Creep-rupture properties and grain-boundary sliding in wrought cobalt-base HS-21 alloys at high temperatures

The effects of serrated grain boundaries on the creep-rupture properties of wrought cobaltbase HS-21 alloys were investigated at 1311 and 1422 K. The amount of grain-boundary sliding and the initiation and growth of grain-boundary cracks were also examined during creep at 1311 K. Specimens with serrated grain boundaries exhibited longer rupture life and larger rupture ductility than those with straight grain boundaries, but these specimens had almost the same rupture life and rupture ductility under lower stresses at 1422 K, because serrated grain boundaries were also formed in specimens with originally straight grain boundaries. The average amount of grain-boundary sliding during creep at 1311 K increased with time (or with creep strain), but was almost the same in both specimens with serrated grain boundaries and those with straight grain boundaries at the same creep strain. Grain-boundary cracks or voids initiated in the early stage of creep in those specimens at 1311 K. Therefore, the strengthening by serrated grain boundaries at high temperatures above about 1311 K was attributed to the retardation of growth and linkage of grain-boundary cracks and voids.     

The creep-rupture properties and the initiation and growth of the grain-boundary cracks in the cobalt-base HS-21 alloy

The effect of the grain-boundary microstructures on the creep-rupture properties and the initiation and growth of the grain-boundary cracks was investigated using four kinds of specimen of various grain-boundary microstructures in the cobalt-base HS-21 alloy at 1089 K in air. Both the rupture strength and the creep ductility increased with increasing mean value of the fractal dimension of the grain boundaries, Dgb. The strain to crack initiation was largest in the specimen of the highest value (1.241), while the strain was much the same in the specimens of the Dgb value less than 1.162. This was explained by the local variation in the grain-boundary microstructures in these specimens. The mean value of the fractal dimension of the grain-boundary fracture, Df, was close to the value of Dgb, although the value of Df was a little higher than that of Dgb in the specimens of the lower Dgb values. The fracture appearance changed from a brittle grain-boundary fracture to a ductile one with increasing values of Dgb and Df. The crack-growth rate is the surface-notched specimens decreased with increasing value of Dgb. The threshold stress intensity factor for crack growth was higher in the specimens with the higher Dgb values. This revised version was published online in November 2006 with corrections to the Cover Date.     

As-cast microstructures and behavior at high temperature of chromium-rich cobalt-based alloys containing hafnium carbides

Hafnium is often used to improve the high temperature oxidation resistance of superalloys but not to form carbides for strengthen them against creep. In this work hafnium was added in cobalt-based alloys for verifying that HfC can be obtained in cobalt-based alloys and for characterizing their behavior at a very temperature. Three Co–25Cr–0.25 and 0.50C alloys containing 3.7 and 7.4 Hf to promote HfC carbides, and four Co–25Cr– 0 to 1C alloys for comparison (all contents in wt.%), were cast and exposed at 1200 °C for 50 h in synthetic air. The HfC carbides formed instead chromium carbides during solidification, in eutectic with matrix and as dispersed compact particles. During the stage at 1200 °C the HfC carbides did not significantly evolve, even near the oxidation front despite oxidation early become very fast and generalized. At the same time the chromium carbides present in the Co–Cr–C alloys totally disappeared in the same conditions. Such HfC-alloys potentially bring efficient and sustainable mechanical strengthening at high temperature, but their hot oxidation resistance must be significantly improved.     

Deformation of Cu–P alloys at high temperatures

Abstract

The deformation behaviour of Cu–P alloys has been investigated by torsion and tensile testing over a range of strain rates and temperatures. The torsion flow curves are interpreted in terms of dynamic softening processes, and the curves obtained during interrupted testing are used to examine static-restoration behaviour. Constitutive equations relating flow strength to strain rate and temperature are deduced, with allowance made for the effect of deformation heating, and implications of the equation constants are discussed. It is shown from tensile results that a state of superplasticity can be achieved in alloys containing 3·8 and 6·8 wt-%P. Superplasticity can occur only if the small α grain size is stable and if the temperature and strain rate fall within certain limits. The activation energy associated with superplastic flow has been determined.

MST/52     

Elastic properties of two titanium alloys at low temperatures

Sound velocities and elastic constants were determined semi-continuously for two annealed polycrystalline titanium alloys between 4 and 300 K. Results are given for: longitudinal sound velocity, transverse sound velocity. Young's modulus, shear modulus, bulk modulus, Poisson's ratio, and elastic Debye temperature. A pulse-superposition technique was used.     

Thermophysical properties of cobalt at high temperatures

Measured values of the thermal conductivity, total hemispherical and spectral (λ=0.65 μ) emittances, resistivity, and coefficient of linear expansion of cobalt are presented.     

