On precipitation in an Fe-Cr-Ni-Mn-Mo-W-V-Nb-N alloy,following prolonged ageing at 650°C

The main existing intergranular phase in the alloy observed is thick lamellar and small plate-like M₆C. Owing to the presence of M₆C precipitates, intergranular M₂₃C₆ nucleates at these locations and coherently precipitates to form a thin lamellar and large-sized filmy single crystal. The misfit of M₂₃C₆ with M₆C obtained from the moiré pattern is 3%. Intragranular M₆C and M₂₃C₆ twin along their own planes (111), and the resulting twins keep a coherent relationship with austenite. The misfit of VC precipitated along dislocations with the matrix is smaller than that of dispersed VC with the matrix. Intragranular Laves phase together with intragranular M₆C and VC strengthens the alloy grains.     

Formation of aluminide coatings on low alloy steels at 650°C by pack cementation process

Abstract

The pack aluminising process is normally conducted on alloy steels at temperatures higher than 900°C at which mechanical properties of steels would degrade. This study aims to investigate the feasibility of pack aluminising a commercial 9Cr – 1Mo alloy steel at 650°C in an attempt to increase its high temperature oxidation and corrosion resistance without adversely affecting its mechanical properties and consequently to increase its long-term structural operating temperatures to up to 700°C. It was demonstrated that this could be achieved using packs containing AlCl₃ as an activator and elemental Al as a depositing source. The coatings formed under these conditions consisted of an outer Fe₁₄ Al₈₆ layer and an inner FeAl₃ layer with an abrupt interface between the coating and substrate, suggesting that the coating is formed via a mechanism of the inward Al reaction – diffusion. The pack Al content was varied from 1 to 6 wt-% to investigate its effects on the coating formation process. It was found that the pack Al content in this range affected only the coating thickness and therefore the growth rate of the coating, but not the surface Al concentration. A post-aluminising heat treatment study was also undertaken for an aluminised specimen at 650°C under an argon atmosphere to investigate the kinetics of converting the brittle Fe₁₄ Al₈₆ and FeAl₃ phase layers to a more ductile FeAl phase layer. It was observed that this was a slow process requiring 1132 h for an initial coating layer thickness of 33μm. The coating after the conversion consisted of a uniform top FeAl layer with all other alloying elements in the solid solution and a diffusion zone underneath.     

Evolution of M23C6 phase in HR3C steel aged at 650 °C

The evolution of the morphology, composition, and particle size of the M₂₃C₆ phase in HR3C steel aged at 650 °C was analysed, by means of metallographic microscopy, scanning electron microscopy, and transmission electron microscopy. The results showed that the Cr/Fe ratio in the M₂₃C₆ phase, in the form of irregular square and long strip, increased with the ageing time. Fe was gradually replaced by Cr in the M₂₃C₆ lattice. The M₂₃C₆ phase gradually evolved and then became stable after ageing for 2000 h.     

Creep fatigue behaviour of Type 321 stainless steel at 650°C

Abstract

Type 321 austenitic stainless steel has been used in the UK’s advanced gas cooled reactors for a wide variety of thin section components which are within the concrete pressure vessel. These components operate at typically 650°C and experience very low primary stresses. However, temperature cycling can give rise to a creep fatigue loading and the life assessment of these cycles is calculated using the R5 procedure. In order to provide materials property models and to validate creep fatigue damage predictions, the available uniaxial creep, fatigue and creep fatigue data for Type 321 have been collated and analysed. The analyses of these data have provided evolutionary models for the cyclic stress strain and the stress relaxation behaviour of Type 321 at 650°C. In addition, different methods for predicting creep fatigue damage have been compared and it has been found that the stress modified ductility exhaustion approach for calculating creep damage gave the most reliable predictions of failure in the uniaxial creep fatigue tests. Following this, validation of the new R5 methods for calculating creep and fatigue damage in weldments has been provided using the results of reversed bend fatigue and creep fatigue tests on Type 321 welded plates at 650°C in conjunction with the materials properties that were determined from the uniaxial test data.     

Precipitation hardening in an Al-Cu-Si-Mg alloy at 130 to 220° C

The variation of the hardness of a complex Al alloy, of the L70 type, (Al 4% Cu 0.8% Si 0.8% Mg 0.7% Mn 0.5% Fe) with ageing time at 130, 160 and 190° C was measured and correlated with transmission electron microscope observations of preciptate morphology. It was established that the peak hardness, after both 160 and 190° C ageing, was associated with coherent precipitate particles with a diameter of 700 Å and the subsequent limited overageing at these temperatures was controlled by the slow growth of particles. Measurements of particle coarsening at 220° C suggested that the process occurred by Ostwald ripening. In contrast, ageing at 130° C gave a series of hardness—time plateaus, which were ascribed to the sequential precipitation of zones,' and precipitates.     

