Comparison of mechanical properties for equiaxed fine-grained and dendritic high-palladium alloys

Does the casting mode influence microstructure, fracture and properties of different metal ceramic alloys?

The aim of the present study was to evaluate the tensile strength, elongation, microhardness, microstructure and fracture pattern of various metal ceramic alloys cast under different casting conditions. Two Ni-Cr alloys, Co-Cr and Pd-Ag were used. The casting conditions were as follows: electromagnetic induction under argon atmosphere, vacuum, using blowtorch without atmosphere control. For each condition, 16 specimens, each measuring 25 mm long and 2.5 mm in diameter, were obtained. Ultimate tensile strength (UTS) and elongation (EL) tests were performed using a Kratos machine. Vickers Microhardness (VM), fracture mode and microstructure were analyzed by SEM. UTS, EL and VM data were statistically analyzed using ANOVA. For UTS, alloy composition had a direct influence on casting condition of alloys (Wiron 99 and Remanium CD), with higher values shown when cast with Flame/Air (p < 0.05). The factors 'alloy" and 'casting condition" influenced the EL and VM results, generally presenting opposite results, i.e., alloy with high elongation value had lower hardness (Wiron 99), and casting condition with the lowest EL values had the highest VM values (blowtorch). Both factors had significant influence on the properties evaluated, and prosthetic laboratories should select the appropriate casting method for each alloy composition to obtain the desired property.

Ti-Ge binary alloy system developed as potential dental materials

As-cast Ti-xGe (x = 2, 5, 10, 20 wt %) binary alloys were produced in this work, and various experiments were carried out to investigate the microstructure, mechanical properties, in vitro electrochemical and immersion corrosion behaviors as well as cytotoxicity with as-cast pure Ti as control, aiming to study the feasibility of Ti-xGe alloy system as potential dental materials. The microstructure of Ti-xGe alloys changes from single α-Ti phase to α-Ti + Ti(5)Ge(3) precipitation phase with the increase of Ge content. Mechanical tests show that Ti-5Ge alloy has the best comprehensive mechanical properties. The corrosion behavior of Ti-xGe alloys in artificial saliva with different NaF and lactic acid addition at 37°C indicates that Ti-2Ge and Ti-5Ge alloys show better corrosion resistance to fluorine-containing solution. The cytotoxicity test indicates that Ti-xGe alloy extracts show no obvious reduction of cell viability to L-929 fibroblasts and MG-63 osteosarcoma cells, similar to pure Ti which is generally acknowledged to be biocompatible. Considering all these results, Ti-2Ge and Ti-5Ge alloys possess the optimal comprehensive performance and might be used as potential dental materials.

Mechanical properties, fracture surface characterization, and microstructural analysis of six noble dental casting alloys

STATEMENT OF PROBLEM

Because noble dental casting alloys for metal ceramic restorations have a wide range of mechanical properties, knowledge of these properties is needed for rational alloy selection in different clinical situations where cast metal restorations are indicated.

PURPOSE

The purpose of this study was to compare the mechanical properties and examine both the fracture and polished surfaces of 6 noble casting alloys that span many currently marketed systems. Five alloys were designed for metal ceramic restorations, and a sixth Type GPT has Type IV alloy for fixed prosthodontics (Maxigold KF) was included for comparison.

MATERIAL AND METHODS

Specimens (n=6) meeting dimensional requirements for ISO Standards 9693 and 8891 were loaded to failure in tension using a universal testing machine at a crosshead speed of 2 mm/min. Values of 0.1% and 0.2% yield strength, ultimate tensile strength, elastic modulus, and percentage elongation were obtained. Statistical comparisons of the alloy mechanical properties were made using 1-way ANOVA and the REGW multiple-range test (α=.05). Following fracture surface characterization using scanning electron microscopy (SEM), specimens were embedded in epoxy resin, polished, and again, examined with the SEM.

