http://www.uniroma2.it/didattica/tbs2/deposito/Manufacture,_characterisation_and_application_of_cellular_metals_and_metal_foams.pdf

Manufacture, characterisation and application of cellular metals and metal foams

The role of deformation twinning in fracture of hexagonal close-packed metals is reviewed from a theoretical point of view. Strength and ductility are correlated with the intrinsic physical and metallurgical variables. The importance of c + a slip and of both “tension” and “compression” twins as independent modes for a generalized polycrystalline deformation is emphasized. Effects of slip-twin and twin-twin interactions on crack initiation and high-order twinning are reviewed. The competitive role of twin nucleationvs crack initiation is discussed. Shortcomings of our current understanding of the role of twinning are indicated, and some futher studies are recommended.

Slip, twinning, and fracture in hexagonal close-packed metals

Diamond may be grown at low pressures where it is the metastable form of carbon. Recent advances in a wide variety of plasma and electrical discharge methods have led to dramatic increases in growth rates. All of these methods have certain aspects in common, namely, the presence of atomic hydrogen and the production of energetic carbon-containing fragments under conditions that support high mobilities on the diamond surface. Some understanding of the processes taking place during nucleation and growth of diamond has been achieved, but detailed molecular mechanisms are not yet known. Related research has led to the discovery of a new class of materials, the "diamondlike" phases. Vapor-grown diamond and diamondlike materials may have eventual applications in abrasives, tool coatings, bearing surfaces, electronics, optics, tribological surfaces, and corrosion protection.

https://science.sciencemag.org/content/sci/241/4868/913.full.pdf

Low-Pressure, Metastable Growth of Diamond and "Diamondlike" Phases

http://meyersgroup.ucsd.edu/papers/journals/meyers 213.pdf

The onset of twinning in metals: a constitutive description

Bonding mechanism in cold gas spraying

We present detailed analyses of x-ray double-crystal rocking curve measurements of superlattices. The technique measures depth profiles of structure factor, and profiles of perpendicular and parallel strains relative to the underlying substrate. In addition to providing a detailed picture of the state of stress, the profiles are a direct measure of the composition modulation. The thickness of the period of modulation and the average strain are determined with a precision of ~1%. The detailed structure of the period is determined to ~5%. We obtain an expression relating the structure of the rocking curve to the structure of the period. This expression allows analytic determination of the structure without Fourier transformation or computer fitting. We show the influence of small random fluctuations in layer thicknesses and strains. The technique is applied to a 15-period GaAlAs/GaAs and a ten-period AlSb/GaSb superlattice grown on GaAs and GaSb substrates, respectively. In the former, the thickness of the period was 676 A and the perpendicular strain varied between zero for the GaAs layer and 0.249% for the layer with peak (93%) Al concentration. Transition regions, ~100 A thick, with continuously varying composition, were found between the GaAs and the Ga0.07 Al0.93As layers. Fluctuations in structural properties were less than 5% of the average. The AlSb/GaSb superlattice had a period of 610 A with sharp transition regions between the layers and negligible fluctuations from period to period. The perpendicular strains were −0.03% and 1.25%, respectively, for the GaSb and AlSb layers. A uniform parallel strain of 0.03% was found throughout the superlattice. Nonzero parallel strain indicates that a small fraction of the misfit between the superlattice and the substrate is plastically accommodated by net edge dislocations lying in a narrow region (a few hundred A thick) at the interface with the substrate. The net number of edge dislocations was calculated to be ~1×10^4/cm^2. The measured perpendicular strains were in excellent agreement with the values calculated from bulk lattice parameters, elastic properties, and the parallel strain. For both superlattices, the standard deviation of random atomic displacements away from perfect crystal sites was below 0.1 A, in agreement with reported ion channeling and electron diffraction measurements of superlattices. The rocking curve method is a major tool for quantitative analysis of superlattices.

