Effect of heat treatment on precipitation behaviour in a Cu-Ni-Si-P alloy

The effects of applying different solution and ageing conditions on the electrical resistivity and precipitation behaviour of a Cu-1.3Ni-0.3Si-0.03P (wt%) alloy were studied. The electrical resistivity of solution-treated material is greatly reduced, by about 50%, by the ageing processes. The reduction in resistivity is due to depletion of solute atoms from the copper matrix by the formation of precipitates. Double ageing peaks appeared during isothermal ageing due to the formation of Ni₃P and Ni₂Si precipitates. The first maximum, due to the precipitation of Ni₃P, appeared at about 1 h of ageing time, while the second peak, due to Ni₂Si, appeared at around 10 h of ageing time when aged at 450°C. The precipitate Ni₃P forms early and the alloy starts to over-age before Ni₂Si precipitates and the alloy reaches maximum hardness. The maximum hardness produced by the precipitations of Ni₃P and Ni₂Si decreased with increasing ageing temperature from 450 to 550°C. The time to reach the maximum hardness due to Ni₃P precipitation became shorter, while that of Ni₂Si became longer, as the solution treatment temperature increased from 780 to 1020° C. The apparent activation energy for Ni₂Si precipitation was found to be about 80 kJ mol^–1 while that for Ni₃P precipitation was about 25 kJ mol^–1.     

Effect of notch on fatigue behaviour of a directionally solidified superalloy at high temperature

The effect of notch types and stress concentration factors (K_t) on low cycle fatigue life and cracking of the DZ125 directionally solidified superalloy has been experimentally investigated. Single‐edge notched specimens with V and U type geometries were tested at 850 °C with stress ratio R = 0.1. High temperature in situ optical method was used to observe crack initiation and short crack propagation. Scanning electron microscope observation of fracture was used to analyse the failure mechanism. The results reveal that fatigue resistance decreases with K_t increasing from 1.76 to 4.35. The ratcheting is found to be affected by both K_t and the nominal stress from the displacement–force curve. In situ observations indicate that the cracking does not occur at the notch apex but at the location where the max principal stress or Hill's stress is the highest. According to the scanning electron microscope observations, the failure of the notched specimens strongly depends on the anisotropy microstructures.     

Carbide behaviour during high temperature low cycle fatigue in a cobalt-base superalloy

The carbide behaviour of a directionally solidified cobalt-base superalloy has been investigated after low cycle fatigue at 900°C. During fatigue, primary carbides, M₇C₃ and MC, decomposed sluggishly and a great amount of secondary carbide, chromium-rich M₂₃C₆ precipitated. The inhomogeneous distribution of M₂₃C₆ brought about a different dislocation substructure. In the vicinity of the primary carbides, densely-distributed fine M₂₃C₆ pinned up dislocations effectively, resulting in a uniform distribution of dislocations, while in the interior of grains, since precipitates were coarse and scarce, dislocations were arranged in a planar array and piled up in the front of the precipitates. M₂₃C₆ also acted as an obstacle deflecting fatigue crack. Primary carbides on the surface of specimens were oxidizied preferentially, causing a precipitate depletion around them. The oxidized primary carbides were crack initiation sites. The primary carbides hindered fatigue crack propagation, causing the formation of shear steps.     

Effects of heat treatment on prior particle boundary precipitation in a powder metallurgy nickel base superalloy

PPB precipitation in powdered Rene'95 is analyzed. The effects of PHT on PPB precipitation are studied. It is found that fracture along PPB is ductile in microscale. During PHT, the phase transformation on powder surface is similar to PPB interface. PHT cannot reduce the degree of contamination on powder surface. But the precipitates on surface are coarsened. The coarsening enlarges distances between precipitates. As the result, microfracture property is improved at PPB.     

Effect of heat treatment on mechanical and ballistic properties of a high strength armour steel

In the present study an ultra high strength armour steel was austenatised at 910°C followed by tempering at 200, 300, 400, 500 and 600°C. After heat treatment the properties of tensile strength, ductility, charpy impact strength, hardness and microstructure were evaluated from the mechanical tests and metallographic analysis respectively. The ballistic behavior of the heat-treated plates was evaluated impacting against non-deformable hard steel core projectiles at 840 ± 15 m/s at normal angle of attack. The changes in the microstructure and mechanical properties with heat treatment have been correlated with ballistic performance of the steel. Experimental results showed that 200°C tempering gives the best ballistic performance.     

