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.     

Effects of Au/Cu ratio and gallium content on the low-temperature age-hardening in Au-Cu-Ga alloys

The effects of Au/Cu ratio and gallium content on the low-temperature age-hardening were examined in Au-Cu binary and Au-Cu-Ga ternary alloys by hardness testing, X-ray diffraction and differential thermal analysis. In alloys in which the Au/Cu ratios greatly deviated from the equiatomic one, age-hardenability at 37 °C significantly decreased. Gallium addition lowered the liquidus temperature and increased the age-hardening rate. It is considered that the diffusion rate of the constituents increased with lowering of the liquidus temperature, and that consequently, gallium addition improved the age-hardenability.     

Age-hardening behaviors and grain boundary discontinuous precipitation in a Pd-free gold alloy for porcelain bonding

Isothermal age-hardening behaviors at 400° and 450 °C and discontinuous precipitation reaction at 450 °C in a commercial Pd-free gold alloy for porcelain bonding were investigated by hardness testing, X-ray powder diffraction, and light microscopy. Variations of electrical resistivity during continuous heating and cooling processes were also measured. The alloy exhibited pronounced age-hardening in the early stage of aging and the maximum hardness exceeded twice that of the solution-treated sample. Precise lattice parameter measurements and investigations of full width at half maximum values for the X-ray Bragg reflections implied that nonuniform strains due to the pre-precipitation or zone formation was responsible for the quick and pronounced age-hardening at 450 °C. Discontinuous precipitation reaction, producing a mixture of a small amount of Pt₃In-phase with the L1₂-type superstructure and a large amount of (Pt, In)-depleted solid solution, started at grain boundaries in the late stage of aging process at 450 °C. The growth of the grain boundary discontinuous precipitates toward the intragrain area led to a gradual decrease in hardness of the alloy.     

Strengthening mechanisms in Elgiloy

The effects of cold-working and age-hardening on the microstructure of Elgiloy (composition in wt %, 40 Co, 20 Cr, 15 Ni, 7 Mo, 2 Mn, 0.1 C, balance Fe) have been established. 36% and 72% cold reduction of the solution treated (ST) alloy (f c c) produced a network of thin f c c deformation twins and h c p-martensite platelets. The subsequent age-hardening of the cold-worked strip noted at 500° C was attributed to the formation of additional e-phase (via the _{f c c h c p} transformation), whereas the ST condition did not harden significantly at this temperature. In contrast, ageing the ST condition at 800° C for periods >1000 h caused an increase in hardness due to the formation of a coarse Mo-Co-Cr intermetallic compound together with a smaller amount of M₂₃C₆ carbide, while age softening of the cold-worked strip occurred at this temperature as a result of recovery and dissolution of the deformation induced-phase.     

Influence of aging treatments and alloying additives on the hardness of Al–11Si–2.5Cu–Mg alloys

This study investigated the effects of cooling rate, heat treatment as well as additions of Mn and Sr on hardness and hardening characteristics in Al–11Si–2.5Cu–Mg alloys. The results of scanning electron microscopy reveal that the age-hardening behaviour is related to the precipitation sequence of alloy. An energy dispersive spectroscopy analysis was used to identify the precipitated phases. The results also show that the hardness of the solution heat-treated samples is higher in air-cooled alloys than in furnace-cooled ones. Furthermore, the hardness observed in solution heat-treated samples is higher than in as-cast samples for air-cooled alloys, with the highest hardness level in the non-modified alloys. The highest hardness levels among the artificially aged samples were observed in the non-modified, air-cooled alloys. These levels occur after aging for longer times at lower temperatures (e.g. 30 h at 155 °C). The alloys studied did not display any softening after 44 h at 155 °C, whereas at 180 °C, softening was noted to occur after 10–15 h. At short aging times of 5–10 h, high hardness values may be obtained by aging at 180 °C. At aging temperatures of 200 °C, 220 °C and 240 °C, softening began after 2 h had elapsed. The cooling rate during solidification does not appear to have any significant effect on the precipitation characteristics and hardness of the Sr-modified alloys at certain aging temperatures. On the other hand, the effects of cooling rate may be clearly observed in the non-modified alloys. Manganese has a minimal effect on the hardness of the aged samples as it diminishes the potential action of age-hardening, while strontium lessens the hardness of the artificially aged samples. The effect of strontium, however, is more pronounced in the air-cooled alloys than in the furnace-cooled alloys. Strontium also has a noticeable effect on the reduction of hardness in aged Mg-containing Al–Si–Cu alloys, in that it affects the precipitates containing Cu and Mg.     

