Phase transformation behaviour of a low carat gold alloy for porcelain bonding during continuous heating

The phase transformation associated with hardening in a low carat gold alloy for porcelain bonding during continuous heating was studied by resistivity measurements, hardness tests, X-ray diffraction and scanning and transmission electron microscopies. A short-range order structure, based on Au-Pd, was produced at a lower temperature. At an elevated temperature, two types of precipitation were observed. One is a nodular reaction consisting of an L1₂ ordered structure based on Pd₃Sn and a disordered FCC structure at grain boundary. The other is a formation of spherical L1₂ ordered precipitates based on Pd₃Sn within the grain followed by Ostwald ripening. A fine dispersed precipitate of the latter contributed to the remarkable hardening.     

Phase transformation of the Ti-5553 titanium alloy subjected to rapid heating

Journal of Materials Science - The α?→?β phase transformation upon heating in the Ti-5553 alloy with lamellar-nodular bimodal microstructure was tracked in situ with...     

Phase transformation in Cu-Cr-Zr-Mg alloy

After solid solution treatment, the effects of aging processes on the microstructure and electrical conductivity of Cu-Cr-Zr-Mg lead frame alloy were investigated in order to determine the phase transformation and the time-temperature-transformation (TTT) behavior. The results show that aged at 470 °C the fine precipitation of an ordered CrCu₂(Zr,Mg) phase is found in copper matrix as well as fine Cr and Cu₄Zr, aged at 550 °C the phase CrCu₂(Zr,Mg) is decomposed into Cu₄Zr and Cr phase, aged at 600 °C the precipitates is bcc Cr only. The phase transformation kinetics equation was deduced from the Avrami empirical formula based on the linear relationship between the electrical conductivity and the volume fraction of the phase transformation.     

Cu-Cr-Zr合金连续加热时效动力学研究

本文通过测试铜合金在连续加热过程中电阻率的变化研究了铜合金的连续加热时效动力学.提出了用时效过程中电阻率的减小值△ρ作为描述时效过程的参数.在此基础上,建立了连续加热时效动力学图.在等速加热时效过程中,利用电阻率与温度呈线性关系这一特点,提出了一种计算△ρ的方法.测出了Cu-Cr-Zr合金以0.075～1.6℃/min加热速率进行加热时的△ρ与温度的关系.     

Age-hardening behavior of a low-gold dental alloy

The age-hardening behaviors of a low gold dental alloy were studied by means of differential scanning calorimetry, hardness testing, X-ray diffraction, optical microscopy and transmission election microscopy. Two distinct hardening behaviors were found at two different aging temperatures. Age-hardening at 290°C was attributed to the formation of the metastable AuCuI′ ordered phase, and the gradual softening in the overaging stage resulted from the slow growth of this phase. The rapid increase in hardness in the early stage at 495°C was due to the precipitation of the metastable AuCuI′ or/and AuCuII′ ordered phases, and the rapid decrease in hardness in the overaging stage was a consequence of the growth of these phases and the loss of the coherency strain at the interface between the spindal-like AuCuI platelets and the matrix.     

Phase transformation and microstructure evolution of pearlite heat-resistant steel during heating

ABSTRACT

The phase transformation and microstructure evolution of pearlite heat-resistant steel during heating were observed with an ultra-high temperature confocal scanning laser microscope. The α-ferrite completely disappeared earlier than Fe₃C during the formation of γ-austenite, which is inconsistent with the fact that the Fe₃C should disappear completely earlier under equilibrium conditions. After the Fe₃C?+?α→γ transformation, static recrystallisation of γ-austenite occurred, accompanied by the dissolution of cementite. During the γ→δ transformation, the δ-cell first precipitated at the triple point of the γ-austenite grain boundaries, and then the δ-cell platelet with one tip appeared in the γ-austenite grain. The law of δ/γ inter-phase boundaries was analysed based on inter-phase boundary types and element diffusion.     

Phase transformation behavior of porous NiTi alloy fabricated by powder metallurgical method

Nickel titanium shape memory alloys (NiTi-SMAs) were successfully produced from elemental Ni/Ti powders by powder metallurgical method and then subjected to age treatment. Microstructure was examined by SEM and XRD and phase transformation temperatures were measured by dilatometric method. The phase transformation temperatures increased with both duration and temperature of the age treatment. The porous product exhibited desirable shape memory effect.     

Phase transformation and precipitation hardening behavior of Cr and Fe in BS40CrFeSn alloy

Phase transformation and precipitation hardening behavior of the water atomized copper alloy powder was studied by aging treatment, to develop high strength Cu–40Zn–X (X: Cr, Fe, Sn) alloys by powder metallurgy process. Super-saturated solid solution elements of Cr and Fe are formed in the brass matrix, and single β phase was retained in the raw powder after water atomization. Solid solubility of Cr and Fe decreased with increase of aging temperature, and phase transformation evolved from single β phase to α + β duplex phase structure after aged at the elevated temperature of 773 K and over. It was clarified that Cr showed higher precipitation potential than Fe in the brass matrix. The hardness depended strongly upon solid solubility of Cr and Fe, and upon phase transformation.     

Ageing reactions in a high carat gold alloy for dental porcelain bonding

The phase transformation during continuous heating of a high carat gold alloy used for porcelain bonding was investigated by electrical resistivity measurements, hardness tests, X-ray diffraction and scanning and transmission electron microscopy. Four reaction stages (I, II, III and IV) were found. Stage I corresponded to the formation of a short-range order. A discontinuous precipitation took place in stage II, which contributed to remarkable hardening. Stages III and IV were reactions to the stable phases at each temperature region, and resulted in softening. The activation energies for stages I, II and III are 27.1, 33.8 and 58.2 kcal/mol, respectively.     

