Volume 32, Issue 4

Thermal fatigue damage of high-temperature titanium alloys is of great concern for severe temperature-fluctuating environment, and the thermal fatigue crack initiation stage plays a crucial role in thermal fatigue life. In present study, thermal fatigue tests keeping 55 seconds at 700 °C followed by water cooling 15 seconds at 20 °C were performed for the laser deposited high-temperature titanium alloy Ti60A (Ti5.54Al3.38Sn3.34Zr0.37Mo0.46Si). Thermal fatigue cracks initiate after 800 thermal fatigue cycles with a length of 20 µm. Subsequently numerous cracks grow to 500 µm and cause severe degradation after 1000 cycles. To investigate the crack initiation behavior, microstructural changes during thermal fatigue process were examined by OM, SEM, EPMA and TEM. Thermal fatigue cracks initiate preferably at grain boundaries, α/β interfaces, microvoids, and abnormal coarsened α produced by oxygen interstitial solution. Mechanisms of thermal fatigue crack initiation are related to compatibility of local deformation and microstructural changes during thermal fatigue process.

In this study, the effect of different heat treatment processes applied to AA7075 alloys on the fatigue behavior was examined. The processes applied to AA7075 aluminum included annealing (O), high temperature pre-precipitating (HTPP), artificial aging (T6), retrogression and re-aging (RRA). The annealing heat treatment was performed for 2 hours at 500°C and samples were cooled in the furnace. In the artificial aging (T6) process, after the samples were solution treated for 2 hours at 500°C, they were quenched at room temperature and aged for 24 hours at 120°C. In the retrogression and re-aging process, samples were solution treated for 1 hour at 220°C after the T6 process and then re-aged for 24 hours at 120°C. In the high temperature pre-precipitating, pre-precipitates were formed for 30 minutes at 450°C and then, it was aged for 24 hours at 120°C. All samples were characterized through the scanning electron microscope (SEM + EDS), hardness measurements and X-ray difraction (XRD) techniques. At the end of experimental studies, SEM and EDS examinations XRD results revealed that η (MgZn 2 ) phase formed in the microstructure following the HTTP, RRA and T6 heat treatment processes. As a result of the fatigue tests, the highest fatigue strength was measured in samples treated with artificial aging (T6), the lowest fatigue strength was measured in the annealed (O) samples.

Directional solidification technique was successfully used to prepare highly reactive NiTiAl-based alloys using novel Y 2 O 3 -coated crucibles. Compared with the as-cast alloys fabricated by vacuum arc melting, the microstructure of the directionally solidified alloys was optimized and mechanical properties remarkably improved. After directional solidification, the microstructure of the alloys became much finer and preferentially grew along [001] orientation. The tensile strength and strain of the directionally solidified alloys at 1550°C were promoted to 1240 MPa and 2.1% respectively, which were 91.4% and 75% higher than that of the as-cast alloys. The strength increased up to 1328 MPa and 1419 MPa after homogenizing treating. The Y 2 O 3 -coated crucibles assured significant purification effect of the alloys. The maximum oxygen increase in the steady-state region of the ingot obtained at 1750°C was no more than 0.014 wt.%.

The low ambient tensile ductility and inadequate high temperature strength and creep resistance have limited the applications of intermetallic compound NiAl as a structure material at high temperatures. A small addition of Ag could increase the strength of NiAl. In this study, hot corrosion behavior of NiAl and NiAl-Ag containing 1 and 5%Ag (in weight percent), respectively, in molten (Na,K) 2 SO 4 at 1173 K has been examined by thermogravimetry and physical analysis techniques. The experimental results indicated that the corrosion rates of NiAl and NiAl-Ag tended to increase with exposure time, accompanied with the formation of nodules. Fast localized corrosion is the typical feature of the corrosion of both NiAl and NiAl-Ag. The addition of Ag to NiAl accelerated its corrosion, with a higher corrosion rate observed for the 5%Ag addition. The two-phase microstructure of NiAl-Ag alloys accelerated the formation of nodules in the regions close to Ag phases.

Extracting aluminum from aluminum alloys in AlCl 3 -NaCl molten salts was investigated in this paper. The influences of experimental parameters such as electrolyte composition, cathodic current density and electrolysis time on the deposits morphology were discussed. The results show that the grain size of the deposits decreases with the increase of AlCl 3 content in the electrolyte. Current density has a big effect on the morphology of the deposits. The particle size of deposits increases with the increase of current density, and dendritic morphology forms at high current density. High nucleation rates are achieved at high current densities above the limiting diffusion current density, and will result in a finer grain size. A non-dendritic deposit of aluminum was obtained at 170°C at 50 mA cm −2 cathodic current density for 1 h in the electrolyte having a 1.3 molar ratio of AlCl 3 /NaCl. The purity of the aluminum deposit is about 99.79% analyzed using inductively coupled plasma.

