Fatigue limits and SEM/TEM observations of fracture characteristics for three Pd—Ag dental casting alloys

The fatigue limits and fracture characteristics for three Pd–Ag dental casting alloys (Super Star, Heraeus Kulzer; Rx 91, Pentron; W-1, Ivoclar Vivadent) were studied. Specimens meeting the dimensions for ADA Specifications No. 5 and 38, and having the as-cast surface condition, were subjected to heat treatment simulating dental porcelain firing cycles and fatigued in air at room temperature under uniaxial tension-compression at 10 Hz. A ratio of compressive stress amplitude to tensile stress amplitude (R-ratio) of −1 was used. Alloy microstructures and fracture surfaces were examined with a scanning electron microscope and a transmission electron microscope. Fatigue limits for the three alloys had low values of approximately 15% of the yield strength for 0.2% permanent tensile strain. Complex fracture surfaces with characteristic striations were observed for all three fatigued alloys. Planar slip of dislocations occurred in the Pd solid solution matrix, along with dislocation-precipitate interactions and dislocation networks in the interfaces between the precipitates and surrounding matrix. Twinning occurred in the Pd solid solution matrix of Rx 91, and within discontinuous precipitates in Super Star and Rx 91. The low fatigue limits for these alloys are attributed to their complex microstructures and perhaps to casting defects.     

Electron microscopic observations of dislocations in nickel and NiMo alloys

Dislocations induced by deformation of disordered f.c.c. 10 and 20 at. % MoNi alloys are closely restricted to the slip planes and pile-up readily at barriers. It is believed that Mo has lowered the stacking fault energy to cause sufficient separation of partial dislocation pairs so that cross-slip is difficult; however, the stacking faults were not resolvable, indicating that the separation was less than about 10 Å. The microstructure of the deformed ordered Ni₄Mo was characterized by numerous long ribbons which were assumed to be contrast effects caused by the formation of antiphase boundaries as dislocations passed through the domains. Dislocations were restricted to single slip planes, and appeared nearly always in pairs, which were concluded to be superdislocations.     

Mechanical properties of Al–Li alloys Part 2 Fatigue crack propagation

Abstract

Mechanisms influencing the ambient temperature mechanical properties of commercial Al–Li alloys 2090, 2091, 8090, and 8091 are examined, with specific emphasis on the role of microstructure. In Part 2, results on fatigue crack propagation behaviour are presented for both ‘long’ (≥ 5 mm) and ‘microstructurally small’ (~1–1000 μm) cracks and compared with behaviour in traditional high strength aluminium alloys. In general, it is found that the growth rates of long fatigue cracks in Al–Li alloys are up to two to three orders of magnitude lower than in traditional 7000 and 2000 series alloys, when compared at an equivalent stress intensity range ?K. By contrast, corresponding growth rates of microstructurally small fatigue cracks were up to two to three orders of magnitude higher than the long crack results. Such observations are attributed to the prominent role of crack tip shielding in Al–Li alloys resulting from the tortuous and deflected nature of the crack paths which results in a reduced crack tip ‘driving force’ from crack deflection and, more importantly, from the consequent crack closure induced by the wedging of fracture surface asperities. Since microstructurally small cracks are unable to develop the same level of shielding from crack closure by virtue of their limited wake, small crack growth rates are significantly accelerated. Unlike fracture toughness behaviour, artificial aging of commercial Al–Li alloys to peak strength has a mixed influence on the (long crack) resistance. Although behaviour at higher growth rates is relatively unaffected, in 2091 nominal threshold ?K_TH values are increased by 17%, whereas in 8090 and 8091 they are decreased by 16–17%. However, all alloys show reduced effective fatigue thresholds at peak strength after correcting for crack closure.

MST/926b     

Annealing study of palladium–silver dental alloys: Vickers hardness measurements and SEM microstructural observations

Three Pd–Ag dental alloys for metal-ceramic restorations, W-1 (Ivoclar Vivadent), Rx 91 (Pentron) and Super Star (Heraeus Kulzer), were subjected to isothermal annealing for 0.5 hr periods in a nitrogen atmosphere at temperatures from approximately 400 ^∘ to 950 ^∘ C. The annealing behavior was investigated by Vickers hardness measurements (1 kg load) and SEM microstructural observations. The highest Vickers hardness occurred at approximately 700 ^∘ C for W-1 and 650 ^∘ C for Rx 91. For Super Star, there were two peaks in hardness at approximately 500 ^∘ and 650 ^∘ C. Additional use of light indenting loads (25 g for W-1; 10 g for Rx 91 and Super Star) revealed that hardness variations during annealing for W-1 and Rx 91 were related to the palladium solid solution matrix phase. For Super Star, the lower-temperature peak was controlled by multi-phase regions and the higher-temperature peak by the matrix phase. While microstructural changes due to annealing were evident with the SEM for Rx 91 and Super Star, no correlation was possible for W-1 because of its finer-scale microstructure. Although commercial Pd–Ag alloys have a relatively narrow composition range, their microstructures and annealing behavior can vary because of differences in proportions of secondary elements utilized for porcelain adherence and grain refinement elements, as well as other proprietary strategies employed by the manufacturers.     

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 × 10⁶ 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.     

