A study on dental nonprecious cast alloys

Tarnish tests conducted in a 0.1% sodium sulphide solution have shown that ternary nickel-copper-manganese alloys, such as 40Ni-30Cu-30Mn and 50Ni-30Cu-20Mn, have superior tarnishment resistance than other alloys, e.g. 20Ni-40Cu-40Mn, 30Ni-30Cu-40Mn and 30Ni-40Cu-30Mn. It was also found that 40Ni-30Cu-30Mn and 50Ni-30Cu-20Mn alloys have lower values of colour change vector than the other alloys given above.     

A study on age hardening in cast Fe-Mn-Al-Si-C alloys

Phase transformations in a group of cast and solution treated (at 1100° C) Fe-Mn-Al-Si-C alloys during isothermal ageing have been investigated as a function of (a) ageing temperature (500 to 800° C), (b) aluminium (5 to 10%) and carbon (0.27 to 0.9%) contents for fixed manganese ( 30%) and silicon ( 1.5%) levels. Irrespective of the ageing temperature and carbon content, 5% Al alloys do not undergo appreciable ageing. Ageing tendency increases with increase in aluminium and carbon contents. Hardening is caused by the precipitation of Al(Fe, Mn)C _x phase inside the austenite grain and Mn₁₂Si₇Al₅ phase at the grain boundaries. The sequence of precipitation of the phases depends on the ageing temperature and composition. Cold working by rolling, after homogenization at 1100° C, accelerates ageing.     

Influence of boron addition on microstructure and mechanical properties of dental cast titanium alloys

As-cast titanium alloys prepared using a dental cast machine with a series of boron additions have been studied using an optical microscope, XRD, SEM, and hardness and tensile testing. It has been shown that a small amount of boron addition induces a significant refinement of as-cast structure and improvement of mechanical properties. Titanium borides, TiB particles, are observed in Ti matrix. Tensile ductility of cast Ti–B and Ti–0.5Si–B alloys is improved obviously when boron content is about 0.086–0.14 mass%. This is primarily due to the role of borides precipitated at the prior β boundary and refinement of the prior β grains. Cast Ti–B alloys with a good combination of greater tensile ductility and strength can be obtained with very low boron addition.     

Evaluation of cast Ti–Fe–O–N alloys for dental applications

Good mechanical properties, biocompatibility and corrosion resistance make titanium an excellent material for biomedical applications. However, when better mechanical properties than those offered by commercially pure titanium (CPTi) are needed, Ti–6Al–4V is sometimes a good alternative. Some new titanium alloys, developed as industrial structural materials, aim at an intermediate range of strength between that of CP Ti and Ti–6Al–4V. Two of these alloys are Super-TIX800™ (Ti–1% Fe–0.35% O–0.01% N) and Super-TIX800N™ (Ti–1% Fe–0.3% O–0.04% N) (both produced by Nippon Steel Corp., Japan). Besides being stronger than CP Ti, the cost of manufacturing these alloys is reportedly lower than for Ti–6Al–4V since they do not contain any expensive elements. In addition, they are not composed of elements such as aluminum or vanadium, which have caused biocompatibility concerns in medical and dental appliances. To evaluate these alloys as candidates for dental use, it is helpful to compare them to CP Ti (ASTM Grade 2) and Ti–6Al–4V (ASTM Grade 5), which have already been employed in dentistry. We evaluated the tensile properties, mold filling capacity, corrosion characteristics and grindability of these industrial alloys prepared by investment casting. Compared to the strengths of cast CPTi, the yield strength and tensile strength of these cast alloys were more than 20% and approximately 30% higher, respectively. On the other hand, both of these properties were 30% lower than for Ti–6Al–4V. Better grindability and wear resistance were additional benefits of these new alloys for dental applications.     

Structural refinement of cast magnesium alloys

Abstract

The structure of cast magnesium alloys (grain size and precipitate morphology and size) affects the properties of the products and the scope for use of the alloys. The structure can be controlled by minor additions of inoculants, which are largely determined on the basis of the composition of the alloy concerned. The present paper reviews the scientific background of structural refinement by inoculation and its application to Mg–Zn, Mg–Al, and Mg–Al–Si alloys.     