Transition in the creep failure mode of an alloy 800H weld at 850°C

Creep tests were conducted in air and helium on Alloy 800H welded with a high strength filler. Shorter tenn tests produced ductile parent metal failures, but long-tenn samples (>5000 h) failed near the weld with low ductility. Crack distributions suggested that this transition arose because restraint of the parent by the more creep-resistant weld caused triaxial tensile stresses to develop near the weld. At short times, these reduced local creep rates in the parent so that fracture occurred away from the weld. However, given sufficient time, restraint promoted diffusion-assisted cavitation near the weld. Implications for other weldments are discussed.     

The formation of AIN in austenitic FeMnAl alloy at 1000°C

When heated in 1000°C air and nitrogen, plate-shaped AIN formed in austenitic Fe-31.3 wt% Mn-7.6 wt% Al-0.82 wt% C alloy. The AlN formed along the (111) habit plane of the fcc matrix has a hexagonal crystal structure. The orientation relationship between AIN and the fcc matrix is: (0001)_AIN∥(11̄1)_γ, [101̄0]_AIN∥[1̄12]_γ (21̄1̄0)_AIN∥(110)_γ. The formation of AIN induced both the depletion and an uneven distribution of Al in the matrix, which resulted in a poor high-temperature oxidation resistance of the alloy in nitrogen-containing environment.     

Decomposition of the Heusler alloy Cu2MnAl at 360°C

During thermomagnetic studies, the as-quenched Heusler alloy Cu_2.00 Mn_1.00 Al_1.01 exhibits two Curie temperatures \(\theta _{C_I } \) and \(\theta _{C_{II} } \) . After 98 h at 360° C, above the Curie point, X-ray diffraction patterns indicate two ferromagnetic phases: Cu₂MnAl (I) witha _I=5.8707 Å and \(\theta _{C_I } \) = 151 ± 10° C; Cu₂MnAl (II) witha _II=5.9656 Å and \(\theta _{C_{II} } \) = 332 ± 4° C, whereas the 850° C as-quenched alloy exhibitsa=5.9612 Å andθ _c=341±5° C. The results are discussed in terms of the early stages of the decomposition of the Heusler phase. A mechanism, involving mainly the atomic migration of manganese, is suggested in order to reach the equilibrium phases of the ternary Cu-Mn-Al diagram. The kinetics of the decomposition should be ruled by the nucleation and the growth of the Cu₃Mn₂Al intermetallic compound.     

Compressive deformation of an Mg-Al-Si-RE alloy between 120 and 180 °C

The deformation behaviour of an Mg-Al-Si-RE (ASE210) alloy between 120 and 180 °C was investigated by means of uniaxial compression tests to identify possible differences in the deformation response compared with uniaxial tensile data. Early fracture was observed in the low-temperature/high strain rate regime, fracture occurring by crack propagation at 45° with respect to the compression axis. In the high-temperature/low strain rate regime, the flow curves exhibited the typical shape that is usually observed in materials where deformation is controlled by recovery of substructure. The peak flow stresses obtained in this regime of temperature and strain rate were compared with other data obtained by testing the same alloy in tension. The strength of the alloy was found to be slightly greater in compression than in tension, this difference gradually disappearing as strain rate decreased.     

Flow behaviour and microstructural evolution in cold worked 70∶30 α-brass

Abstract

Deformation behaviour and microstructures at failure were investigated in a mill cold worked 70∶30 α-brass over the test temperature range of 298–973 K and strain rate range of 10^?5–5×10^?3 s^?1. Tensile properties as a function of temperature revealed three distinct regions, with their temperature sensitivity being maximum at intermediate temperatures (553–673 K) and much less towards the lower and higher temperature ranges. Two values of activation energy for high temperature deformation Q were obtained to be 117·5 kJ mol^?1 below 623 K and 196·4 kJ mol^?1 above this critical temperature. In the respective temperature range the values of stress exponent n were 5·6 and 3·8. Based on the values of Q and n, the deformation mechanism was suggested to be dislocation climb creep with a probable contribution from dislocation pipe diffusion on lowering the temperature. Both grain size and cavity size were found to increase with increasing test temperature, suggesting them to be interrelated and act as an alternative steps for accommodating grain boundary sliding. Static grain growth study, over the temperature range of 773 to 1073 K, led to activation energy for grain growth to be 71 kJ mol^?1, with the time exponent of 0·37.     