RESULTS

When the multiple comparisons were considered, there were generally no significant differences in the elastic modulus, 0.1% and 0.2% offset yield strength, and ultimate tensile strength for the d.SIGN 91 (Au-Pd), d.SIGN 59 (Pd-Ag), Capricorn 15 (Pd-Ag-Au) and Maxigold KF (Au-Ag-Pd) alloys, except that the ultimate tensile strength was significantly lower (P<.05) for Maxigold KF than these other 3 alloys. These 4 mechanical properties were generally significantly lower (P<.05) for Aquarius XH (Au-Pt-Pd) and Brite Gold XH (Au-Pt). The d.SIGN 59 (14.6%) and Capricorn 15 (13.8%) alloys had the highest values of mean percentage elongation, which were not significantly different. Aquarius XH (6.0%) and Maxigold KF (4.2%) had the lower mean values of percentage elongation, which were also not significantly different. The polished and etched surfaces for all alloys revealed equiaxed, fine-grain microstructures, and all fracture surfaces contained casting porosity. Incomplete solidification suggestive of dendritic structures was observed for some alloys. Fracture surfaces were complex, with characteristic features of both brittle and ductile fracture. Precipitate particles on the fracture surfaces indicated the multi-phase character of the alloys.

CONCLUSIONS

For the important mechanical property of yield strength, there were generally no significant differences among the Au-Pd, Pd-Ag, Pd-Ag-Au and Au-Ag-Pd alloys. Wide variation was found in percentage elongation, with the Pd-Ag and Pd-Ag-Au alloys having the highest values and the Au-Pd-Pt and Au-Ag-Pd alloys having the lowest values.

Study of Pd-Ag dental alloys: examination of effect of casting porosity on fatigue behavior and microstructural analysis

The goals of this study were to investigate the fatigue limits of two Pd-Ag alloys (Ivoclar Vivadent) with differing mechanical properties and varying proportions of secondary alloying elements, examine the effect of casting porosity on fatigue behavior, and determine the effect of casting size on microstructures and Vickers hardness. The alloys selected were: IPS d.SIGN 59 (59.2Pd-27.9Ag-8.2Sn-2.7In-1.3Zn); and IS 64 (59.9Pd-26.0Ag-7.0Sn-2.8Au-1.8 Ga-1.5In-1.0Pt). Tension test bars, heat-treated to simulate dental porcelain application, were subjected to cyclic loading at 10 Hz, with R-ratio of -1 for amplitudes of compressive and tensile stress. Two replicate specimens were tested at each stress amplitude. Fracture surfaces were examined with a scanning electron microscope (SEM). Sectioned fatigue specimens and additional cast specimens simulating copings for a maxillary central incisor restoration were also examined with the SEM, and Vickers hardness was measured using 1 kg load. Casting porosity was evaluated in sectioned fatigue fracture specimens, using an image analysis program. The fatigue limit (2 × 10(6) loading cycles) of IS 64 was approximately 0.20 of its 0.2% yield strength, while the fatigue limit of d.SIGN 59 was approximately 0.25 of its 0.2% yield strength. These relatively low ratios of fatigue limit to 0.2% yield strength are similar to those found previously for high-palladium dental alloys, and are attributed to their complex microstructures and casting porosity. Complex fatigue fracture surfaces with striations were observed for both alloys. Substantial further decrease in the number of cycles for fatigue failure only occurred when the pore size and volume percentage became excessive. While the heat-treated alloys had equiaxed grains with precipitates, the microstructural homogenization resulting from simulated porcelain firing differed considerably for the coping and fatigue test specimens; the latter specimens had significantly higher values of Vickers hardness.

Transmission electron microscopic studies of deformed high-palladium dental alloys

Objective. The purpose of this study was to employ transmission electron microscopy (TEM) to investigate the microstructures of Pd-Cu-Ga and Pd-Ga dental alloys that had been permanently deformed, in order to obtain information about the deformation behavior of individual phases and changes in microstructure brought about by that deformation.Methods. Heavily deformed regions taken from fractured tensile test bars of the two alloys in the as-cast condition were prepared for TEM analysis, using mechanical grinding and polishing, ion milling, and plasma cleaning. The specimens were examined in the TEM using bright-field and dark-field diffraction contrast imaging. Selected-area and convergent-beam electron diffraction patterns were employed to analyze the structures of the phases, and standardless energy-dispersive X-ray spectrometry was used to determine their mean compositions.Results. For both alloys, tweed structures underwent permanent deformation by twinning, whereas dislocation movement occurred in the face-centered cubic (fcc) palladium solid solution matrix. A body-centered cubic (bcc) phase, previously unreported in our TEM studies and containing a high density of dislocations, was identified in the Pd-Cu-Ga alloy, while fine-scale, stress-induced precipitates were found in some regions of the fcc matrix in the Pd-Ga alloy.Significance. The present results have provided novel information about the mechanical deformation behavior of high-palladium alloys. The stress-induced precipitation in the Pd-Ga dental alloy studied may be a critical component of strengthening mechanisms.

