Age-hardening associated with precipitation reaction and spinodal decomposition in a commercial dental low-carat Au–Ag–Cu–Pd alloy

Two distinguishable hardening mechanisms, depending on the temperature, were found by isothermal ageing in a commercial dental low-carat Au–Ag–Cu–Pd alloy. Age-hardening was attributed to the precipitation of the metastable AuCu I and equilibrium AuCu I ordered phases and spinodal decomposition depending on the ageing temperature. It was clearly visible, by using the direct-ageing method, that the XRD peaks of the parent phases showed a shift during transformation.     

The effect of δ alumina fibre arrays on the age-hardening characteristics of an Al-Mg-Si alloy

The effect of alumina fibre arrays on the age-hardening characteristics of an Al-Mg-Si alloy (6061) has been investigated by means of hardness and electrical resistivity measurements, and optical, scanning and transmission electron microscopy. It is shown that the fibre array can have a considerable effect on the age-hardening response of the matrix alloy in metal matrix composites, causing suppression of GP zone formation which inhibits natural ageing and considerably reduces the peak hardening produced during artificial ageing. The reduced hardening potential of the composites during artificial ageing is shown to result from a competition between GP zone formation and heterogeneous nucleation of the intermediate precipitate on lattice defects. The most probable cause of both phenomena is shown to be lack of quenched-in vacancies following solution treatment, due to the availability of a large number of vacancy sinks at the fibre-matrix interfaces.     

Age hardening and precipitation in a cast magnesium-rare-earth alloy

The precipitation sequence responsible for the age-hardening behaviour of a cast Mg-1.3 wt% rare-earth alloy has been investigated by analytical electron microscopy. Very fine intermediate precipitates formed at an early stage of ageing. Plate-shaped Mg₃MM precipitates (MM = misch metal) and hexagonal prism-shaped Mg₁₂MM precipitates were primarily responsible for age hardening. Precipitate morphologies, crystal structures and crystallographic orientation relationship were determined for the various types of precipitates that formed during ageing at different temperatures.     

γ′ particle coarsening and yield in Alloy 800

The age strengthening of Alloy 800 by ordered particles of-Ni₃(Al, Ti) phase has been studied by using both transmission electron microscopy and room-temperature tensile tests on aged specimens. The samples have been aged in the temperature range 525 to 650° C up to a maximum time of 10⁴ h. Two groups of samples with different Ti/Al ratios have been investigated in order to obtain more reliable information on the role played by these alloying elements on the age-hardening behaviour. The linear dimension increases with time,t, ast ^1/3 and an activation energy of 70.0 kcal mol^–1 was derived from the temperature dependence of the coarsening rate. The particle volume-fraction, as measured by electron microscopy, has been found to remain constant on ageing in the temperature range 525 to 600° C. The increase in the critical shear stress due to the particles is found to agree quantitatively with the equations of Brown and Ham which describe hardening by ordered particles. Antiphase boundary energies of 227 and 279 mJ m^–2 have been measured, respectively, for the two groups of samples investigated.     

Atomic ordering and α′-Cr phase precipitation in long-term aged Ni3Cr and Ni2Cr alloys

Phase instabilities of binary Ni₂Cr and Ni₃Cr alloys after long-term exposure at temperatures between 450 and 600 °C were studied by monitoring lattice parameter, electrical resistivity and microhardness variations and by analysing the microstructural evolution of the alloys at interrupted annealing times. Both materials undergo two metallurgical changes: atomic ordering, based on the Ni₂Cr superlattice, and the precipitation of the -Cr phase. Short-range order develops in both alloys during the first's hours of ageing. The degree of order and kinetics of ordering transformation depend on the alloy composition, time and temperature of ageing. In Ni₂Cr, the short-range ordered structure transforms to long-range order after ageing for 100–1000 h below 525 °C, but in Ni₃Cr the transformation occurs after 30 000 h. The embrittling -Cr phase precipitates at grain boundaries in both alloys after long-term ageing, mainly at 550–600 °C. The amount and size of particles increase with time and temperature of ageing.     