Evolution of Fe–Co rich particles in Alnico 8 alloy thermomagnetically treated at 800°C

Abstract

The morphology and size of Fe–Co rich particles in Alnico 8 specimens thermomagnetically treated at 800°C for various times have been investigated by means of transmission electron microscope (TEM) observations and small angle X-ray scattering (SAXS). Phases rich in Fe–Co and Ni–Al were formed during spinodal decomposition. With longer treatment times, as Fe–Co rich particles coarsened to a critical size, a first splitting took place. When the resultant small particles originating from the first splitting also coarsened, and they reached critical size, a similar secondary splitting occurred. This is attributable to perfect maintenance of the coherent relationship between the Fe–Co and Ni–Al rich phases, as well as the negative interaction energy that arose from the large lattice misfit. The splitting resulted in a sharp decrease in the mean particle diameter and an increase in the mean aspect ratio of the larger particles studied, which is expected to increase the coercive force of the Alnico 8. In addition, the mean particle length always decreased with increasing treatment time.     

Characterization of alloy 3033 after exposure to superheated steam at 800°C

Alloy 3033 was evaluated in superheated steam (SHS) at 800°C for a duration of 3000 hours. The SHS was able to simulate the supercritical water (SCW) condition at higher temperatures which no commercial SCW rig is currently capable of reaching. After exposure to the SHS, the weight change and surface oxide formation of Alloy 3033 were analyzed. Alloy 3033 had an initial weight gain after 1000 hours; however, the net weight gain reduced after 2000 and 3000 hours of exposure, suggesting oxide spallation. Formation of both Cr₂O₃ and MnCr₂O₄ was observed on the surface after 2000 and 3000 hours of SHS exposure. However, as exposure progressed, the XRD peak intensity ratio of MnCr₂O₄ to Cr₂O₃ decreased, in addition to the observation of more exposed Cr₂O₃. Based on this preliminary investigation, Alloy 3033 may not be suitable for extended use in SHS due to weight loss associated with oxide spallation.     

Effect of nanocrystallisation on oxide composition of Ni-20Cr alloy at 1000°C

Abstract

A Ni-20Cr nanocrystalline coating was obtained by means of magnetron sputtering. The oxide composition of the Ni-20Cr alloy and its nanocrystalline coating after treatment in air at 1000°C was analysed. The results indicated that the oxide scale formed on Ni-20Cr alloy is composed of two layers. The external layer consists of NiO containing some Cr₂O₃ particles and the internal layer is continuous Cr₂O₃ with a little NiCr₂O₄, whereas the scale formed on the sputtered Ni-20Cr nanocrystalline coating consists mainly of Cr₂O₃ with a little NiCr₂O₄. The mechanism for the effect of nanocrystallisation on the oxidation of the Ni-20Cr alloy is discussed.     

Effect of vanadic corrosion on creep-rupture properties of Superni-600 at 650–750°C

The effect of vanadic corrosion on creep-rupture properties of a nickel base superalloy Superni-600, at 650–750°C, has been investigated. Sodium metavanadate and sodium metavanadate plus 15 wt% sodium sulphate were used as the corrodent salts. Weight change studies have also been made to understand the mechanism of corrosion. Both sodium metavanadate and the sodium metavanadate/sodium sulphate mixture are found to be aggressive and to reduce the creep-rupture life. The degradation of creep-rupture properties and possible hot corrosion reactions are discussed. Cracking and fluxing of the protective scales, together with easier crack nucleation and growth at grain boundaries in the presence of a liquid deposit accounts for the enhanced creep rates and reduced rupture lives.     

The decomposition of the solid solution state in the temperature range 20 to 200° C in an Al-Zn-Mg alloy

The decomposition of the supersaturated solid solution of an Al-3.2wt% Zn-2.2 wt% Mg alloy has been investigated in the temperature of 20 to 200° C, by small-angle X-ray scattering, electrical resistivity and mechanical measurements. On the basis of the results obtained, three subsequent stages of the decomposition process can be distinguished. Between 20 and 70° the basic process is the nucleation and growth of G.P. zones, the volume fraction of which increases logarithmically with time. A transition stage is observed between 80 and 100° in which the volume fraction increases linearly with time. Above 90° C, the growth kinetics of the volume fraction shows a definite incubation period at the beginning of ageing, while the yield stress increases monotonically. In the temperature range 100 to 160° C, the formation of the phase takes place. Below 100° a linear connection between the yield stress and (fR)^1/2 is found from which the specific surface energy to cut a G.P. zone is calculated to be ₀= 0.21 Nm m^–2.