Ronald O. Scattergood

Ultrahigh strength and high ductility of bulk nanocrystalline copper

We have synthesized artifact-free bulk nanocrystalline copper samples with a narrow grain size distribution (mean grain size of 23nm) that exhibited tensile yield strength about 11 times higher than that of conventional coarse-grained copper, while retaining a 14% uniform tensile elongation. In situ dynamic straining transmission electron microscope observations of the nanocrystalline copper are also reported, which showed individual dislocation motion and dislocation pile-ups. This suggests a dislocation-controlled deformation mechanism that allows for the high strain hardening observed. Trapped dislocations are observed in the individual nanograins.

Ductile-regime machining model for diamond turning of brittle materials

Abstract A new machining model has been developed for single point diamond turning of brittle materials. Experiments using the interrupted cutting method allow model parameters to be determined that provide a quantitative method for determining the machinability of a material with respect to the rake angle, tool nose radius and machining environment. The model uses two parameters, the critical depth of cut and the subsurface damage depth, to characterize the ductile-regime material removal process. Also included in the model is a parameter used to set a process limit defined as the maximum feed rate. Machining experiments have verified the model, and allow for determination of optimum machining conditions.

Ultratough nanocrystalline copper with a narrow grain size distribution

We report a unique way of using mechanical milling/in situ consolidation at both liquid-nitrogen and room temperature to produce artifact-free nanocrystalline Cu(23nm) with a narrow grain size distribution. This nanocrystalline Cu exhibits an extraordinarily high yield strength (770MPa), as predicted from a Hall–Petch extrapolation, along with good ductility (comparable with ∼30% uniform tensile elongation). Possible factors leading to this excellent optimization of strength and ductility are discussed.

Effect of erodent hardness on erosion of brittle materials

Abstract Data in the literature suggest that the relative hardness values of the erodent particles and the target sample may play a significant role in the erosion behavior of ceramics. This effect was investigated systematically. When the ratio of the target hardness to the hardness of the erodent particle approaches unity, there is a sharp transition in erosion rates. Relatively soft erodents lead to less erosion. Based on scanning electron microscopy observations, it is suggested that softer erodents can fragment and crush on impact. In steady state, it appears that the lateral-crack-based erosion mechanisms still operate, but for softer erodents damage accumulation is necessary to build up the requisite stresses to produce lateral cracks. Changes in the erosion velocity exponent can also occur. They have been attributed to particle blunting effects and/or R-curve behavior.

Flux effects in solid particle erosion

Abstract Systematic measurements of the steady state erosion rate as a function of erodent particle flux were made for a range of different erosion conditions. A significant decrease in erosion rate with increasing flux was observed. A first-order particle collision model was developed that provides a physical basis for describing the flux effect. The data confirm the exponential dependence of the erosion rate on flux predicted by the model. Further analysis of the data within the context of the collision model also provides new insight into the particle impact process.

Development of the Cylindrical Wire Electrical Discharge Machining Process, Part 1: Concept, Design, and Material Removal Rate

Results of applying the wire Electrical Discharge Machining (EDM) process to generate precise cylindrical forms on hard, difficult-to-machine materials are presented. The design of a precise, flexible, and corrosion-resistant underwater rotary spindle is first introduced. A detailed spindle error analysis identifies the major sources of error at different frequency spectrum. The spindle has been added to a conventional two-axis wire EDM machine to enable the generation of free-form cylindrical geometries. The mathematical model for material removal rate of the free-form cylindrical wire EDM process is derived. Experiments were conducted to explore the maximum material removal rate for cylindrical and 2D wire EDM of carbide and brass work-materials. Compared to the conventional 2D wire EDM of the same work-material, higher maximum material removal rates may be achieved in the cylindrical wire EDM, possibly due to better debris flushing condition.

Development of the Cylindrical Wire Electrical Discharge Machining Process, Part 2: Surface Integrity and Roundness

This study investigates the surface integrity and roundness of parts created by the cylindrical wire EDM process. A mathematical model for the arithmetic average surface roughness on the ideal surface of a cylindrical wire EDM workpiece is first derived. Effects of wire feed rate and part rotational speed on the surface finish and roundness for brass and carbide work-materials at high material removal rates are investigated. The pulse on-time and wire feed rate are varied to explore the best possible surface finish and roundness achievable by the cylindrical wire EDM process. This study has demonstrated that, for carbide parts, an arithmetic average surface roughness and roundness as low as 0.68 and 1.7 mm, respectively, can be achieved. Surfaces of the cylindrical EDM parts were examined using Scanning Electron Microscopy (SEM) to identify the macro-ridges and craters on the surface. Cross-sections of the EDM parts are examined using the SEM to quantify the sub-surface recast layers and heat-affected zones under various process parameters. This study has demonstrated that the cylindrical wire EDM process parameters can be adjusted to achieve either high material removal rate or good surface integrity and roundness. [DOI: 10.1115/1.1475989]

Thermodynamic stabilization of nanocrystalline binary alloys

The work presented here was motivated by the need to develop a predictive model for thermodynamic stabilization of binary alloys that is applicable to strongly segregating size-misfit solutes, and that can use available input data for a wide range of solvent-solute combinations. This will serve as a benchmark for selecting solutes and assessing the possible contribution of thermodynamic stabilization for development of high-temperature nanocrystalline alloys. Following a regular solution model that distinguishes the grain boundary and grain interior volume fractions by a transitional interface in a closed system, we include both the chemical and elastic strain energy contributions to the mixing enthalpy ΔHmix using an appropriately scaled linear superposition. The total Gibbs mixing free energy ΔGmix is minimized with respect to simultaneous variations in the grain-boundary volume fraction and the solute contents in the grain boundary and grain interior. The Lagrange multiplier method was used to obtain n...

Erosion of 304 stainless steel

Abstract Erosion rate measurements and scanning electron microscopy observations were made for the steady state erosion of 304 stainless steel eroded by sharp alumina particles. Both the velocity and the particle size dependence of the erosion rates were similar at all angles of impact between 10° and 90°. Micrographic observations of the steady state erosion surfaces disclosed similar overall features at low and high angles of impact. Results reported in the literature for aluminum tend to confirm these observations. It was concluded that a single erosion mechanism can be operative at all impact angles 7in ductile metals such as stainless steel, rather than a superposition of different mechanisms for the low angle and high angle range. The physical basis for a single mechanism of erosion by sharp particles was discussed.

Phase transformations during microcutting tests on silicon

Controlled slow‐speed microcutting tests were made on single crystal silicon. Micro‐Raman spectroscopy confirmed the presence of amorphous silicon within the microcutting grooves as well as in the debris particles removed from the grooves. These results indicate that pressure‐induced transformation to metallic silicon can occur during microcutting and the ductile metallic phase will facilitate the cutting process. Raman spectroscopy further indicated the presence of large residual tensile strains in some areas of the microcutting grooves.