Mechanical and wear behavior of quenched and tempered alloyed hypereutectic gray cast iron

The effect of tempering temperature (100–600 °C) on the hardness and wear resistance of a series of quenched and tempered hypereutectic alloyed gray cast irons has been studied in this work. Hardness was observed decreases with increase in tempering temperature and this trend is influenced by alloying additions and the volume of graphite flakes. Hardness of alloyed gray irons is also influenced by solid solution strengthening of tempered ferrite and carbide content and their distribution. The wear loss of alloyed cast irons was found to be lowest at a tempering temperature of 100 °C and 400 °C. The optimum tempering temperature is 400 °C with moderate hardness and low wear rate. This has been attributed to strengthening of the matrix at this temperature. Beyond 400 °C, the wear rate increases significantly due to carbide coagulation.     

Precipitation behavior of Ag–Pd–In dental alloys

Age-hardening characteristics and precipitation behavior of Ag–25%Pd–3%In–1%Zn–0.5%Ir alloy were investigated in detail by means of hardness testing, X-ray diffraction, electron microscopy and resistivity measurement. The solution treating could be accomplished at 980 °C and the aging in the temperature range from 950 to 850 °C occurred by continuous precipitation. The aging in the temperature range from 850 to 450 °C occurred first, forming GP-zones with a hardness increase and then in overaging stage by forming discontinuous precipitation, which consisted of lamellae of solute (Pd, In, Zn) depleted Ag-rich phase and (Pd,Ag)₃(In,Zn) intermetallic phase. The hardness increased very fast to its peak in 10 min during aging at temperatures between 450 and 550 °C.     

Effects of ageing treatments on transformation temperatures and precipitation kinetics in a Cu-Zn-Al shape-memory alloy

The effects of ageing treatments on transformation temperatures, hardness, and precipitation kinetics in a Cu-14.2Zn-8.5Al (wt%) shape-memory alloy were investigated. Quench-ageing treatment temperatures varied from 100 to 500° C with times up to 200 h after the solution treatment. The martensitic transformation temperature, M _s, of the hot-rolled material was decreased from 55 to 51 °C by the solution treatment. The temperature hysteresis (A _f-M _f) was 50° C for the hot-rolled condition, but was reduced to 30° C after the solution treatment. The maximum hardness for material aged at 500° C was lower than that for that aged at 300 or 400° C. The apparent activation energy for hardness increase in this alloy was 110 kJ mol^–1, compared with 72 kJ mol^–1 for the similar copper-based shape-memory alloy Cu-21.2 Zn- 6.0 Al. The ordering temperatures for B₂ and DO₃ superlattices were in the neighbourhood of 480 and 260° C, respectively. The tensile ductility and yield strength of this alloy were significantly reduced by the ageing treatment at 400° C.     

Age-hardening associated with ordering and spinodal decomposition in a AgCu-40 at % Au pseudobinary alloy

The age-hardening mechanism in an AgCu-40 at% Au alloy was studied by means of electrical resistivity measurement, hardness tests, X-ray diffraction and electron microscopy. Two stages of hardening were found by isothermal ageing below 648 K, which was higher than the critical temperature of ordering, T _c=620 K, in the present alloy. The first stage of hardening took place by formation of a modulated structure resulting from spinodal decomposition. Further hardening was brought about by ordering, yielding metastable AuCu I and/or AuCu II ordered platelets grown from the copper-rich portion of the modulated structure. Transitional ordering which gave rise to a marked hardening of the second stage was found, even though the temperature of below 648 K was higher than the T _c of the present alloy. Drastic softening was also found on disappearance of the transitional ordered phases. Although the modulated structure was observed by ageing at 773 K, there was no age-hardening.     

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.     

