W. A. T. Clark

Dislocation and grain boundary interactions in metals

Abstract The passage of dislocations through grain boundaries in face centered cubic and body centered cubic polycrystalline metals was studied using dynamic in situ high voltage electron microscopy (HVEM), static transmission electron microscopy (TEM), and anisotropic elastic stress analysis. Several conclusions were reached: (1) when dislocations propagate across grain boundaries, the activated slip system can be predicted from pile-up properties and grain boundary orientation using a combined criterion based on boundary geometric factors and internal stresses; (2) different grain boundaries impede dislocation slip propagation to different degrees, the calculated value of the pile-up obstacle stress varying from 280 to 870 MPa for dislocation transmission through a grain boundary in 304 stainless steel; (3) dynamic in situ straining of miniature tensile specimens reveals additional modes of dislocation and grain boundary interactions that were hidden from static TEM observations. In connection with the last conclusion, simultaneous dislocation transmission and reflection was activated by a stressed pile-up and a complex mechanism involving coordinated movements of four sets of dilocations in and near a grain boundary was observed.

Orientation determination of subsurface cells generated by sliding

Abstract The near-surface material of copper samples has been examined by transmission electron microscopy (TEM) and STEM (scanning TEM) techniques after sliding tests of varying duration. The development of deformation substructure is described, and detailed orientation information is obtained with the aid of an improved technique which uses a minicomputer to analyse Kikuchi line patterns from individual cells or sub-grains. Large plastic strains and large rotation angles are achieved after very short sliding distances. The principal rotation is about the transverse axis, i.e. the axis normal to the sliding direction and parallel to the sliding interface. No tendency for alignment of close-packed planes with the sliding interface has been found.

Metallurgical Characterization of a New Nickel-Titanium Wire for Rotary Endodontic Instruments

INTRODUCTION A novel thermomechanical processing procedure has been developed that yields a superelastic (SE) nickel-titanium (NiTi) wire (M-Wire) that laboratory testing shows has improved mechanical properties compared with conventional SE austenitic NiTi wires used for manufacture of rotary instruments. The objective of this study was to determine the origin of the improved mechanical properties. METHOD Specimens from 2 batches of M-Wire prepared under different processing conditions and from 1 batch of standard-processed SE wire for rotary instruments were examined by scanning transmission electron microscopy, temperature-modulated differential scanning calorimetry, micro-x-ray diffraction, and scanning electron microscopy with x-ray energy-dispersive spectrometric analyses. RESULTS The processing for M-Wire yields a microstructure containing martensite, that the proportions of NiTi phases depend on processing conditions, and that the microstructure exhibits pronounced evidence of alloy strengthening. CONCLUSIONS The presence of Ti(2)Ni precipitates in both microstructures indicates that M-Wire and the conventional SE wire for rotary instruments are titanium-rich.

Dissociated and faceted large-angle coincident-site-lattice boundaries in silicon

Abstract The interaction of three large-angle near-coincident-site-lattice grain boundaries ([Sgrave]3, [Sgrave]9 and [Sgrave]27) in a polycrystal of silicon, has been observed both optically and by transmission electron microscopy (TEM). [Sgrave]3 boundaries were found to be formed on {111} and {112} planes, [Sgrave]9 on {122}, {411} and {111}/{115} planes, and [Sgrave]27 on {255} planes. On a macroscopic scale, [Sgrave]9 {122} dissociated repeatedly into two coherent [Sgrave]3 boundaries which, when contacted with a third non-coherent [Sgrave]3 boundary, formed a triangular twin grain with an average size of 0·3 mm. Further TEM observations showed the [Sgrave]27 boundary to dissociate into a series of 100nm triangular grains, consisting of a coherent [Sgrave]3, a {122} [Sgrave]9 and a {111}/{115} [Sgrave]9 boundary. The [Sgrave]9 boundary {111}/{115} dissociated further to form still smaller 10nm twin grains consisting of two coherent and one noncoherent [Sgrave]3 boundaries. The dissociation reactions ...

The nucleation kinetics, crystallography, and mechanism of the massive transformation

Experimental and theoretical evidence on the role of special crystallographic orientation relationships in massive transformations is critically reviewed. The implications of these and of the results of interfacial structure and morphological studies for the nucleation mechanism of the massive transformation are considered. The role of crystallography during the growth process of massive transformation is also briefly discussed. The conclusion is reached that orientation relationships capable of producing low energy, zero mobility interfaces play the same type of dominant roles in the nucleation and growth of the massive transformation as they do in precipitation from solid solution.