/pdf/x-ray-rocking-curve-analysis-of-superlattices-142i1hsv0v.pdf

X‐ray rocking curve analysis of superlattices

A theory for the shock-wave consolidation of powders

The velocities of individual dislocations of edge and mixed types in pure aluminum single crystals were determined as a function of applied‐resolved shear stress and temperature. The dislocation velocities were determined from measurements of the displacements of individual dislocations produced by stress pulses of known duration. The Berg‐Barrett x‐ray technique was employed to observe the dislocations, and stress pulses of 15 to 108 μsec duration were applied by propagating torsional waves along the axes of [111]‐oriented cylindrical crystals. Resolved shear stresses up to 16×10^6 dynes∕cm^2 were applied at temperatures ranging from −150° to +70°C, and dislocation velocities were found to vary from 10 to 2800 cm∕sec over these ranges of stress and temperature. The experimental conditions were such that the dislocation velocities were not significantly influenced by impurities, dislocation curvature, dislocation‐dislocation interactions, or long‐range internal stress fields in the crystals. The velocity of dislocations is found to be linearly proportional to the applied‐resolved shear stress, and to decrease with increasing temperature. Qualitative comparison of these results with existing theories leads to the conclusion that the mobility of individual dislocations in pure aluminum is governed by dislocation‐phonon interactions. The phonon‐viscosity theory of dislocation mobility can be brought into agreement with the experimental results by reasonable choices of the values of certain constants appearing in the theory.

/pdf/mobility-of-dislocations-in-aluminum-2dzjw5429o.pdf

Mobility of Dislocations in Aluminum

The velocity of selectively-introduced edge dislocations in
99.999 percent pure copper crystals has been measured as a function
of stress at temperatures from 66°K to 373°K by means of a torsion
technique. The range of resolved shear stress was 0 to 15 megadynes/
cm^2 for seven temperatures (66°K, 74°K, 83°K, 123°K, 173°K, 296°K, 296°K, 373°K. Dislocation mobility is characterized by two distinct features;
(a) relatively high velocity at low stress (maximum velocities of about
9000 em/sec were realized at low temperatures), and (b) increasing
velocity with decreasing temperature at constant stress. The relation between dislocation velocity and resolved shear
stress is: v = v_o(τ_r/τ_o)^n where v is the dislocation velocity at resolved shear stress τ_r, v_o
is a constant velocity chosen equal to 2000 cm/ sec, τ_o
is the resolved shear stress required to maintain velocity v_o, and n is the mobility coefficient. The experimental results indicate that τ_o decreases from 16.3 x 10^6 to 3.3 x 10^6 dynes/cm^2 and n increases from about 0.9 to 1.1 as the temperature is lowered from 296°K to 66°K. The experimental dislocation behavior is consistent with an
interpretation on the basis of phonon drag. However, the complete
temperature dependence of dislocation mobility could not be closely
approximated by the predictions of one or a combination of mechanisms.

/pdf/an-experimental-study-of-the-mobility-of-edge-dislocations-4v6xnvztn7.pdf

An experimental study of the mobility of edge dislocations in pure copper single crystals

The velocity of dislocations of mixed edge-screw type in copper crystals of 99.999% purity has been measured as a function of stress at room temperature. Dislocation displacements produced by torsion stress pulses of microsecond duration were detected by etch pitting {100} surfaces. A nearly linear relationship between dislocation velocity and resolved shear stress was found. Stresses from 2.8×10^6 to 23.1×10^6 dyn/cm^2 produced velocities from 160 to 710 cm/sec. These data give a value of the damping constant for high-velocity dislocations of 7×10^(-4) dyn·sec/cm^2, in good agreement with the values deduced from internalfriction measurements. The results also agree, within experimental and theoretical uncertainties, with the phonon viscosity model for the mobility of dislocations.

Thad Vreeland

Papers

X‐ray rocking curve analysis of superlattices

A theory for the shock-wave consolidation of powders

Mobility of Dislocations in Aluminum

An experimental study of the mobility of edge dislocations in pure copper single crystals

Dislocation Mobility in Copper