Coarsening kinetics of γ′ precipitates in Ni-Al and Ni-Al-Mo alloys

The ordered L1₂ precipitate coarsening kinetics without the influence of an external stress were studied in a Ni-Al(13.5 at%) alloy at 1413 K and a Ni-Al(13.8 at%)-Mo(5.9 at%) alloy at 1443 K. The Ni-Al-Mo alloy has a lattice mismatch of about –0.3% at the ageing temperature while the Ni-Al has a positive lattice mismatch of about 0.25% at the ageing temperature. For both alloys, the precipitates were initially cuboidal. After ageing for 3–10 min, the precipitates in the Ni-Al-Mo alloy split mostly into two parallel plates (doublets) or eight sub-cubes (octets), but the initial cuboidal precipitates in the Ni-Al alloy only showed the tendency to split into doublets. After further ageing, the precipitates in both alloys eventually aligned and agglomerated into groups consisting of many particles separated by a small distance of 30 nm, and the distribution of the precipitates became inhomogeneous. There was no linear relationship between the cube of the average precipitate size and the ageing time as predicted by the classical Lifshitz-Slyozov-Wagner theory. Instead, a retardation of the coarsening process is found.     

Ordering within precipitates in copper-nickel-titanium alloys

The precipitation process in alloys containing 4% Ti and 5 to 16% Ni was investigated using transmission electron microscopy, hardness and electrical conductivity measurements. After ageing, spherical -Ni₃Ti precipitates with ordered DO₂₂ superlattice were observed in alloys CuTiNi5 to 10 and the L1₂ superlattice in the CuTi4Ni16 alloy. With increasing nickel content the size of precipitates decreases; this has only a minor influence on the age hardening. The CuTiNi10 alloy has the best electrical conductivity approaching 18m^–1mm^–2.     

Dental gold alloys with age-hardenability at intraoral temperature

Dental gold alloys with age-hardenability at intraoral temperature were developed. Either 3 or 6 at% Ga, Al, Zn, In, Ni or Pd were added to an equiatomic AuCu alloy and the effect of additives on the low-temperature age-hardenability was examined. Alloys containing Ga, Al or Zn exhibited excellent low-temperature age-hardenability. The hardness of an alloy containing 6 at% Ga or 6 at% Al was doubled in one to ten days by ageing at 37 °C. Pronounced hardening was not demonstrated in an as-cast alloy, but solution treatment for only a few minutes provided enough age-hardenability. The age-hardening rate at low temperature related closely with the melting temperature of the alloy. Experimental gold alloys exhibited electrochemical behaviour similar to that of a commercial Type IV gold alloy. It is therefore concluded that AuCu alloy with added Ga, Al or Zn is expected to have enough age-hardenability and corrosion resistance for clinical use in the oral environment.     

The role of silicon in the formation of the (Al5Cu6Mg2) σ phase in Al-Cu-Mg alloys

The role of silicon in the precipitation of the phase (Al₅Cu₆Mg₂) has been investigated through comparative studies on Al-3.63Cu-1.67Mg (wt%) and Al-3.63Cu-1.67Mg-0.5Si alloys. Both alloys were extensively examined after solution treating at 525°C for 2.5 h followed by ageing at 265°C for times up to 650 h. Limited studies were also undertaken on both alloys after ageing at 200 and 305°C. Precipitation of was observed in Al-3.62%Cu-1.66%Mg-0.5%Si for all ageing conditions studied but was absent in Si-free Al-3.62%Cu-1.66%Mg. In addition, S and phases were observed in both alloys. The volume fraction of phase in the Si containing alloy was substantially reduced by a pre-age stretch followed by ageing for 24 h at 265°C with S being the dominant precipitate type. The volume fraction of phase in the Si containing alloy was lower after ageing 24 h at 200°C than after 24 h at 265 and 305°C. Peak hardness was higher for the Si free alloy on ageing at 200 and 265°C, but the Si free alloy softened more rapidly, reflecting the more rapid coarsening kinetics of S compared with .     