Fracture toughness of cast aluminium alloys

An investigation was carried out to evaluate the fracture toughness of cast aluminium alloys of different microstructural complexity, brought about by alloy constitution and cooling rate of castings. In all cases the three-point bend specimens, which had a thickness of 15 mm, did not provide valid plane — strain stress intensity factor values. The fracture susceptibility at a given stress level reckoned in terms of the conditional plane strain stress intensity factor (K _Q) was found to be lowest in aluminium-4.5% copper alloy castings and the susceptibility increased with increase in microstructural complexity. Casting cooling rate in these castings is likely to affect the damage potential of a given defect at yield stress to a greater extent than the fracture susceptibility at a given stress.     

Phenomenological study of elastic and anelastic properties of dental alloys

Measurements of Young's modulus, mechanical damping, stress and unit damping energy have been made as a function of temperature on five dental amalgams with the PUCOT (piezoelectric ultrasonic composite oscillator technique). In Dispersalloy amalgam the mechanical damping changed by a factor of 4.3 and Young's modulus was reduced by 0.5% as the temperature increased from 20 to 80° C. The effective activation energy for the change in damping was 0.42 eV. For the five amalgams the curves of unit damping energy against stress, damping against modulus, and (damping/modulus) against modulus had a variety of forms: one, two, three or four straight lines, with or without hysteresis effects. All the curves for Tytin amalgam showed a sharp change near 68° C. The data are examined phenomenologically to show that they lend themselves well to the detection and monitoring of transformations taking place in dental alloys.     

Analytical electron microscopy of cast Au-Ag-Cu alloys

The microstructure in an as-cast Au-Ag-Cu-Pd alloy has been examined by electron microscopy (TEM) combined with quantitative X-ray microanalysis (EDS). The primary phase formed during solidification is enriched in silver and palladium, while gold and copper segregate to grain boundaries, where substantial amounts of lamellar eutectic colonies are formed. The compositions of the lamellae were determined by EDS analysis to be 56% Au-13% Ag-26% Cu-6% Pd and 50% Au-42% Ag-5.5% Cu-2.5% Pd (wt%). Ordering and decomposition reactions take place during cooling in the solid state: plate-like precipitates of tetragonal AuCu-l on {001} planes were found within the primary phase and in silver-rich lamellae in eutectic colonies.     

Pore formation in cast metals and alloys

Porosity occurs in cast solidifying metals and alloys due to negative pressures generated during solidification contraction, and pressure developed by gases dissolved in the motten metal. Both the above processes may act either together or separately to produce such shrinkage or gas defects (collectively termed pores). They are generally unwanted and constitute a major industrial problem. This paper is an attempt to review up-to-date knowledge of the conditions of pore formation in cast metals and alloys. Various mechanisms responsible for pore nucleation and growth are summarized, and experimentally evaluated using an unfed type of mould with aluminium alloy castings. The observations are in support of a non-nucleation mechanism of pore formation playing a major role in the occurrence of such defects in cast metals. Further, in gas-containing alloy melts the critical amounts of gas required for single and multiple pore nucleation have been determined quantitatively and are listed in the text. The gas contents of the melts were measured using an apparatus based on the first bubble technique. It is also experimentally observed that under poor feeding conditions more than one of the non-classical nucleation mechanisms may be functional at the same time for the formation of such defects.Nomenclature P _e Sum of external forces which tend to collapse a pore - P _h Hydrostatic pressure - P _g Internat gas pressure in liquid metal - P _a Atmospheric pressure applied during solidification - P _s Shrinkage pressure - Surface tension at gas-metal interface - P _f Fracture pressure of liquid metal - 2_/r Surface tension resistance - V _i Volume of initial gas content of liquid Metal - P _i Total internal pressure due to gas and/or shrinkage - K _s,K _L Gas solubility constants of solid and liquid metal, respectively - K K_s/K_L - r Radius of liquid channel - R Radius of cylindrical casting - L Length of liquid channel - L _c Length of cylindrical casting - f ₁ Fraction of liquid left in the casting of pure and short freezing-range alloy, r ² L/R ² L _c - V _i ^* Critical gas content - t Tortuosity of channel - n Number of channels (approximately number of dendrite arms per unit area) - f _L Fraction of liquid left in the casting of mushy freezing alloys, r ² nt/R ² - B K _Lnt(1 -K) - nb _i ² B² (approximately) - r* Critical radius for pore nucleation (pore belowr* size cannot exist) - Surface tension of liquid - N Avogadro's number - k Boltzmann constant - h Planck's constant - T Temperature (K) - a,b Inner and outer radii of solidified shell - Y Uniaxial yield stress - , _s Energy per unit area of solid-liquid and solid-vapour interface, respectively - h Thermal diffusivity of solid metal - C_p Specific heat of solid metal - m,K ₁,K ₂ Constants - Fractional contraction of liquid phase on solidification [(solid density — liquid density)/solid density] - /(1 –) - Vortex circulation factor - d Density of liquid - Metal viscosity - h ₁ Thermal conductivity of mould - H Heat of fusion of metal     

