nano-scale, deformation twinned copper microstructures exhibit a rare combination of strength and stability. While storing less energy in their interfaces than other nanostructured metals, they also exhibit lower vacancy supersaturations, reducing the driving force and mobility for microstructure evolution. From a thermal stability perspective, the nano-twinned microstructure may thus be preferred over the more commonly produced nano-scale equiaxed microstructures.

Deformation field heterogeneity in punch indentation

Plastic heterogeneity in indentation is fundamental for understanding mechanics of hardness testing and impression-based deformation processing methods. The heterogeneous deformation underlying plane-strain indentation was investigated in plastic loading of copper by a flat punch. Deformation parameters were measured, in situ, by tracking the motion of asperities in high-speed optical imaging. These measurements were coupled with multi-scale analyses of strength, microstructure and crystallographic texture in the vicinity of the indentation. Self-consistency is demonstrated in description of the deformation field using the in situ mechanics-based measurements and post-mortem materials characterization. Salient features of the punch indentation process elucidated include, among others, the presence of a dead-metal zone underneath the indenter, regions of intense strain rate (e.g. slip lines) and extent of the plastic flow field. Perhaps more intriguing are the transitions between shear-type and compression-type deformation modes over the indentation region that were quantified by the high-resolution crystallographic texture measurements. The evolution of the field concomitant to the progress of indentation is discussed and primary differences between the mechanics of indentation for a rigid perfectly plastic material and a strain-hardening material are described.

Deformation and Microstructure in Machining

Deformation history and state of chip and machined surface in low-speed cutting have been characterized using image analysis, complemented by microstructure and hardness. Fine scale details of the deformation field of relevance to machining modelling are highlighted. The severe plastic deformation inherent to chip formation results in microstructure changes which can be controlled through appropriate process parameters selected with the aid of machining simulations. Scaling of subsurface strain distribution is observed. Similarity in deformation history of chip and near-surface suggests opportunities for engineering surfaces with controlled deformation levels and microstructures, directly, by machining. The deformation characterization offers substantial scope for improvement and validation of machining models, and enhancement of machining process capability.

Modulation-Assisted Machining: A New Paradigm in Material Removal Processes

Modulation Assisted Machining (MAM), based on controlled superimposition of low-frequency modulation to conventional machining, effects discrete chip formation and disrupts the severe contact condition at the tool-chip interface. The underlying theory of discrete chip formation and its implications are briefly described and illustrated. Benefits such as improved chip management and lubrication, reduction of tool wear, enhanced material removal, particulate manufacturing and surface texturing are highlighted using case studies. MAM represents a new paradigm for machining in that it deliberately employs ‘good vibrations’ to enhance machining performance and capability.

Effect of Controlled Modulation on Chip Formation and Interface Tribology in Machining

Based on consideration of mechanics of chip formation, it is shown that the application of a controlled modulation fundamentally changes the nature of the extreme deformation underlying chip formation, and the severe contact conditions at the tool-chip interface. Important consequences are significant reduction in the energy of chip formation, and control of chip shape and size for improved chip management. Implementation of modulation-assisted machining for industrial machining processes is discussed.Copyright

Deformation Field in Indentation of Granular Materials

A preliminary study has been made of the deformation field in indentation of a model granular material using particle tracking optical flow analyses. A continuum composed of spherical sand particles with average size of 0.4 mm is indented with a flat punch under plane‐strain conditions. The region around the indenter/indentation is imaged in situ using a Charge‐Coupled Device (CCD) imaging system. By applying this hybrid analysis technique to image sequences of the indentation parameters of the deformation field such as displacement, velocity, and velocity gradient are measured at high spatial resolution. Implications of the measurement technique for accurate determination of local strain and strain rate, and exploring phenomena such as friction shear bands, in granular solids are briefly discussed.

Deformation heterogeneity and texture in surface severe plastic deformation of copper

Comprehensive understanding of thermomechanical response and microstructure evolution during surface severe plastic deformation (S2PD) is important towards establishing controllable processing frameworks. In this study, the evolution of crystallographic textures during directional surface mechanical attrition treatment on copper was studied and modelled using the visco-plastic self-consistent framework. In situ high-speed imaging and digital image correlation of surface deformation in circular indentation were employed to elucidate mechanics occurring in a unit process deformation and to calibrate texture model parameters. Material response during directional surface mechanical attrition was simulated using a finite-element model coupled with the calibrated texture model. The crystallographic textures developed during S2PD were observed to be similar to those resultant from uniaxial compression. The implications of these results towards facilitating a processing-based framework to predict deformation mechanics and resulting crystallographic texture in S2PD configurations are briefly discussed.