Precipitation hardening of Cu-4Ti-1Cd alloy

Precipitation hardening of Cu-4Ti-1Cd alloy has been studied using hardness measurements and transmission electron microscopy. This alloy exhibited hardness of 238 Hv in solution treated (ST) condition and attained peak hardness of 318 Hv after ageing at 450°C for 40 h. Electrical conductivity of Cu-4Ti-1Cd alloy increased from 5.7 %IACS (International Annealed Copper standard) in ST condition to 8.9 %IACS on ageing at 450°C for 16 h. This alloy exhibited markedly higher yield strength (751 MPa in the peak-aged condition) compared to Cu-4.5Ti alloy but the increase in UTS due to cadmium addition was less significant. The higher yield strength of ternary alloy in peak aged condition is due to the solid solution strengthening of cadmium as well as the presence of -Cu₄Ti precipitate. On over-ageing the alloy showed a decrease in hardness as a result of formation of equilibrium precipitate, -Cu₃Ti. Optical microscopy reveals single phase with equiaxed grains in solution treated condition. A coherent, metastable phase -Cu₄Ti is responsible for high strength and hardness in peak aged condition. The over-ageing in this alloy shows the formation of cellular structure at the grain boundaries of the matrix phase.     

On the ageing of an aluminium-lithium-zirconium alloy

Electron microscopy has been used to follow the ageing of an aluminium alloy containing 3.08 wt % lithium and 0.19 wt % zirconium over the temperature range 433 to 553 K. A dispersion of Al₃Zr particles was present before these ageing treatments and is unmodified by them. Two dispersions of Al₃Li () are produced by these ageing treatments, one is formed homogeneously in the matrix while the other nucleates and coarsens on the Al₃Zr/matrix interface. From the data it appears that there is little interaction between the lithium and zirconium in solution and that the precipitation processes occur chemically independently. The coarsening characteristics of both dispersions of have been investigated as has the discontinuous precipitation of .     

Microstructural changes during ageing of Cu-2.5 wt% Ti

Microstructural changes in Cu-2.5wt % Ti during ageing at 573 and 773 K have been studied by transmission electron microscopy. Ageing times ranged from 60 sec to 200 h. Ordering of the precipitates was observed after very short ageing treatments at 773 K, and coarsening according tot ^1/3 was also observed from very early times. The particles were observed to become increasingly aligned into rod-like groups along 10 0 as ageing progressed. These results permit a new interpretation of the strengthening mechanisms in this alloy.     

Josephson transport in Tl-cuprate bicrystal weak links

Superconducting epitaxial Tl₂Ba₂CaCu₂O₈ (Tl-2212) and Tl₂Ba₂CuO₆, (Tl-2201) thin films are synthesised on single crystal and bicrystal LaAlO₃ substrates by an ex-situ process. Tl-2212 films were optically smooth, with T_cs from 105 to 108K and T_c <2K, J_c reaching 2·10⁶A/cm² at 77K. Tl-2201 films exhibited tetragonal or orthorhombic symmetry depending on the thallium content. The T_c of the as-synthesised Tl-2201 films was in the range 40–55K. By oxygen depletion, T_c was increased by 15 – 20 K. Properties of weak links in Tl-2212 and Tl-2201 thin films are reported.     

Age-hardening and the associated phase transformation in an Au-55.2 at% Cu-17.4 at% Ag ternary alloy

To develop a low gold content dental alloy, age-hardening characteristics in an Au-55.2 at% Cu-17.4 at% Ag alloy were studied by means of hardness, electron microscopy and X-ray diffraction examination. Three distinct age-hardening behaviours depending on temperature were found in the alloy, i.e. (i) a dual mechanism of spinodal decomposition and Cu₃Au ordering below 673 K, (ii) a single mechanism of spinodal decomposition at 693 K, and (iii) a single mechanism of nucleation and growth of silver-rich precipitate at 773 K. A marked over-ageing was observed by lengthy ageing over the whole range of temperature. The long-period superstructure of Cu₃Au(II) was found only in the grain boundary product at temperatures between 623 and 633 K.     

Band-theoretical calculation for the anomalous resistivity of Ni near the Curie temperature

The anomalous resistivity of nickel near the Curie temperature T _c is investigated using the itinerant model of the magnetic electron. The 3d-band wave functions are used to calculate the form factor. The temperature derivative of resistivity diverges positively when T approaches T _c from below and negatively when it approaches from above. The calculation of the correlation function shows that both short-range and long-range orders exist. The critical exponents = 1/2 and = 0 are equal to the Ornstein—Zernike values in the paramagnetic region, but in the ferromagnetic region in addition to these values ₁ = 1/2 and ₁ = 0 there is simultaneously a second set of values ₂ = and ₂ = –1.     