Computerized method to determine crystal orientations from Kikuchi patterns

Abstract A computerized method for determining crystal orientations from Kikuchi line patterns is described. The method is rapid and easy to use. When the Bravais lattice is known, the user need only specify a reference direction and the coordinates of the origin and of two points on each of six Kikuchi lines (three pairs). The programs yield the hkl indices of the Kikuchi lines, the beam direction, and the rotation matrix relating the crystal to a chosen reference frame. This rotation matrix can be used for various purposes, e.g., to specify an angle of rotation around a chosen axis or to calculate the misorientation between any two crystals.

Microstructure of a quenched and tempered Cu-bearing high-strength low-alloy steel

The grain structure, inclusion content, and precipitate types were characterized for six heat treatments of a copper-bearing HSLA steel. A higher austenitizing temperature combined with water-quenching resulted in an acicular ferrite microstructure, while lower temperatures produced equiaxed ferrite or ferrite-pearlite structures. Refinement of equiaxed ferrite was observed in material austenitized at a high enough temperature to dissolve a portion of the carbonitrides, allowing reprecipitation during the austenite-ferrite transformation. Age-hardening precipitated body-centered cubic copper clusters; face-centered cubic copper precipitates were observed in overaged material.

X-ray diffraction study of low-temperature phase transformations in nickel–titanium orthodontic wires

OBJECTIVES Employ conventional X-ray diffraction (XRD) to analyze three clinically important nickel-titanium orthodontic wire alloys over a range of temperatures between 25 and -110 degrees C, for comparison with previous results from temperature-modulated differential scanning calorimetry (TMDSC) studies. METHODS The archwires selected were 35 degrees C Copper Ni-Ti (Ormco), Neo Sentalloy (GAC International), and Nitinol SE (3M Unitek). Neo Sentalloy, which exhibits superelastic behavior, is marketed as having shape memory in the oral environment, and Nitinol SE and 35 degrees C Copper Ni-Ti also exhibit superelastic behavior. All archwires had dimensions of 0.016in.x0.022in. (0.41 mm x 0.56 mm). Straight segments cut with a water-cooled diamond saw were placed side-by-side to yield a 1 cm x 1cm test sample of each wire product for XRD analysis (Rint-Ultima(+), Rigaku) over a 2theta range from 30 degrees to 130 degrees and at successive temperatures of 25, -110, -60, -20, 0 and 25 degrees C. RESULTS The phases revealed by XRD at the different analysis temperatures were in good agreement with those found in previous TMDSC studies of transformations in these alloys, in particular verifying the presence of R-phase at 25 degrees C. Precise comparisons are not possible because of the approximate nature of the transformation temperatures determined by TMDSC and the preferred crystallographic orientation present in the wires. New XRD peaks appear to result from low-temperature transformation in martensite, which a recent transmission electron microscopy (TEM) study has shown to arise from twinning. SIGNIFICANCE While XRD is a useful technique to study phases in nickel-titanium orthodontic wires and their transformations as a function of temperature, optimum insight is obtained when XRD analyses are combined with complementary TMDSC and TEM study of the wires.

Activation of slip in lamellae of α2-Ti3Al in TiAl alloys

The activation of slip at room temperature in lamellae of α2-Ti3Al in a Ti- 48 at.% Al alloy has been studied using transmission electron microscopy. In common with other studies, it appears that slip activity occurs mainly at, or near to, the intersection of certain cross-lamellar twins, formed by deformation in γ-TiAl, with the α2 lamellae. In polycrystalline samples, heat treated to a duplex microstructure with a significant lamellar component, only dislocations with Burgers vectors given by b = 1/3 have been activated, and essentially no dislocations with b = 1/3 , that is c -component dislocations, have been observed after deformation in tension at room temperature. It has been found that, for these samples slip activity in α2-Ti3 Al may occur either by a slip transmission process or by stress-induced activation of sources in the interfaces between the two phases (α2-Ti3Al and γ-TiAl). The resultant defects are superdislocations with Burgers vectors b = 1/3 gliding on prismatic {1100...

In Situ observation of copper oxidation at high temperatures

The high temperature oxidation of OFHC copper was studied inside an environmental hot-stage scanning electron microscope (HSESEM). The evolution of oxidation morphologies was observed at temperatures above 850 °C for a maximum oxygen pressure of 3 × 10-4 atm. The oxidation products were characterized by high resolution SEM, STEM, EELS, and microdiffraction analysis. Two simultaneous mechanisms were observed: the growth of solid cuprous oxide crystals, and between and on top of these grains, the formation of a solid copper oxide fume. The formation of this oxide fume is very dependent upon the oxygen pressure, the temperature, and the experimental treatment of the specimen. Upon cooling, the cuprous oxide fume undergoes a reversible transformation to cupric oxide at about 519 °C, and thus cannot be retained unchanged for post-oxidation analysis. The formation of microfissures in the scale, rapid oxide grain growth, and scale spallation upon thermal cycling were observedin situ. The initial heating of the copper in hydrogen (as contrasted to the SEM vacuum) resulted in more rapid scale growth and reduced fume formation.