Paul Shewmon

A new model for the erosion of metals at normal incidence

Abstract A new theoretical model is proposed for the erosion of metals by particles at normal incidence. The model employs a criterion of critical plastic strain to determine when the material will be removed. This critical plastic strain is defined as the strain at which the deformation in the target localizes and hence results in the lip formation. It is shown that, under typical erosion conditions, the “localization” model is more appropriate than the “fatigue” models. Finally, it is demonstrated that the new model predicts quite well all the essential features of the normal impact erosion process.

Hydrogen attack of carbon steel

A model is developed for the kinetics of nucleation and growth of methane bubbles in the hydrogen attack of carbon steel. It is concluded that at high temperatures the time to incubate fissuring along grain boundaries is determined by the rate of iron diffusion away from microscopic growing bubbles. At lower temperatures and/or higher hydrogen pressures carbon supply is limiting. The equations fit the observed incubation times if the bubble density is high (~107/cm2) and essentially independent of temperature (T) and hydrogen pressure (P) over a wide range. It is postulated that the number of growing bubbles is limited at high nucleation rates (lowT and highP) by carbon starvation. At hiT and lowP chemisorption to lower the solid-vapor surface energy or fine inclusions are required to aid nucleation. A quantitative analysis of these processes leads to several predictions which can be checked experimentally.

Erosion of a strong aluminum alloy

Abstract The erosion of a strong aluminum alloy (7075-T6) and annealed commercially pure aluminum was modelled with impacts by ball bearings of diameter 5 mm at velocities of 140–185 m s −1 . Single impacts caused significant erosive loss only at low impact angles (20°–35°). Single impacts normal to the surface removed negligible amounts of metal. Erosion by impacts normal to the surface occurred only when the deformation field from one impact overlapped that of an earlier impact. Erosion resistance, as measured by velocity threshold for weight loss, was much lower for the high strength 7075-T6 alloy than for pure aluminum. Thus alloying for strength diminishes erosion resistance. Treatments that improve the fracture toughness of the 7075 alloy did not improve erosion resistances. Metal left the target as one or two chips per impact. These separated along narrow bands of intense shear. While particle impacts always deform the surface, it is postulated that this deformation results in metal loss (erosion) only when a shear instability (adiabatic shear) develops under the ball prior to the tensile stresses set up in the surface upon particle rebound.

Diffusion driven grain boundary migration

Abstract If the grain boundary diffusion coefficient of the two species in a binary alloy differ, diffusion driven grain boundary motion should lead to predictable surface relief and shape changes. This leads to predictions and equations which would allow the determination of the separate diffusion coefficient of each species as well as the chemical diffusion coefficient for the boundary, Db. For a thick sample the stress gradients built up by differences in the fluxes of the two species can lead to a reduced apparent Db, especially if the solute boundary diffusion coefficient, D′2, is appreciably greater than that of the solvent. D′1. For Zn diffusion into Fe it appears that D′Fe ⪢ D′Zn. The stress built up will also slow the boundary motion.

Energy and structure of (001) twist interphase boundaries in the Ag/Ni system

Abstract Computer calculations of coincidence (001) Ag/Ni twist interphase boundaries were performed with the Embedded Atom Method (EAM). The results show the variation of interfacial energy with azimuthal misorientation in the range 0–45°. Relative minima in the energy-misorientation curve are predicted; the lowest minimum is at 26.56°. Predicted low-energy boundaries at 4.4 and 26.56° are confirmed experimentally by the rotating crystallite method. The rotations of Ag crystallites on the (001) surface of a Ni thin film are determined by transmission electron microscopy with special use of double diffraction.

Investigation of low energy interphase boundaries in agni by computer simulation and crystallite rotation

Using the embedded atom method, boundary energy calculations were done for (111)Ag//(001)Ni twist boundaries and interfaces between low-index planes of Ag and Ni. The E(θ) curve for (111)Ag//(001)Ni boundaries shows the existence of one deep cusp at θ = 0°, or at the variant θ = 30°. Experiments using the rotation of (111)Ag crystallites on (001)Ni film confirm this single cusp. Calculations for AgNi interfaces containing low-index planes show that those containing the (111) plane have the lowest energy while those with the (110) are highest.

Void nucleation and cracking at grain boundaries

Abstract Dimpled grain boundary fracture occurs in creep tests, stress relief cracking and hydrogen attack of steels. While the growth of these voids by grain boundary diffusion is well established, the mode of void nucleation is uncertain. It is shown that the reduction of surface energy by solute adsorption plays an essential role in giving easy void nucleation. A calculation is given for the stress needed for this mode of nucleation using data for phosphorous in steel. Numerous examples exist of strongly adsorbing solute inducing elevated temperature grain boundary cracking. A row of voids growing by stress driven boundary diffusion is shown to develop a tensile stress maximum which aids void nucleation, giving rise to dimpled grain boundary fracture. Our cracking model involves repeated void nucleation and is thus fundamentally different from the steady-state Hull–Rimmer model. At times and temperatures too low for void formation, adsorption can lead to smooth grain boundary cracking at a rate controlled by solute diffusion (adsorption).

Diffusion induced recrystallization of NiO

Abstract It is shown that changing the composition of a sample of NiO from that in equilibrium with air at 1200°C to that in equilibrium with oxygen saturated Ni at 800–900°C will recrystallize the surface to a much finer grain size. Annealing back at 1200°C in air will again recrystallize the surface layer. This type of diffusion induced recrystallization has often been observed in metals, but has never before been reported in ceramics. Its occurrence in NiO is interpreted as a demonstration that diffusion induced grain boundary motion is driven directly by the free energy of mixing defects into the matrix instead of indirectly as suggested by others.

Adiabatic shear localization and erosion of strong aluminum alloys

Abstract A high strength aluminum alloy, 7075-T651, was eroded by ball-bearing impact at high velocities (about 180 m st1) at normal and oblique incidence. Scanning electron microscope studies of the target and ejected chip show that the fracture surface is covered with myriad small globules (roughly 1 μm in dimension) whose form was clearly established by the effect of surface tension on a fluid metal. It is argued that for high strength age-hardened alloys like this, chips are formed by fracture across molten adiabatic shear bands which then quickly solidify after separation to give the globular surface. Such structures were not found on eroded surfaces of the much softer commercially pure aluminum.

A mechanism for hydrogen-induced intergranular stress corrosion cracking in alloy 600

Intergranular stress corrosion cracking (IGSCC) of Alloy 600 in high-temperature, deaerated water or steam has been termed “hydrogen-induced IGSCC.” We believe these cracks are initiated by the nucleation of a high density of bubbles on the grain boundary under the combined action of the applied stress and high pressure methane formed from carbon in solution reacting with hydrogen injected by corrosion. The bubbles then grow together by grain boundary diffusion to give local failure. This is supported by transmission electron microscope (TEM) observations of two-stage replicas, which show the subsurface formation of closely spaced (0.2μm) bubbles along boundaries, and their growth into fine cracks before they open to communicate with the corroding atmosphere. The kinetics are examined and shown to be in quantitative agreement with several experimental observations. This mechanism involves no grain boundary dissolution of the metal, the only role of corrosion being the injection of hydrogen at a high fugacity. It predicts an activation energy roughly equal to that for grain boundary dψusion of nickel in Alloy 600.