On the evaluation of stability of rare earth oxides as face coats for investment casting of titanium

Attempts have been made to evaluate the thermal stability of rare earth oxide face coats against liquid titanium. Determination of microhardness profiles and concentration profiles of oxygen and metallic constituents of oxide in investment cast titanium rods has allowed grActation of relative stability of rare earth oxides. The relative stability of evaluated oxides in the order of increasing stability follows the sequence CeO2 — ZrO2 — Gd2O3 — didymium oxide — Sm2O3 —Nd2O3 — Y2O3. The grading does not follow the free energy data of the formation of these oxides. A better correlation with the experimental observations is obtained when the solubility of the metallic species in titanium is also taken into consideration.

Electrical transport in magnesium aluminate

The conductivity of MgAl2O4 has been measured at 1273, 1473 and 1673 K as a function of the partial pressure of oxygen ranging from 105 to 10−14 Pa. The MgAl2O4 pellet, sandwiched between two platinum electrodes, was equilibrated with a flowing stream of either Ar + O2, CO + CO2 or Ar + H2 + H2O mixture of known composition. The gas mixture established a known oxygen partial pressure. All measurements were made at a frequency of 1 kHz. These measurements indicate pressure independent ionic conductivity in the range 1 to 10−14 Pa at 1273 K, 10−1 to 10−12 Pa at 1473 K and 10−1 to 10−4 Pa at 1673 K. The activation energy for ionic conduction is 1·48 eV, close to that for self-diffusion of Mg2+ ion in MgAl2O4 calculated from the theoretical relation of Glyde. Using the model, the energy for cation vacancy formation and activation energy for migration are estimated.

Effect Of Electroslag Refining On Microstructure And Mechanical-Properties Of Is-7670 Alloy

The electroslag refining technique is one of the modern tools which is capable of imparting superior mechanical and chemical properties to metals and alloys. Refining usually results in the elimination of a number of casting or solidification defects, such as shrinkage porosity, gas porosity, pipe, micro- and macro segregation. Remelting also imparts a directional grain structure apart from refining the size of the inclusions, grains and precipitates. This technique has over the years been used widely and successfully to improve the mechanical and chemical properties of steels and alloy steels which are used in the nuclear, missile, aerospace and marine industries for certain critical applications. But the application of ESR to aluminium and its alloys is only recent. This paper investigates the response of an aluminium alloy (corresponding to the Indian Specification IS: 7670) to ESR. Based on theoretical considerations and microstructural evidence it elucidates how ESR of aluminium alloys differs from that of ferrous alloys. The improvement achieved in mechanical properties of the alloy is correlated with the microstructure.

Crystallization studies on amorphous AI-Y-Ni and AI-Y-Cu alloys

AI83Y10Ni7, AI80Y10Ni10 and AI80Y10Cu10 alloys were studied by the rapid solidification processing route. The glass-forming ability was found to decrease in the order of alloys mentioned above. Differential scanning calorimetry (DSC) of these amorphous alloys showed that the amorphous phase in AI-Y-Ni alloys has a higher thermal stability when compared to that in AI-Y-Cu alloys. A four-stage crystallization sequence could be identified for the AI-Y-Ni amorphous alloys. Even though the AI80Y10Cu10 alloy showed four exothermic peaks in the DSC study, a definite crystallization sequence could not be arrived at due to the coexistence of many crystalline phases along with the amorphous phase in the melt-spun condition.

Magnetoresistance of FexNi80-xCr20 (50alloys

The authors have measured longitudinal and transverse magnetoresistance (MR) of crystalline pseudo-binary alloys FexNi80-xCr20 (50alloys have compositions close to the critical composition regime for ferromagnetism. While the Fe-Ni-Cr alloys have a FCC gamma -phase, the binary Fe-Cr alloy has a BCC alpha -phase. This gives an opportunity to compare magnetoresistances for gamma -phase and alpha -phase Fe alloys when they lie close to the critical composition. The experiments were conducted at 4.2 K with magnetic fields up to 7 T. The authors observed that all the alloys show negative magnetoresistance at 4.2 K in fields up to 7 T. However, for the gamma -phase alloys the typical maximum MR ( Delta rho / rho ) is about 1%, while for the alpha -phase alloy it is 10%. In the gamma -phase alloys there is a small but finite anisotropy of MR in the phases with long-range magnetic order which gradually vanishes near the critical region (x=xc approximately=59-63) when it becomes a spin glass. In the range x approximately=xc, Delta rho / rho varies as Mn (M=magnetization) with n approximately=2.