Deep cryogenic treatment of AISI 302 stainless steel: Part II – Fatigue and corrosion

During the last decade, the interest about deep cryogenic treatment (DCT) is grown beyond its successful application on tool steels. The use of such cold treatment has shown positive effects on carburized steel fatigue life and some promising results were also noticed on stainless steels and on other materials. In this article, the DCT effects on fatigue and corrosion resistance of the AISI 302 austenitic stainless steel are analysed starting from the results of an extensive experimental campaign that was performed on both hardened and solubilized material conditions. The analysis includes an evaluation of the influence that the most important treatment parameters have on the final result. Considering their statistical significance at different reliability levels, the results show that the DCT can improve the fatigue behaviour of the solubilized AISI 302, while no important changes are noticed on the hardened material. Moreover, no difference was detected on the corrosion resistance of the cryotreated material, ensuring that such key-property for this class of steel is not compromised by the cryogenic treatment.     

Transmission electron microscopic investigation of the β1→β phase transformation in a Mg–Dy–Nd alloy

The phase transformation of β₁→β in a Mg–Dy–Nd alloy was studied by TEM. The β₁ phase, forming as plates parallel to {11?00}_α, has an f.c.c structure, with a ～ 0.74 nm, and an orientation relationship that may be described by (11?1)_β1//(112?0)_α, (110)_β1//(0001)_α. The equilibrium phase β, forming as plates parallel to {11?00}_α, has an ordered f.c.c structure, with a ～ 2.223 nm, and the orientation relationship between β and the matrix phase is identical to that observed between β₁ and matrix phase. Both phases are interconnected according to the orientation relationship: (11?1)_β//(11?1)_β1 and (110)_β//(110)_β1.     

Design of novel NiSiAlY alloys in marine salt-spray environment:Part Ⅱ.Al-Ni-Si-Y thermodynamic dataset

The NiSiAlY material is a promising candidate to replace NiCrAlY,which can withstand the harsh salt-spray conditions in marine environment.To efficiently design...     

Tissue reactions of subcutaneously implanted mixture of ε-caprolactone-lactide copolymer and tricalcium phosphate. An electron microscopic evaluation in sheep

Biodegradable polymers, mainly derivates of -hydroxy acids, are widely used today in oral- and maxillofacial surgery, orthopedics, and other fields of surgery. These biomaterials are well tolerated by living tissue and fracture fixation devices made of polylactic or polyglycolic acid are clinically widely used today. Still, there are some problems in application of biodegradable polymers. Abacterial inflammatory reactions have been noticed after the clinical introduction of these devices. Both swelling and pain at the site of implantation have also been reported. The etiology of this inflammatory reaction is still unknown, despite the numerous studies. Therefore, the aim of the present study was to further characterize this inflammatory reaction in detail, by electronmicroscopy. We prepared a mixture of -caprolactone–lactide copolymer and tricalcium phosphate and placed it in the dermis in 12 sheep. Follow-up times were 9, 14, 24, and 52 weeks. We found that the mixture caused a mild inflammatory reaction. There were no signs of cell damage. Fibroblasts, macrophages, and eosinophils were found adjacent to the copolymer. The mixture is easy to handle and can be moulded into different shapes in room temperature. The results encourage us to continue our studies to develop a filling material for bone defects.     

Evaluation of Compression Behavior of Paracetamol Tablets Produced with β Cyclodextrin Dispersions. Part II: Energy Distribution Study of Tablets

Abstract

The compression behavior of three paracetamol/β-cyclodextrin solid dispersions (PAR/β-CD, ratio 1: 1 w/w), which differ in particle size, shape, and crystallinity, were studied using force-displacement measurements (F-D plots). The measured parameters included: effective work, expansion work, friction work, useful work, and plasticity derived from F-D plots at the tableting stage. The PAR/β-CD spray-dried solid dispersion has the best compressional behavior, compared to other dispersions (PAR/β-CD physical mixture and PAR/β-CD kneaded solid dispersion) and PAR alone. Energy distribution study also showed that β-CD has positive influence on the PAR compression characteristics.     

Impact failure of beams using damage mechanics: Part II—Application

The analytical model developed in Part I (Int J Impact Eng 2002;27(8):837–61) using continuum damage mechanics is used in this paper to predict the displacement to failure of beams subjected to static or dynamic loads producing large plastic strains. The beams, which were made of a ductile, strain rate sensitive material, were loaded at different positions on the span, by two tup geometries travelling with initial impact velocities up to 15 m/s. A reasonable correlation is obtained between the theoretical predictions and the experimental results for the displacements to failure.     

Postbuckling behavior of a pressurized stiffened composite panel – Part II: Numerical analysis. Effect of the geometrical imperfections

Numerical simulations of the experimental tests performed with a pressurized composite stiffened panel are presented in this paper. As a consequence of the high slenderness of this structural typology, pressure caused the panel to enter the postbuckling regime. Previous experimental tests showed that the panel had a large safe postbuckling range, the experimental collapse pressure being over four times the first buckling load. Due to the relevant influence of the geometric imperfections on the global response, a procedure for taking into account actual imperfections in the development of the models is proposed. This procedure can be used as a tool to facilitate the modeling of the actual geometry of the panel, mainly the zone of the skin located between the stringers. A satisfactory agreement with experimental results has been reached using the proposed procedure.     

Several Theoretical Perspectives of Ferrite-Based Materials—Part 1: Transmission Line Theory and Microwave Absorption

A theoretical model for microwave absorption from both forced non-resonance oscillation and resonance is constructed using the transmission line theory familiar to microwave engineers. The model covers both the single-phase ferrite and its composites of interest to material scientists, and can be applied to a variety of different absorption mechanisms. The transmission line theory is also shown to be consistent with the band theory of solids, a relationship that has not been revealed previously. The work bridges the gap between the interests of microwave engineers and material scientists.