Effect of thermal ageing on hardness,tensile and impact properties of an alumina microsphere-reinforced aluminium metal-matrix composite

Thermal ageing studies have been carried out with an alumina microsphere-reinforced 6061 aluminium metal-metrix composite (MMC). A solution treatment temperature of 530°C for 1.5 h and ageing temperature 175°C with ageing time ranging between 0 and 12 h have been used. It was observed that the hardness achieves a peak value in about 8 h; the ultimate tensile strength shows an increase with increasing ageing time, and reaches a plateau at about 10 h. On the other hand, elongation to failure and impact properties show a sharp decline at approximately 4 h of ageing time. Also, a limited amount of experiments using 175°C/8 h ageing after solution treatment at 510, 490, 470 and 430 °C for 1.5 h show that the hardness of the MMC deceases steadily as the solution treatment temperature is decreased.     

Improvement of age-hardening process of a nickel-base superalloy, IN738LC, by induction aging

Influences of induction heating on the age-hardening process of a Nickel-base cast superalloy, IN738LC, was investigated. In this study cast specimens were undergone a solution treatment process in an argon atmosphere controlled furnace at 1125°C for 2 hours. Then, they were quenched in oil to room temperature to obtain supersaturated solid solution. These samples then subjected to two types of aging with equal heating rate, 30°C/Sec. One was induction aging and the other salt bath aging. Effects of these types of aging on the structure were analyzed and compared with normal aging having an average heating rate of 400°C/hr. The age-hardening behavior and microstructural characteristics were studied by hardness testing, scanning electron microscopy (SEM), electron image analyzing, X-ray diffractometery (XRD) and transmission electron microscopy (TEM) with replica method. According to the results obtained by these experiments, although the rate of heating of the specimens in induction and salt bath aging were equal, the diffusion-control process of nucleation and growth of precipitates in induction aging were considerably accelerated. Furthermore, desirable characteristics of precipitates were achieved in induction aging at lower time and temperature in comparison with times and temperatures of other types of aging. Improvement of microstructural characterization obtained in induction aging was related to the existence of an external electromagnetic force produced by induction heating. This electromagnetic force raised the effective driving force necessary for age-hardening process and intensified the nucleation and growth of precipitates remarkably.     

Effect of Zn addition on two-step aging behaviour of Al–Mg–Si–Cu alloy

This study investigates the effect of Zn addition two-step behaviour in an Al–Mg–Si–Cu alloy. During pre-aging at 100°C for 3?h, the Zn can partition into clusters because of the strong Zn–Mg interaction, prompting the formation of clusters. During subsequent artificial aging at 180°C for up to 240?min (peak hardness condition), the Zn does not significantly partition into clusters or precipitates, and the majority of Zn remains in the Al matrix. However, the presence of Zn in the matrix stimulates the transformation from clusters to GP zones to β′′ phases. The enhanced formation of GP zones and β′′ phases correlates well with the remarkable age-hardening response.     

Precipitation in Cu–Ni–Si–Zn alloy for lead frame

The aging of Cu–Ni–Si–Zn alloy for lead frame is investigated. The results showed that the peak of hardening effect occurs after aging for about 1 h and the electrical conductivity increases continuously with aging times. The hardness of the alloy reached a peak at 430–460 °C for 2 h and electrical conductivity reached a peak at 500–550 °C and continuously decreased afterwards. The cold rolling prior to the aging treatment was used to increase the precipitation rate. The precipitates responsible for the age-hardening effect are disc-shaped δ-Ni₂Si, which has an orthorhombic structure.     

Precipitation and its effect on age-hardening behavior of as-cast Mg–Gd–Y alloy

Microstructures evolution of Mg–7Gd–3Y–0.4Zr (wt.%) alloy during aging at 200 °C was investigated by using optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results showed that the alloy could exhibit remarkable age-hardening response by optimum solid solution and aging conditions. Especially, the highest Vickers hardness (HV) of this alloy was obtained when it was aged at 200 °C for 120 h, which was mainly attributed to a dense distribution of β′ precipitation in the matrix.     