Processing map for hot working of as cast magnesium

The constitutive flow behaviour in hot working of as cast magnesium has been studied with the help of a processing map developed in the temperature range 300-550°C and strain rate range 0·001-100 s−1. The map, interpreted using the dynamic materials model, revealed that the material undergoes dynamic recrystallisation at 425°C and 0·3 s−1, which are the optimum parameters for hot working. Ai temperatures higher than 450°C and strain rates lower than about 0·1 s−1, wedge cracking occurs in as cast magnesium. The wedge cracking domain has a high efficiency of power dissipation (60%), whereas the dynamic recrystallisation domain has a value of 34%. At temperatures below 450°C and strain rates above 10 s−1, the material exhibits flow instability in the form of mechanical twinning. At higher temperatures and strain rates, instability is manifested by flow localisation.

Processing maps for hot working of Cu-Ni-Zn alloys. Part I - Alpha-nickel silver

The constitutive behaviour of agr — nickel silver in the temperature range 700–950 °C and strain rate range 0.001–100 s–1 was characterized with the help of a processing map generated on the basis of the principles of the ldquodynamic materials modelrdquo of Prasadet al Using the flow stress data, processing maps showing the variation of the efficiency of power dissipation (given by 2m/(m+1) wherem is the strain-rate sensitivity) with temperature and strain rate were obtained, agr-nickel silver exhibits a single domain at temperatures greater than 750 °C and at strain rates lower than 1s–1, with a maximum efficiency of 38% occurring at about 950 °C and at a strain rate of 0.1 s–1. In the domain the material undergoes dynamic recrystallization (DRX). On the basis of a model, it is shown that the DRX is controlled by the rate of interface formation (nucleation) which depends on the diffusion-controlled process of thermal recovery by climb. At high strain rates (10 and 100s–1) the material undergoes microstructural instabilities, the manifestations of which are in the form of adiabatic shear bands and strain markings.

Processing maps for hot working of Cu-Ni-Zn alloys Part II Alpha-Beta nickel silver

The constitutive behaviour of agr-beta nickel silver in the temperature range 600�850 °C and strainrate range 0.001�100s�1 was characterized with the help of a processing map generated on the principles of the dynamic materials model. On the basis of the flow-stress data, processing maps showing the variation of the efficiency of power dissipation (given by [2m/(m+1)], wherem is the strain-rate sensitivity) with temperature and strain rate were obtained, agr-beta nickel silver exhibits a single domain at temperatures greater than 700 °C and at strain rates lower than 1 s�1 with a maximum efficiency of power dissipation of about 42% occurring at about 850 °C and at 0.1 s�1. In the domain, the agr phase undergoes dynamic recrystallization and controls the deformation of the alloy, while the beta phase deforms superplastically. Optimum conditions for the processing of agr-beta nickel silver are 850 °C and 0.1 s�1. The material undergoes unstable flow at strain rates of 10 and 100 s�1 and in the temperature range 600�750 °C, manifestated in the form of adiabatic shear bands.

Measurement for Thermal Effusivity of AlxGa1-xN Alloys Using Thermoreflectance with Periodic Heating

AlxGa1-xN alloys with x=0.375, 0.398, 0.401, 0.592 and 0.696 were deposited on sapphire substrate by the hydride-vapor-phase epitaxy (HVPE) method. Thermal effusivity measurements were carried out on AlxGa1-xN alloys using a thermal microscope at room temperature. The lag between sinusoidal heating laser wave and thermoreflectance wave was used to measure the thermal diffusivity. Thermal conductivity values of the AlxGa1-xN alloys were also obtained as a function of AIN mole fraction in the alloy. The thermal conductivity was found to decrease with increasing AIN fraction and the experimental data agree with values estimated using the virtual crystal model.

The Determination of the Surface Tensions of Liquid-Iron, Nickel and Iron-Nickel Alloys using the electromagnetic Oscillating Droplet Technique

An oscillating droplet method combined with electromagnetic levitation has been applied to determine the surface tensions of liquid pure iron, nickel and iron-nickel alloys as a function of the temperature. The natural frequency of the oscillating droplet is evaluated using a Fourier analyser. The theoretical background of this method and the experimental set-up were described, and the influence of magnetic field strength was also discussed. The experimental results were compared with those of other investigators and interpreted using theoretical models (Butler's equation, subregular and perfect solution model for the surface phase).