Hardening of Dental Gold-Copper Alloys
X-ray diffraction studies of several proprietary dental gold alloys demonstrated that hardening probably is a dual mechanism of precipitation and order-hardening. Lattice parameter measurements showed that the ordered structure was AuCu. In addition, it was shown that maximum hardness occurs at about 75% full order.
Order-hardening in CuAu
Single crystals of CuAu were used to examine the changes in hardness, flow stress, and microstructure resulting from heat treatment at 150 to 340° C, which produces the tetragonal superlattice. The crystals, quenched from a high temperature, were in the disordered cubic form to begin with; some were also cold-rolled, to permit comparison of the ageing characteristics of as-quenched and of cold-worked specimens. The asquenched crystal hardened much more than the cold-rolled ones; a peak hardness was attained and followed by a fall in hardness. At 150° C, no microstructural changes were seen; at 240° C, spontaneous recrystallisation took place at grain boundaries; while at 340° C, there was twinning and spontaneous grain-boundary fracture. The observations are rationalised in terms of the various mechanisms for relief of the large microstrains that accompany ordering in this alloy.
Strain-ageing of ordering alloys, with special reference to the nickel-iron system
Abstract Nickel-iron alloys containing 20–34 at. % iron were strain-aged just below the critical ordering temperature (500°C), and indentation hardness changes determined. All alloys hardened rapidly during the first few minutes, but further hardening during prolonged annealing was restricted to alloys in which considerable long range order developed (approximate range, 23–30 at. % nickel). Other alloys progressively recrystallised and softened; the origin of this relationship between ordering behaviour and recrystallisation is analysed. Tensile tests showed that the short term hardening is due to a change in flow stress, while the long term hardening is caused by a greatly enhanced rate of work-hardening. Both forms of hardening became more pronounced with increasing prestrain. Strain-ageing experiments on Ni-Fe-Al alloys, which order more rapidly than the binary alloys, show that their strain-age hardening is also more rapid. Tensile tests on Cu3Au and Au3Cu also showed enhanced work-hardening rates after strain-ageing. The nature of the increase of flow stress in briefly strain-aged Ni-Fe alloys is discussed, with special reference to the possible role of antiferromagnetic bonds; short range order is the most probable cause of this prompt hardening. The increase of work-hardening rate resulting from prolonged anneals is interpreted in terms of the production of antiphase domain boundaries by intersecting superdislocations.