Inductively coupled plasma-mass spectroscopy measurements of elemental release from 2 high-palladium dental casting alloys into a corrosion testing medium

STATEMENT OF PROBLEM

The biocompatibility of high-palladium alloy restorations has been of some concern due to the release of palladium into the oral environment and sensitivity reactions in patients.

PURPOSE

This study measured the in vitro elemental release from a Pd-Cu-Ga alloy and a Pd-Ga alloy into a corrosion testing medium.

MATERIAL AND METHODS

Both alloys were cast into 12-mm-diameter x 1-mm-thick disks, subjected to heat treatment that simulated porcelain firing cycles, polished to a 0.05-mm surface finish, and ultrasonically cleaned in ethanol. Two specimens of each alloy were immersed 3 times (at 7, 70, and 700 hours) in an aqueous lactic acid/NaCl solution used for in vitro corrosion testing and maintained at 37 degrees C. The specimens were removed after each immersion time, and the elemental compositions of the solutions were analyzed with inductively coupled plasma-mass spectroscopy (ICP-MS). Elemental concentrations for the 2 alloys at each immersion time were compared with Student t test (alpha=.05).

RESULTS

No significant differences in palladium release were found for the 7- and 70-hour solutions, but significant differences were found for the 700-hour solutions. Mean concentrations of palladium and gallium in the 700-hour solutions, expressed as mass per unit area of alloy surface, were 97 (Pd) and 46 (Ga) microg/cm(2) for the Pd-Cu-Ga alloy and 5 (Pd) and 18 (Ga) microg/cm(2) for the Pd-Ga alloy.

CONCLUSION

Relative proportions of the elements in the solutions were consistent with the release of palladium and breakdown of microstructural phases found in the alloys. The results suggest that there may be a lower risk of adverse biological reactions with the Pd-Ga alloy than with the Pd-Cu-Ga alloy tested.

Potentiodynamic polarization study of the in vitro corrosion behavior of 3 high-palladium alloys and a gold-palladium alloy in 5 media

STATEMENT OF PROBLEM

Corrosion of cast alloy restorations may lead to their failure or adversely affect their biocompatibility. Although some documentation of the corrosion behavior of the high-palladium dental alloys exists, questions remain about their corrosion resistance and mechanisms.

PURPOSE

This study compared the in vitro corrosion characteristics of 3 high-palladium alloys and 1 gold-palladium alloy in simulated body fluid and oral environments.

MATERIAL AND METHODS

Two Pd-Cu-Ga alloys and 1 Pd-Ga alloy were selected; an Au-Pd alloy served as the control. The corrosion behavior for the as-cast and simulated porcelain-firing (heat-treated) conditions of each alloy (N = 5) was evaluated in 0.9% NaCl, 0.09% NaCl, and Fusayama solutions. Heat-treated specimens of each alloy (N = 5) were also tested in N(2)-deaerated 0.09% NaCl and Fusayama solutions (pH 4). After immersion in the electrolyte for 24 hours, the open-circuit potential (OCP) was measured, and linear polarization was performed from -20 mV to +20 mV (vs. OCP) at a scanning rate of 0.125 mV/s. Cyclic polarization was performed from -300 mV to +1000 mV and back to -300 mV (vs. OCP) at a scanning rate of 1 mV/s. Data were evaluated with analysis of variance and the Ryan-Einot-Gabriel-Welsch multiple-range test (alpha=.05).

RESULTS

The OCP of each alloy varied with the condition (as-cast or heat-treated) and electrolyte used. Corrosion resistance was similar for the 4 alloys tested. For cyclic polarization, all alloys showed active-passive or spontaneous passive behavior in nearly all electrolytes. During some reverse scans, the 3 high-palladium alloys displayed 3 or 5 anodic peaks. No positive hysteresis was observed for any of the alloy/electrolyte combinations evaluated.

CONCLUSION

The corrosion resistances of the 3 high-palladium alloys in simulated body fluid and oral environments were comparable to that of the gold-palladium alloy. The similar corrosion resistance for the 3 high-palladium alloys was attributed to their high noble metal content and theorized stable structure at the submicron level. Selective corrosion of different phases and elements, surface enrichment of palladium, and adsorption of species are possible corrosion mechanisms. The cyclic polarization results suggest that none of the 4 alloys would be prone to pitting or crevice corrosion under in vivo conditions, but crevice conditions should nonetheless be avoided for these alloys in the oral environment.