Phase transformation in NiTiHf shape memory alloy thin films

In the present study, amorphous NiTiHf thin films with different compositions were deposited on silicon wafers by using D.C. magnetron sputtering. Crystallization and martensitic transformation characteristics were studied. Crystallization temperatures and activation energy increased with increasing Hf content. The addition of Hf caused larger atomic size mismatch and stronger interactions among constituent elements, thus, increasing the thermal stability of amorphous thin films. With increasing annealing temperature or Hf content, martensitic transformation temperature increased. The results imply that NiTiHf thin films may be used as the potential candidates for high temperature applications in microactuators.     

The effects of casting techniques on the mechanical properties of a dental cobalt-chromium alloy

Tensile test pieces were cast by four selected sprueing techniques and studied for density and tensile properties. One of the sprueing systems referred to as horizontal twin feed produced the most consistent and the highest values for mechanical properties. This same series exhibited the lowest percentage elongation and the highest density.     

Variations in steel cleanness during transient continuous casting practices

Abstract

During sequenced continuous casting, concerns have been expressed about the density of nonmetallic inclusions present in steel slab produced during the transition period as successive ladles are teemed. Consequently, for critical applications such as thin wall tinplate products, with can wall thicknesses of ～0.26 mm, only steel cast during 'steady state' conditions is selected. In contrast, slab cast over defined lengths before and after a ladle change is downgraded to less demanding applications, irrespective of cleanness. To evaluate this practice, steel inclusion levels have been assessed during ladle change operations, when the steel reservoir in the tundish fell to below 20 t. This plant based study confirmed that, during the transition period, marked variations occur in the size and morphology of the nonmetallic inclusions, as well as in the oxygen content of the steel slab.     

钢铁连铸用铜合金结晶器

结晶器是钢铁连铸设备的重要部件，其用途是把浇注的钢水成形并生成足够厚度的凝壳，防止铸锭带移向２次冷却带时跑钢。以往，结晶器是选择导热性良好的脱氧铜制作的。实践表明，脱氧铜结晶器在使用中易变形和磨损，寿命很短。因此，研制、开发新型铜合金结晶器材料，便成为钢铁连铸工程中的重要课题。本文综合性地介绍了这方面的发展。     

Age-hardening reactions and microstructures of a dental gold alloy with palladium and platinum

The age-hardening reactions and microstructures of a dental casting gold alloy with some palladium and platinum were investigated by means of hardness tests, X-ray diffractometry, electrical resistivity measurements, and scanning and transmission electron microscopy. Ageing reactions during isothermal annealing were completed by two stages. The first stage corresponded to the formation of a metastable CuAu I phase within grains, and the second stage to a cellular reaction at grain boundaries. The former contributed to the hardening and the latter to the softening. An activation energy of 128 kJ mol^–1 for hardening was obtained. Two types of cell growth were observed and could be distinguished in terms of the homologous temperature.     

Joule heating in electromagnetic casting

According to the experimental analysis of temperature distribution in an Al ingot of electromagnetic casting, the induction thermal profile in the horizontal and vertical directions is studied. The influence of inductor currents on the thermal distribution of the ingot is also analyzed. The relation between the average temperature in the ingot cross-section and the metal height is studied. Finally, the gross Joule heating is calculated from the experimental and regression curve.     

The microstructure and property of Al-Si alloy and Al-Mn alloy bimetal prepared by continuous casting

The Al-10%Si alloy and Al-1%Mn alloy bimetal slabs are prepared by continuous casting. The microstructures around the interface are investigated, chemical composition distributions across the interface are detected, and tensile strengths are evaluated at the top, center and bottom regions of the bimetal slab. The results show that the Al-1%Mn alloy serves as a substrate of heterogeneous nucleation of Al-10%Si alloy at the bimetal interface. The metallurgical bonding is excellent without any discontinuities along the interface due to the diffusions of Si and Mn elements. The thickness of diffusion layers is about 40 μm on average. The tensile strengths of the top, center and bottom regions of the bimetal slab tend to be uniform.     

Phase transformation in nanoscale indium-tin alloy particles embedded in metallic matrices

The interfaces influence the phase stability of small particles. For embedded indium-tin alloy particles, the interface among the phases present within the particles and the matrix contribute significantly to the free energy and hence influences the phase stability at small sizes. Our results show that upon rapid cooling, we obtain a combination of larger two-phase alloy particles consisting of beta- and y-phases and very fine precipitates of In or Sn. Both beta- and gamma-phases possess an orientation relationship with Al. The in situ results suggest that the beta-rich phase gradually dissolved in the eutectic liquid before melting and the particles retain the facets much above the melting temperature of the alloy particle. We have tried to explain the formation and stability of these phases in terms of favorable nucleation kinetics.     

Kinetics of austenite formation during continuous heating in a low carbon steel

The kinetics and microstructural evolution of austenite formation in a low carbon steel, with initial microstructure composed of ferrite and pearlite, were studied during continuous heating, by using dilatometric analysis and measurements of microstructural parameters. The formation of austenite was observed to occur in two stages: (a) pearlite dissolution and (b) ferrite to austenite transformation. The critical temperatures of austenite formation in continuous heating increase with increasing heating rate, with greater influence on the finishing temperature of austenite formation. For both the 1 °C/s and 0.1 °C/s heating rates, the formation rate of austenite reaches a maximum at approximately the finishing temperature of pearlite dissolution, and the formation rate of austenite as a function of the temperature is greater at the higher heating rate.