Simultaneous ac Susceptibility and Specific Heat Measurements of YBCO Ceramics

Simultaneous ac magnetic susceptibility and specific heat studies of YBCO ceramics permitted the correct correlation of the corresponding critical temperatures. The midpoint of the real component () and the maximum of the imaginary component () of ac susceptibility curves confirmed the T _c = 92 K critical temperature value determined by dc resistance measurements. The specific heat maximum's temperature (T _cc 93.6 K)—-taken as the calorimetric critical one—fell between T _c and T* (the pseudogap onset temperature). We suggest that T _cc signals the appearance of the first localized coherent pairs, while the temperature T _c marks their percolation. The existence of two different critical temperature values confirms the coexistence of partial coherent and coherent electron pairing phenomena in the region below T _cc of the superconducting phase diagram.     

Effects of Zn addition to AuCu on age-hardening behaviors at intraoral temperature

The effect of Zn addition to AuCu on the age-hardening rate at the intraoral temperature was investigated to find out the proper condition for high age-hardening rate. The increase in hardness of Zn-added alloys during aging at 37 °C was due to the atomic ordering. With an increase in Zn concentration, hardness of a sample under the as-quenched condition decreased, but the age-hardening rate obviously increased. When Zn content was fixed, a higher solution treatment temperature was more effective for the age-hardening at 37 °C. It was suggested that the formation energy of a vacancy considerably decreased with an increase in Zn content. It is reasonable to consider that the amount of quenched-in excess vacancies are markedly increased with an increase in Zn content when the solution treatment temperature was fixed. By transmission electron microscopic observations, it was revealed that the formation of the AuCu II superstructure contributed to the age-hardening at 37 °C in the high zinc content alloy.     

The modulated structure formed by isothermal ageing in ZrO2-5.2 mol % Y2O3 alloy

The modulated structure produced by isothermal ageing of ZrO₂-5.2 mol % Y₂O₃ alloy was examined mainly by electron microscopy. It was found that the modulated structure was formed at ageing temperatures between 1400 and 1600° C, but not at 1700° C. The structure is developed by spinodal decomposition, which produces compositional fluctuation in the elastically soft 111 direction in cubic zirconia. The hardness increase caused by the development of modulated structure during ageing is larger than the hardening by precipitation of tetragonal phase in the cubic matrix.Graduate Student, Tohoku Univerisy, Sendai, Japan.     

Age-hardening by miscibility limit of Au–Pt and Ag–Cu systems in an Au–Ag–Cu–Pt alloy

The age-hardening by miscibility limit of Au–Pt and Ag–Cu systems in an Au–Ag–Cu–Pt alloy was examined by characterizing the hardening behavior, phase transformations and changes in microstructure, and elemental distribution during aging. The hardness increased by the transformation of the parent α₀ phase into the α₁ and metastable AuCu I′ phases, but not by the further transformation of the metastable AuCu I′ phase into the stable AuCu I phase due to the simultaneously initiated lamellar-forming grain boundary reaction. The replacing ratio of matrix by lamellar structure was not directly proportional to the AuCu I phase formation. The relatively high Pt content caused the severe exclusion of Au from the Cu-rich layer of the lamellar structure due to the overlapped miscibility limit of both Au–Pt and Ag–Cu systems.     

Long-term ageing characteristics of some commercial nickel-chromium-molybdenum alloys

The long-term ageing characteristics of some commercial Ni-Mo-Cr alloys (the high-temperature HASTELLOY alloy S and the corrosion resistant HASTELLOY alloys C-4 and C-276) at 810 K were investigated. It was found that the three alloys undergo the following long-range ordering reaction: disordered f c c lattice ordered orthorhombic, Pt₂Mo-type superlattice. Ordering was found to cause considerable strengthening without severe loss of tensile elongation. Deformation in the ordered state occurred predominantly by twinning. The corrosion rates of alloys C-4 and C-276 in boiling sulphuric-ferric sulphate solution did not seem to be greatly affected by the long-range ordering reaction. In addition to ordering, the three alloys were also found to undergo grain boundary reactions. The resulting phase in alloys S and C-4 assumed a dispersed morphology and was identified as carbide, probably of the Type M₁₂C. In alloy C-276, however, which contains higher amounts of iron and tungsten, the boundary precipitate was in the form of a continuous layer consisting of M₁₂C and Mu-phase. This could account for the reduced tensile elongation of alloy C-276 relative to alloys S and C-4 and also to its high corrosion rate.