Correlation between grain boundary misorientation and M23C6 precipitation behaviors in a wrought Ni-based superalloy

The correlation between the grain boundary misorientation and the precipitation behaviors of intergranular M₂₃C₆ carbides in a wrought Ni–Cr–W superalloy was investigated by using the electron backscattered diffraction (EBSD) technique. It was observed that the grain boundaries with a misorientation angle less than 20°, as well as all coincidence site lattice (CSL) boundaries, are immune to precipitation of the M₂₃C₆ carbides; in contrast, the random high-angle grain boundaries with a misorientation angle of 20°–40° provide preferential precipitation sites of the M₂₃C₆ carbides at the random high-angle grain boundaries with a higher misorientation angle of 55°–60°/[2 2 3] turn to retard precipitation of M₂₃C₆ carbides owing to their nature like the Σ3 grain boundaries and retard the precipitation of M₂₃C₆ carbides. The low-angle and certain random grain boundary segments induced by twins were found to interrupt the precipitation of the M₂₃C₆ carbides along the high-angle grain boundaries.     

Effect of high pressure and high temperature on the microstructural evolution of a single crystal Ni-based superalloy

The application of high nearly hydrostatic pressures at elevated temperatures on the LEK94 single crystal (SX) nickel-based superalloy directly affects its microstructure. This is due to a combination of the effect of pressure on the Gibbs free energy, on the diffusion coefficients of the alloying elements, on the internal coherent stresses, and on the porosity distribution. The last effect depends at least on the first three. Therefore, based on the theoretical influences of the pressure, the main objective of this work is to understand, by means of an experimental work, the effect of high pressure at elevated temperature during annealing on the evolution of the phases morphology, and porosity of the high-temperature material LEK94. Specifically, pressures up to 4 GPa, temperatures up to 1180 °C, and holding times up to 100 h were investigated. The main findings are that, porosity can be considerably reduced without affecting significantly the γ/γ′ microstructure by high pressure annealing and the verification that increasing the external pressure stabilizes the γ′-phase.     

Yield strength anomaly and dynamic strain ageing behaviour of recently developed advanced ultra-supercritical boiler grade wrought Ni-based superalloy IN 740H

The yield strength anomaly (YSA) and dynamic strain ageing (DSA) behaviour of advanced ultra-supercritical boiler grade wrought nickel-based superalloy IN 740H is studied by conducting tensile tests in temperature range 28–930°C and by employing strain rates 1 × 10^?2, 1 × 10^?3, 1 × 10^?4 and 1 × 10^?5 s^?1 followed by extensive electron microscopic examination. Increase in yield strength accompanied by impairment of ductility indicates that YSA exists in alloy IN 740H in temperature range of 650–760°C. The electron microscopic observation confirms that YSA is due to pinning of dislocations by γ′ precipitates and shearing of γ′ precipitates in IN 740H. DSA is observed in the temperature range of 200–500°C and is predominant at 300°C. The nature of serrated plastic flow due to DSA is dependent on the temperature and strain rate.     

Effect of Re content on precipitation behaviour of secondary phases in a single-crystal Ni-based superalloy during high-temperature thermal exposure

The precipitation of secondary phases in single-crystal Ni-based superalloys with different contents of Re are investigated using TEM and SEM. The results show that after full heat treatment, the size of secondary γ′ phase decreases gradually with increasing Re content. During long-term thermal exposure at 1093 °C, some needle-like and rod-like secondary phases precipitate from the γ matrix. With increasing Re content and prolonging of the thermal exposure, the amounts of precipitated phases clearly increase. In the alloy with 2·5 wt-% Re, the needle-like phases are identified as M₆C carbides. In the alloy with 3 wt-% Re, both M₆C carbides and μ phase are found. In the alloy with 3·5 wt% Re, the needle-like and rod-like precipitated phases are identified as μ and P phases. Stacking fault was found in the P phase and that part was identified as μ phase by SAD; that is, there is intergrowth of P and μ phases.     

