The thin foil transmission electron microscope technique has been used to study the ordering of CuAu I. At low annealing temperatures ordered plate-like CuAu I nuclei are formed which are coherent with the disordered matrix and parallel to the (101) planes. At higher annealing temperatures short range strain accommodations in the form of microtwinning occur at first and later long range accommodations in the form of coarse twin lamellae. The latter result from the accommodations to strains set up by the growth of ordered lattice domains whose nucleation centers had different c-axis orientations. The orientation of the twin planes is (101) and the twins appear to be nucleated by other twins or on free surfaces and not by dislocations. The ordered domains become disordered at the twin interfaces but remain intact within the lamellae. The increase in mechanical strength upon ordering is related to the coherency strains between ordered nuclei and disordered matrix and the decline of the mechanical strength is due to the elimination of coherency strains by twinning.

Study of CuAu I by Transmission Electron Microscopy

Publisher Summary This chapter discusses vibrational properties of shape-memory alloys. The crystal structure of a given material depends on its energy and also its entropy, which plays an increasingly important role as temperature is raised. In the case of nonmagnetic metallic systems, the entropy has two major contributions: the electronic contribution arising from electronic states near the Fermi level and the vibrational contribution, related to the vibrations of the atoms around their equilibrium positions in the crystal lattice. The present chapter is concerned with a class of bcc-based metallic alloys, commonly called shape-memory alloys. These materials have the unique property of being able to recover from large deformations by slightly increasing their temperature. The physical mechanism behind this effect is a diffusionless, first-order structural transition, usually referred to as the martensitic transition. Martensites form when a high-temperature crystalline phase undergoes a first-order phase transition to a lower symmetry crystalline phase. This structural phase transition usually proceeds by means of the nucleation of the new phase within the initial one, and may be induced either by changing the temperature or by applying a mechanical stress.

Vibrational properties of shape-memory alloys

A kinetic model based on the theory of strain-induced interactions is developed to describe the effect of elastic strain on structural transformation kinetics. An ordering transition resulting in a reduction in the crystal lattice point-group symmetry in a substitutional solid solution is considered. Our computer simulation of a two-dimensional model binary alloy revealed the detailed kinetics of formation of tweed morphologies and the tweed → twin coarsening process driven by the transformation-induced elastic strains.

Kinetics of tweed and twin formation during an ordering transition in a substitutional solid solution

Origin of chessboard-like structures in decomposing alloys. Theoretical model and computer simulation

Two complete classes of freshman dental and dental hygiene students, 120 men and 102 women (mean age 23.9 years), were assessed for the presence of masticatory pain or dysfunction by questionnaire, clinical examination, and evaluation of dental casts. The purpose of these examinations was to determine potential relationships between clinical muscle tenderness, occlusal relationships, and signs of TMJ dysfunction. Awareness of muscle tenderness increased with the number of muscle sites involved ( p ≤ .025) but 80% of clinically tender subjects were unaware of any tenderness ( p ≤ .01). In comparison, subjects with generalized clinical muscle tenderness more often reported TMJ clicking that was not verified at the time of clinical examination ( p ≤ .001). Occlusal factors, except in highly selective categories, were not associated with muscle tenderness. All subjects with moderate or severe TMJ tenderness had clinically tender muscle sites, whereas subjects with generalized muscle tenderness (≥4 sites) had more severe TMJ tenderness ( p ≤ .01). Subjects with localized ( p p p ≤ .001). TMJ dysfunction was verified more often in subjects with more localized muscle tenderness ( p ≤ .025). Although occlusal factors were not good predictors of muscle tenderness, intracapsular signs of TMJ disorders and muscle tenderness were often associated.

Temporomandibular disorders. Part III: Occlusal and articular factors associated with muscle tenderness

The correlation between microstructure and hardness of dental gold alloy, Au-35.7 at.% Cu-11.2 at.% Ag, was studied by electron microscopy. The age-hardening peak was due to the formation of plate-like AuCu I on the matrix {100} planes. The microstructure at this stage showed a striation contrast, being parallel to the 〈111〉 directions on the (110) plane, as found in the AuCu alloy. The structure of the final state was identified as AuCu I-type ordered platelets which had the c -axis distributed in the three cube directions and had the [011] twin relation, although relatively higher hardness was maintained at the later stage of ageing.

Structural changes caused by age-hardening in a dental gold alloy

Electron Microscope Observation in an Age-Hardenable Dental Gold Alloy

The composition and temperature range of the AuCu-type superlattice formation in the Au-Cu-Ag ternary system was determined by X-ray powder diffraction. Four distinguishable phase transformations were detected, i.e., 1) α (disordered FCC phase) → AuCu I, 2) α → AuCu II, 3) α → α1 (Cu-rich FCC phase) + α 2 (Ag-rich FCC phase), and 4) α → AuCu3. The stable temperature range of the AuCu I-type superlattice was restricted to a lower temperature range by adding silver to stoichiometric and off-stoichiometric Au-Cu alloys. Depending on the increase in silver content, the co-existing region of the AuCu II-type superlattice and the silver-rich α2 phase extended to the lower temperature and over a wide composition range.

Determination of the AuCu Superlattice Formation Region in Gold-Copper-Silver Ternary System

Transmission electron microscopy and electron diffraction studies have been carried out in order to make clear the correlation between the microstructure and age-hardening of a dental gold alloy Au-43.2 at.% Cu13.8 at.% Ag. At lower ageing temperatures the hardening is due to the development of a long period antiphase domain structure of the AuCu-II type. The strain field induced by the AuCu-I type superlattice coexisting with the AuCu-II type superlattice also contributes to the. age-hardening. An alternating coarse lamellar structure is formed by the mechanism of discontinuous decomposition at higher ageing temperatures. The phases composing this structure have been identified as the copper-rich α 1 phase and the silver-rich α 2 phase, which are both f.c.c. in structure. No age-hardening was observed in this temperature range. In the middle temperature range (350–400 °C) the α 1 phase becomes ordered, resulting in the AuCu-II structure type, but remains intact within the lamellae. The increase in hardness on ordering is related to the volume fraction of the ordered phase.

Yasuo Kanzawa

Papers

Structural changes caused by age-hardening in a dental gold alloy

Electron Microscope Observation in an Age-Hardenable Dental Gold Alloy

Determination of the AuCu Superlattice Formation Region in Gold-Copper-Silver Ternary System

Structure and morphology of an age-hardened goldcopper-silver dental alloy