Effects of HIPping on high-cycle fatigue properties of investment cast A356 aluminum alloys

This study is concerned with the effects of HIPping on high-cycle fatigue properties of investment cast A356 Al alloys. Tensile and high-cycle fatigue tests were conducted on cast alloys, two of which were HIPped, and then the test data were analyzed in relation with microstructures, tensile and fracture properties, and fatigue fracture mode. Eutectic Si particles were homogeneously dispersed in the matrix of the casting A356 Al alloys, but there were many large pores formed as casting defects. The high-cycle fatigue test results indicated that fatigue strength of the HIPped alloys was higher than that of the non-HIPped alloys because of the significant reduction in volume fraction of pores by HIPping. In the non-HIPped specimens, fatigue cracks initiated at large pores adjacent to the specimen surface and then propagated down to several hundreds micrometers depth while coalescing with other large pores. On the other hand, the HIPped specimens, where pores did not affect the fatigue much, fatigue cracks initiated at eutectic Si particles and propagated along them, thereby leading to improved fatigue strength by 40 to 50% over the non-HIPped specimens.     

铸造镁硅合金组织和阻尼性能研究

利用动态机械热分析仪研究了四种镁硅合金的阻尼性能.研究表明:铸造镁硅合金具有良好的阻尼性能.并且体现出有别于纯镁的新阻尼特征,铸造镁硅合金具有较纯镁在更大应变振幅范围内的弛豫型内耗.由于Mg2 Si对晶界的钉扎作用,界面阻尼峰出现在较纯镁更高的温度.少量钙的添加改善了Mg2 Si析出的形态,但对阻尼性能的影响似乎不明显.     

Creep and creep recovery of cast aluminum alloys

Constant load uniaxial creep tests were performed on four aluminum alloys (designated M4032-2, 332, 332RR, and 333) at stresses of 31.5 MPa, 56.5 MPa, and 73 MPa and temperatures of 220°C and 260°C. Of the four materials, M4032-2 had the greatest resistance to creep, while 332RR alloy had the least. In addition to creep, the creep recovery phase was observed as well. It was found that, even for short loading periods, much of the time-dependent strain was not recoverable for all of the materials studied. Hardening was observed to occur in each of the alloys, resulting in a reduced creep rate on subsequent loadings. A constitutive equation for creep and recovery incorporating both stress and temperature dependence was developed for each of the alloys tested based on a viscous-viscoelastic model.     

Study on hardness and microstructural characteristics of sand cast Al-Si-Cu alloys

In this study, the influence of Cu content on the hardness and microstructural characteristics of sand cast Al-Si-Cu alloys have been investigated. Al-Si alloys with 2% and 5% Cu have been utilized for this purpose. Solidification of Al-Si-Cu alloys have been realized by melting in a gas furnace with a crucible and casting in green sand molds at 690°C. The solution treatment has been performed at 500°C for 7 h and then specimens were quenched in water. The samples have been aged at 190°C for 15 h to observe the effect of aging on mechanical properties.     