Effect of heat treatment on microstructure and mechanical properties of a new β high strength titanium alloy

Microstructure and mechanical properties of a new β high strength Ti–3.5Al–5Mo–6V–3Cr–2Sn–0.5Fe titanium alloy were investigated in this paper. Both the α/β and β solution treatment and subsequent aging at temperatures ranging from 440 °C to 560 °C for 8 h were introduced to investigate the relationship between microstructures and properties. Microstructure observation of α/β solution treatment plus aging condition shows that the grain size is only few microns due to the pinning effect of primary α phase. The β solution treatment leads to coarser β grain size and the least stable matrix. The size and volume fraction of secondary α are very sensitive to temperature and strongly affected the strength of the alloy. When solution treated at 775 °C plus aged at 440 °C, the smallest size (0.028 μm in width) of secondary α and greatest volume fraction (61%) of α resulted in the highest yield strength (1624 MPa). And the yield strength decreased by an average of 103 MPa with every increase of 40 °C due to the increase of volume fraction and decrease of the size of secondary α. In β solution treatment plus aging condition, tensile results shows that the strength if the alloy dramatically decreased by an average of 143 MPa for every increase of 40 °C because of larger size of secondary α phase than α/β solution treated plus aged condition.     

Low temperature effects on the fracture behaviour of a nickel base superalloy

The mechanical properties of a solution treated and double aged nickel - 18% iron - 18% chromium alloy (Inconel 718) were studied to assess its utility at temperatures in the ambient-to-cryogenic range. Uniaxial tensile property measurements using unnotched specimens at decreasing temperatures between 295 and 4 K show that yield and ultimate strengths increase by 20% and 29%, respectively, while ductility remains virtually constant. Fracture mechanics tests using 2.54 cm thick compact specimens revealed that the fatigue crack growth resistance of this alloy improves slightly at extreme cryogenic temperatures, and its plane strain fracture toughness, K_lc, increases from 96.3 MPa m^{$\text{1}\text{2}$} at 295 K to 112.3 MPa m^{$\text{1}\text{2}$} at 4 K. These results are compared with similar data for Inconel 750 alloys     

Effect of temperature on the strength and fatigue behaviour of optical fibres

The fatigue behaviour of a polymer-coated fibre and a silicon oxynitride-coated fibre was measured in water between 25 and 90° C by the dynamic fatigue test technique where strength is measured as a function of stressing rate. The results are analysed in terms of stress corrosion theory. It was found that the silicon oxynitride coating reduced fatigue significantly, but caused a large reduction in strength of the fibre. The implications of these results to the design of optical fibre communication cables, where a high reliability must be assured, is discussed.     

Effect of prestrain on cyclic creep behaviour of a high strength spring steel

Cyclic creep and fracture behaviour of SAE spring steel 5160 under two prestrain conditions were studied. The cyclic creep rate, cyclic plastic strain range, total plastic strain range and cyclic creep life of the material were all found to depend strongly on the prestrain condition. It was also found that, after a certain number of cycles, cyclic hardening would reach a saturation state, followed by cyclic softening which would last until the final failure. However, a higher prestrain delayed the appearance of cyclic softening. Prestraining was also found to extend considerably the third stage of cyclic creep and thus increase the elongation to fracture of the material to the extent observed in a uniaxial tension test. Fracture surface study exhibited a mixed mode failure process and also a transition from quasi-brittle fracture at the lower prestrain, consisting of both intergranular and transgranular features, to ductile fracture at the higher prestrain, which is mainly made of dimples and is similar to that found in a uniaxial tension test. However, in the ductile fracture area, some transgranular facets and fatigue striation regions could still be observed. Finally, when the prestrain reached a certain value, compressive cyclic creep was detected and this is attributed to a strong Bauschinger effect.     