The solidification structure of two 500 g cylindrical ingots of Fe-36Ni invar alloy, which were melted in a carbon tube furnace and cast adding Ce and Zr-Ti refiners respectively, were investigated. The results showed that the grain size became smaller. With the increase of refiner content, the ratio of equiaxed grains of ingot increased and the equiaxed grain size decreased. As for the 100 kg flat ingot, which was melted in a vacuum induction furnace and cast with Ce and Zr-Ti refiners respectively, the situation was different slightly. When adding Ce the dendritic grain diameter of the flat ingot became finer and the equiaxed grain ratio remained low; however, the mechanical property deteriorated. While adding Zr-Ti complex refiners, the solidification structure of the alloy was improved dramatically. Therefore, the mechanical properties at room temperature of as-cast flat ingots were improved. Furthermore, its high temperature ductility range expanded and the expansion coefficient of its hot-rolling plate remained almost the same.

The thermodynamic quantities of the isothermal compressibility, thermal expansion and the specific heat are calculated here as a function of pressure by using the observed Raman frequencies of the lattice modes and vibrons in the η phase of solid nitrogen. The Pippard relations and their spectroscopic modifications are constructed, and the slope dP/dT is deduced from the Raman frequency shifts in this phase of N 2 . It is shown that the thermodynamic quantities can be predicted from the Raman frequency shifts, in particular, in the η phase of solid nitrogen.

Colloidal spherical particles (micrometer size) of zirconia were synthesized at 50°C by homogeneous precipitation method from aqueous solutions, containing appropriate amounts of zirconium chloride, hydrochloric acid, potassium sulphate and urea. Results revealed that reactants concentration significantly affected characteristics of the precipitated solids, which ranged from gel to dispersion of discrete particles. Size uniformity in the resulted particles was obtained only under limited conditions. SEM images revealed that particles of zirconia obtained under the described experimental conditions have spherical morphology, which maintained their original shape after calcination at 700°C. Selected powders were also characterized with XRD, TGA/DTA, and FT-IR techniques. XRD results showed that as-prepared and calcined powders were crystalline. We believe that our method is simple and has the potential for further tailoring of the particles characteristics. Work is in progress in our laboratory in developing Cu-ZrO 2 metal matrix composite coating on steel substrate with better resistant properties than simple Cu electroplating.

The effect of rare-earth element Y on high-temperature oxidation behavior of Cu-Si alloys at 973 and 1073 K in 0.1 MPa flowing pure O 2 has been investigated. Results show at the two temperatures the mass gain of the alloys with different compositions follows the following sequence, Cu-3Si-1.0Y > Cu-3Si-0Y > Cu-3Si-0.5Y alloy. As the Y content increases, the grain size of the alloys decreases, which accelerates the diffusion rate of the alloying elements in the alloys and oxygen under the experimental conditions, increases the ratio of short-path diffusion, and promotes the formation of SiO 2 and Y 2 O 3 . The three alloys do not form continuous oxide scales of SiO 2 or Y 2 O 3 , but their rapid formation and dispersed distribution due to the grain-size reduction may also hinder the diffusion of alloying elements and oxygen, which is beneficial to improve the oxidation resistance of the Y-containing alloys. Thereby, the Cu-3Si-0.5Y alloy has good oxidation resistance. The Cu-3Si-1.0Y alloy exhibits largest mass gain among the three alloys, which is due to the fact that in the alloy with higher Y contend and finer grain size, more amount of Y 2 O 3 is more quickly formed, whose mass occupy a more proportion in the whole mass gain of the alloy.

In this study, experiments were conducted to understand the optimum direct reduction conditions of mill scale which is formed on the surface of the materials produced by continuous casting and which contains iron (II) and iron (III) oxides. Experiments were performed in a rotary kiln and anthracite and metallurgical coke were used as carbon source. The eligible parameters like process temperature, process duration, reductant type and quantity were tried to determine. Obtained reduced iron pellets were characterized by using EPMA, XRD and chemical analysis techniques. The highest metallization degrees were observed as 97.4% for the stoichiometrically 200% anthracite added experiments and as 95.5% for the stoichiometrically 200% metallurgical coke added experiments at 1423 K.

Recrystallization behavior of 20% coldworked alloy D9 (14Cr-15Ni-2Mo austenitic stainless steel) during high temperature tensile deformation has been assessed. The alloy was produced by vacuum induction melting followed either by vacuum arc refining (VAR) or electroslag refining (ESR). Tensile studies were carried out at various temperatures between ambient and 1073 K at an interval of 50 K and at strain rate of 1.2 × 10 −3  s −1 . The peak in the stress-strain curve was observed for the tensile tests conducted at 973–1073 K. This is attributed to dynamic recrystallization during tensile deformation. The values of the apparent activation energy were estimated as 384 kJ/mol for ESR grade alloy D9 and 372 kJ/mol for VAR grade alloy D9 by hyperbolic-sine Arrhenius equation, which agree well with that required for dynamic recrystallization in stainless steels. TEM studies confirmed extensive recovery and dynamic recrystalization in both the alloys.