The effects of temperature on the behaviour of an apatitic calcium phosphate cement

An apatitic calcium phosphate cement is obtained by mixing -tricalcium phosphate (-TCP) and precipitated hydroxyapatite into a cement powder, and by then mixing this powder with an aqueous solution of Na₂HPO₄ as an accelerator. Setting times were reduced by about 30% by increasing the temperature from 22 to 37°C. Compressive strength reached higher intermediate and final values at 37 °C. Degrees of transformation of the -TCP in the resulting calcium-deficient hydroxyapatite (CDHA) were much higher at 37 °C after 24 h of storage in Ringer's solution according to X-ray diffraction. Differential scanning calorimetry indicated that the rate of reaction increased by a factor of about 5 when the temperature was increased from 25 to 37 °C. Scanning electron microscopy showed that the microstructure was more homogeneous and that a more tight entanglement of the precipitated CDHA crystals occurred after storage at 37 °C than at room temperature.     

Influence of high temperature exposure on the mechanical behavior and microstructure of 17-4 PH stainless steel

The mechanical behavior and microstructural evolution of 17-4 PH stainless steels in three conditions, i.e. unaged (Condition A), peak-aged (H900) and overaged (H1150), exposed at temperatures ranging from 200 to 700°C were investigated. The high-temperature yield strength of each condition decreased with an increase in temperature from 200 to 400°C except for Condition A at 400°C with a longer hold time where a precipitation-hardening effect occurred. At temperatures from 500–700°C, the decrease in after-exposure hardness of Condition A and H900 at longer exposure times was caused by a coarsening effect of copper-rich precipitates. A Similar microstructural change was also responsible for the hardness of H1150 exposed at 700°C decreasing with increasing exposure time. Scanning electron microscopy (SEM) observations indicated that the matrix structures of Condition A and H900, when exposed at 600°C and above, exhibited lamellar recrystallized -ferrite in the tempered martensite and the size and quantity of these lamellar ferrite phases increased with exposure time. X-ray diffraction (XRD) analyses showed that the reverted austenite phase in H1150 that formed during the over-aging treatment was stable and hardly affected by deformation at temperatures of 200–400°C.     

Effect of sintering temperature and holding time on the properties of 3Y-ZrO2 microfiltration membranes

Sintering behavior of supported and unsupported microfiltration membranes prepared from 3 mol% yttria doped zirconia powder was investigated as a function of temperature and holding time in non-isothermal and isothermal densification. Shrinkage that started at 1000°C showed the highest rate between 1200°C and 1300°C although the rate decreased above 1300°C. The activation energy of sintering was calculated at 735 kJ/mol, assuming the grain boundary diffusion mechanism for mass transport. Mean pore size decreased in unsupported membranes and increased in supported ones as the sintering temperature increased up to 1200°C. Dimensional shrinkage of unsupported membrane slabs showed an increase in shrinkage first in the lateral dimension and then in the thickness as the sintering temperature increased. Pore growth and lower hardness in supported membranes, can be explained due to the restricted lateral shrinkage in the supported membranes. Removal of porosity was pronounced above 1100°C and the density increased linearly as a function of holding time. Microhardness of membranes sintered above 1100°C increased as a function of sintering temperature and was higher in unsupported membranes. Samples sintered above 975°C had a100% tetragonal phase structure. Permeability of supported membranes increased as a function of sintering temperature due to pore growth despite a decrease in porosity.     

High Strength AA7050 Al alloy processed by ECAP: Microstructure and mechanical properties

Commercial AA7050 aluminium alloy in the solution heat-treated condition was processed by ECAP through routes A and B_C. Samples were processed in both room temperature and 150 °C, with 1, 3, and 6 passes. The resulting microstructure was evaluated by optical microscopy (OM) and transmission electron microscopy (TEM). Only one pass was possible at room temperature due to the low ductility of the alloy under this condition. In all cases, the microstructure was refined by the formation of deformation bands, with dislocation cells and subgrains inside these bands. The increase of the ECAP temperature led to the formation of more defined subgrain boundaries and intense precipitation of spherical-like particles, identified as η′ and η phases. After the first pass, an increase in the hardness was observed, when compared with the initial condition. After 3 passes the hardness reached a maximum value, higher than the values typically observed for this alloy in the overaged condition. The samples processed by route B_C evolved to a more refined microstructure. ECAP also resulted in significant strength improvement, compared to the alloy in the commercial overaged condition.     

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.