High temperature creep behaviour of single crystal superalloy

Abstract

Creep behaviour of a newly developed single crystal nickel base superalloy DMSX-l (Ni-7.7Co-6.4Cr-8.4W-0.3Ti-S.S7 Al-7.6Ta-Nf-Y-Nb-Re, wt-%) with 〈001〉 orientation has been analysed and compared with the reported data ofSRR99. It was concluded that the shear stress based model developed for SRR99 is also applicable for the new alloy. Although the material constants for octahedral slip for the new alloy are not exactly the same as those of SRR99, the estimated values of initial creep rate and softening coefficients are nearly of the same order. Therefore it is concluded that in the absence of a material database for cube slip for this new alloy, those available for SRR99 could be used to predict the orientation dependence of its creep behaviour.     

Notch fatigue crack initiation studies in a high strength nickel base superalloy

It is with great pleasure that I offer this paper for the Special Issue honoring Professor Takeo Yokobori on the occasion of his 70th birthday. I regard Professor Yokobori as a personal friend, having known him for over twenty five years professionally and socially. His impact on the discipline of fracture mechanics has been most significant, not only by his own technical contributions but, perhaps, more so because he put into motion the means whereby those interested in the field of fracture mechanics could meet together and to share their work with others. I refer, of course, to the formation of the International Conference on Fracture at Sendai, Japan in 1965 and to the present international journal, Engineering Fracture Mechanics. The following paper is therefore dedicated to Professor Yokobori as my contribution in honoring his distinguished career.     

Low-cycle-fatigue behaviour of superalloy blade materials at elevated temperature

Abstract

The low-cycle-fatigue (LCF) behaviour of superalloys has been examined with particular emphasis on their early stage strength changes and their response to complex cycling. Using steels and a titanium alloy as comparators, it has been possible to establish some general guidelines regarding their LCF performance. In the normal testing domain (700–1000°C) a high yield strain results in predominantly elastic conditions and cyclic stability. Under strain controlled dwell testing superalloys are fatigue dominated and generally more sensitive to compressive hold periods, whereas under stress control shortest lifetimes result from tensile dwells. Environmental influences are strong especially at temperatures where oxides remain brittle, but at high temperatures (>900°C) overaging effects may be beneficial. During thermal–mechanical cycling in-phase testing appears most deleterious.

MST/516     

Fatigue crack growth characteristics of a new Ni–Co-base superalloy TMW-4M3: effects of temperature and load ratio

The fatigue crack growth (FCG) behavior of a newly developed Ni–Co-base superalloy TMW-4M3 has been investigated in this study. Experiment was carried out in laboratory air with compact-tension specimen. The effects of temperature (400, 650, and 725 °C) and load ratio (0.05 and 0.5) on crack growth rate were studied in the Paris regime. The results revealed that the crack growth rate increased with increasing temperature under a given load ratio. The influence of load ratio on crack growth rate was pronounced, especially at higher temperature and low stress intensity factor range. Fractographic observations showed that fracture mode was transgranular at 400 °C, mixed transgranular and intergranular at 650 °C, and predominantly intergranular at 725 °C. The possible explanations for the crack growth behavior were discussed based on the degradation of mechanical properties and the oxidation assisted crack growth, as well as the crack closure effect. A comparison of FCG rate was also made between TMW-4M3 and the commercial superalloy U720Li.     

Room and high temperature tensile behaviour of a P/M 2124/MoSi2 composite at different heat treatment conditions

In the present work, a 2124/15 vol%MoSi₂ composite was obtained by powder metallurgy. Its microstructure and mechanical properties were investigated at room and at high temperature (up to 200°C) in conditions T351, T4 and after heat treatments at 495°C for up to 100 h. Up to 150°C, tensile properties of 2124/MoSi₂ in T351 resulted similar to those of a ceramic reinforced 2124/SiC composite. Yield stress of the 2124/MoSi₂ material, after heating at 495°C for up to 100 h, resulted higher than that of the monolith 2124 alloy heated for the same periods. No diffusion reaction phases were formed surrounding the MoSi₂ reinforcing particles during such long exposures to high temperature. Only at 100 h, large plate-like precipitates that contain Al, Cu, Mg and Si appeared. The high thermal stability of this 2124/MoSi₂ composite and its good mechanical properties at room and at elevated temperature makes MoSi₂ intermetallic a competitor of ceramic reinforcements.