Comparison between mechanochemical effect on die cast and extruded magnesium alloys

Abstract

The mechanochemical effect (MCE) of several magnesium based alloys, obtained by both die casting and extrusion methods, was studied by potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques. The mechanochemical behaviour of each alloy was evaluated as a function of die cast parameter, environment, and alloy composition. Electrochemical tests were performed in a buffer solution of sodium tetraborate, (Na₂B₄O₇) with and without magnesium hydroxide (Mg(OH)₂). The MCE was correlated with the microstructure of the Mg alloys.     

Comparative corrosion study of Ag-Pd and Co-Cr alloys used in dental applications

The electrochemical behaviour of two Ag-Pd alloys (Unique White and Paliag) used in dental prosthetics construction for crowns and bridges and one Co-Cr alloy (Vitallium 2000) was studied in artificial saliva using the polarization curves and electrochemical impedance spectroscopy (EIS). The corrosion resistance was evaluated by means of the corrosion currents value and by coulometric analysis. The open circuit potential of Ag-Pd are attributed to dealloying followed by surface enrichment with Ag and the possible formation of an insoluble AgCl surface film on the respective alloy surfaces. Our results have shown that these alloys have a somewhat good corrosion resistance in artificial saliva. The corrosion current densities of Unique White and Vitallium 2000 alloys were very low (∼100 nA/cm²). For Ag-Pd alloys, when increasing the content of Cu, corrosion resistance decreases. The passivation of all samples occurred spontaneously at the open circuit potential. The electrochemical properties of the spontaneously passivated electrodes at the open circuit potential were studied by EIS. The polarization resistance (R _p) and the electrode capacitance (C _dl) were determined. The polarization resistance of all the samples increases with the immersion time. The polarization resistances are largest for Unique White (Ag-Pd) and Vitallium 2000 (Co-Cr) alloys. Because the electrochemical behaviour of the Co-Cr alloy was compared with that of Ag-Pd alloy, this type of alloy may be a suitable alternative for use in the manufacture of fixed dental prostheses. The present study, though limited, has shown that electrochemical characteristics can be used to identify such alloys. Knowledge of the in vitro corrosion behaviour of these alloys may lead to better understanding of any biologically adverse effects in vitro.     

Thermoanalytical characteristics of powders in dental cast investment

The present study examined the thermal properties of phosphate-bonded investments, a gypsum-bonded investment and an experimental investment powder when the basic powders were heated to high temperatures by simultaneous differential thermal analysis (DTA) and thermogravimetry (TG). The phosphate-bonded investments showed values of about 59 kcal mol^–1 (247 kJ mol^–1) (thermal decomposition of NH₄H₂PO₄) and about 11 kcal mol^–1 (46 kJ mol^–1) (formation of NH₄MgPO₄). Thermal reactions occurred clearly on the DTA-TG curves for the investment powders, using powders of NH₄H₂PO₄, and MgO with NH₄H₂PO₄/MgO = 1 as main components in the investment.     

The structure of a cast dental Ni-Cr-Be alloy

The structure of a typical dental Ni-Cr-Be alloy with 1.8 wt% Be has been investigated by SEM and TEM as well as by quantitative X-ray microanalyses in both instruments. Due to its low atomic weight the atomic fraction of Be is as high as 0.10. During solidification beryllium segregates substantially, and a large volume fraction of the casting is made up of a eutectic with coarse ( 1 µm diameter) alternating rods of fcc Ni-Cr and NiBe with a CsCl-type structure (ordered bcc). Smaller ( 0.1 µm diameter) rods of NiBe are precipitated in matrix in the solid state. Microanalyses of the NiBe rods show that they have a low chromium content ( 1.5 wt%). The cube boundary planes of the ordered b cc and fcc structures have a slight difference in orientation of about 7° which is most probably due to a small coherency misfit of the two types of lattices. The 100 directions in cube boundary plane of the fcc structure are nearly parallel to the 110 directions of the ordered bcc cube boundary plane. Sometimes another and more complex relationship between the two lattices occurs. The alloy contains 3.9 wt% Al which gives rise to numerous small ( 10 nm), spherical, ordered particles of Ni₃Al both in matrix as well as in the fcc eutectic rods.