Thermophysical properties of lanthanides at high temperatures

Results of measurements of heat capacity, thermal diffusivity, and thermal conductivity of yttrium, gadolinium, holmium, and lutecium in the temperature range 1100–2100°K are presented. The behavior of the thermophysical properties with phase transitions is discussed.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 6, pp. 1010–1012, December.     

Low-temperature elastic properties of four wrought and annealed aluminium alloys

The elastic properties of four annealed polycrystalline commercial aluminium alloys were studied between 4 and 300 K using a pulse-superposition method. Results are given for longitudinal sound velocity, transverse sound velocity, Young's modulus, shear modulus, bulk modulus (reciprocal compressibility), Poisson's ratio, and elastic Debye temperature. The elastic stiffnesses of the alloys increase 4 to 13% on cooling from room temperature to liquid helium temperature. The elastic constant-temperature curves exhibit regular behaviour.     

Integral emittance of alloys of the iron-aluminum system at high temperatures

Experimental data on the integral emittance coefficients of alloys of the iron-aluminum system in the temperature range from 80 to 1700°C are given.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 47, No. 5, pp. 823–824, November, 1984.     

Effects of high-temperature ageing on the creep-rupture properties of cobalt-base L-605 alloys

Effects of high-temperature ageing on the creep-rupture properties of cobalt-base L-605 alloys were investigated at 1089 and 1311 K in air. The specimens with serrated grain boundaries and those with normal straight grain boundaries were aged for 1080ksec at 1273 or 1323 K to cause the matrix precipitates of tungsten-rich b c c phase and M₆C carbide. The creep-rupture strength of both specimens were improved by the high-temperature ageing. The rupture strength at 1311 K was the highest in the specimens with serrated grain boundaries aged at 1273 K, while the specimens with straight grain boundaries aged at 1273 K of the highest matrix hardness had the highest rupture strength at 1089 K. The high-temperature ageing did not decrease the rupture ductility of specimens. The ruptured specimens with serrated grain boundaries exhibited a ductile grain-boundary fracture surface which consisted of dimple patterns and steps, regardless of whether high-temperature ageing was carried out. The fracture mode of the specimens with straight grain boundaries was changed from the brittle grainboundary fracture to the ductile one similar to that of the specimens with serrated grain boundaries by high-temperature ageing, since large grain-boundary precipitates which gave nucleation sites of dimples were formed during the ageing. The grain-boundary cracks initiated in the early stage of creep (transient creep regime) in both non-aged and aged specimens of L-605 alloys in creep at 1089 and 1311 K, although the time to crack initiation is shorter in the specimens with straight grain boundaries than in those with serrated grain boundaries. Thus, the period of crack growth and linkage occupied most of the rupture life. The strengthening mechanisms of the aged specimens were also discussed.     

Plastic deformation and fatigue properties of ternary CoTi based alloys containing Al at room and high temperatures

Mechanical and fatigue properties of ternary CoTi based single crystals containing 2% and 5% Al was investigated. It was shown that in these two alloys the addition of Al improve the strength of CoTi at all tested temperatures, but by increasing Al content the peak temperature shifted to lower temperature and the effect of anomalous strengthening was weakened. The number of cycles to failure in fatigue test for CoTi (2 at.% Al) single crystals showed a minimum, at high temperatures. This phenomenon seems to be due to the activation of new slip systems. Also by increasing Al, the fatigue limit at constant temperature can be changed. It was shown that Al addition is not effective in improving fatigue life at low temperatures but it can improve it at high temperatures.     

Microstructure and mechanical properties of CxHf0.25NbTaW0.5 refractory high-entropy alloys at room and high temperatures

The microstructure and mechanical properties of as-cast and isothermally annealed CxHf0.25NbTaW0.5(x=0,0.05,0.15,0.25)refractory high-entropy alloys(RHEAs)were ...     

Fe-Cr合金钢高温性能研究

本文试验研制了四种Fe-Cr合金材料,其在900 ℃时的热膨胀率在10～11×10-3范围内,其中FeCr-4合金的热膨胀率为10.27×10-3,与钇稳定氧化锆(YSZ)相比,热膨胀率逐渐接近;与传统合金材料相比,热膨胀率之差均有了明显下降,因而降低了界面热应力,有效改善层间的热失配现象.随着温度的升高,各合金试样的电阻率呈平缓增大的趋势.在850℃时,四种合金试样的电阻率在0.95～1.17×10-3 Ω·cm范围内,其中Fe-Cr-4试样的电阻率较低,为1.17×10-3 Ω·cm,满足用作SOFC连接体材料导电性能的要求.Fe-Cr -1、4两种合金试样的抗氧化性能较好,随着氧化时间的延长,试样表面形成稳定的3Cr2 O3 Fe2 O3氧化层,氧化增重逐渐趋于稳定.累计氧化455 h后,Fe-Cr-1、4两种试样对应的氧化增重在0.0002 g/g左右;而Fe-Cr-2、3两种试样仍在0.0004 g/g以上.