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Precipitation hardened nickel-based superalloys are widely used in aero and industrial gas turbine engines due to their excellent high temperature strength and remarkable hot corrosion resistance. A drawback of many of these alloys is that they are very difficult to weld due to their high susceptibility to heat affected zone (HAZ) cracking, both during welding and post weld heat treatments (PWHT). Weld cracking in many of these alloys has been attributed mostly to constitutional liquatioin of grain boundary NbC precipitates. however, HAZ cracking has been observed in carbon-free superalloys as well, Therefore, research was initiated to examine grain boundary liquation and cracking in HAZs in a variety of Ni-based superalloys. It was found that intergranular cracking of grain boundaries involved liquation of several other phases, in addition to NbC, that were present in pre-weld microstructure of the alloy. These even included the primary strengthening phase,γ’, in a very widely used superalloy, Inconel 738. In addition, segregation of melting point depressant element B was also observed at gain boundaries in other superalloys, which also caused grain boundaries in HAZ to liquate and resulting in their cracking. An overview of microstructural aspects of different liquation phenomena involved and characteristics of the liquid film contributing to the HAZ microfissuring of nickel superalloys will be discussed in this presentation

The effect of welding, heat-affected zone (HAZ) simulation, and specimen orientation on the microstructure and fatigue properties of 2195 Al-Li alloy was studied. HAZ simulation and GTA welding with a 4043 filler alloy resulted in a significant change in the microstructure. In the HAZ the primary strengthening phase, T1 (Al2CuLi), in the base alloy in T8 temper was replaced by TB (Al7Cu4Li) phase and voids/microcracks, and the fusion zone (FZ) consisted of T (AlLiSi) phase particles in the matrix, which consisted mainly of the filler alloy. The yield strength and fatigue threshold of the 2195-T8 alloy were observed to be dependent upon the specimen orientation. The HAZ simulation and welding led to a reduction in the tensile properties and fatigue strength. While the post-weld heat treatment resulted in the re-precipitation of T1 phase in the HAZ, but no increase in the fatigue strength was observed due to the presence of microcracks. Fatigue crack initiation was observed to occur at the surface in the base alloy in T8 temper, and at the internal defects after HAZ simulation and welding. Fatigue crack propagation exhibited characteristic striations in the T8 alloy, and brittle cleavage-like feature after HAZ simulation and welding.

Tensile specimens of superplastic forming grade AA 8090 Al–Li alloy were deformed at constant strain rates in the range 1×10-5–1×10-2s-1and at constant temperatures in the range 298–843 K, to investigate their effects on the nature of stress–strain (cs–ecurves and on the concurrent microstructures, substructures, and microtextures. The s–ecurves exhibited flow hardening in the early part of deformation, the rate of which was found to increase with an increase in strain rate and decrease in temperature. Grain growth, cavitation, dislocation interactions, and texture weakening were observed to occur during superplastic deformation. The early part of deformation involved a substantial dislocation slip contribution to flow hardening and to the mechanism for superplastic flow. However, the microstructural evolution facilitated the conventional mechanism of grain boundary sliding and its accommodation by the diffusional process, as suggested by the strain rate sensitivity index and activation energy determined.

Superplastic forming grade sheets of AA 8090 Al–Li alloy were observed to contain layers of different microstructure and microtexture across the thickness cross-section. Superplastic behaviour and its relationship to the concurrent microstructural and microtextural evolution of this sheet were studied at 803 K by tensile testing of specimens taken from the full thickness and the near surface and midthickness layers. Initially, the surface layers contained nearly equiaxed and relatively coarse grains with a strong S {123}634type texture, whereas the midthickness section had elongated fine grains and a dominant Bs {011}211texture. The stress–strain rate (s–e) curves exhibited minimum flow stress for the full thickness material. Varying levels of grain growth and texture weakening occurred in the above two layers, the extent of which depended on whether the layers were in separated form or as coexistents in the full thickness material. The maximum values of strain rate sensitivity index for the full thickness, surface, and centre materials were 0.82, 0.64, and 0.56, respectively. The corresponding ductility values were 475, 420, and 286% at e=1×10-3s-1.

A brief review of orientation imaging microscopy and an overview of the results of investigations related to textural evolution during superplastic deformation of a recent series of superplastic grade aluminium alloys, and specifically the results of the authors’ study on microtextural evolution in AA 8090 Al–Li alloy, are presented. It was observed that the surface layer of the AA 8090 alloy sheet was dominated by S {011}634type texture, whereas the midthickness layer was dominated by brass {011}211type texture. In the course of superplastic deformation, textural weakening occurred, the rate and extent of which were different in the surface and centre regions, depending upon whether the two layers were deformed separately or as a composite in the full thickness tensile specimen. Textural weakening was generally more extensive at higher test temperatures and lower strain rates. Based on textural evolution and the presence of a large proportion of low angle boundaries, and their near elimination at larger strains, dislocation slip could be considered to be the dominant mechanism during the initial stages of deformation, whereas the conventional mechanism for superplastic deformation is thought to be operating during the later stages.

The influence of sulphur on microfissuring has been studied by characterising the hot ductility and heat affected zone (HAZ) microfissuring in welded wrought Inconel 718. A series of Inconel 718 base alloys, with sulphur concentrations in the range 7–110 wt-ppm and with the lowest possible concentrations of carbon, boron, and phosphorus were used in the study. The hot ductility of the alloys was measured using a Gleeble 1500 system, and their weldability was evaluated by bead on plate electron beam welding. The effect of sulphur on microfissuring in the weld HAZ was compared with that of boron in boron containing, but sulphur free, alloys. Sulphur reduced the weldability of the material, but its influence was very moderate in comparison with that of boron. The rate of increase of the brittle temperature range hot ductility parameter with increasing sulphur concentration was significantly smaller compared with that with increasing boron concentration. This may be because sulphur and boron seem to influence grain boundary liquation in the HAZ differently during welding of these alloys. Sulphur influences the weld HAZ microfissuring mainly by reducing the solidus temperature of liquid films during the cooling part of the welding cycle. Boron, however, not only reduces the solidus temperature, but also promotes grain boundary liquation during the heating part, and is therefore more effective than sulphur in promoting HAZ microfissuring.

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The present state of knowledge on the effects of C, B, S, and P on the weldability of Ni base superalloys is reviewed. Consideration is also given to the effects of Mg and the rare earths on weldability, though the data are sparse and the manufacturing route often not defined. From the analysis, B is considered to be the most detrimental element to the weldability, with lesser effects from C and S. The data on P are limited, but recent results show that P has a variable effect on weldability, being most detrimental in low C alloys (0·01 wt-%), and less of a problem in C levels >0·03 wt-% and above. Consideration is also given to the effects of heat treatment in influencing solute segregation to the grain boundary and on grain growth. The potential effects of two or more elements on grain boundary microchemistry and its effects on weldability are also discussed.

Tensile specimens of superplastic forming grade IN718 superalloy, containing banded microstructure in the as received state, were deformed at high temperatures Tto investigate the stress sversus strain rate ? ·behaviour, the nature of the stress versus strain ?curves, ductility, and microstructure upon failure. The log s–log ? ·plot for the ? ·range ~5 × 10-6–3 × 10-2s-1at T= 1173–1248 K exhibited a strain rate sensitivity index m= 0·62 at low strain rates and m= 0·26 at high strain rates, representing region II and III behaviour, respectively. The activation energies were estimated to be 308 and 353 kJ mol-1, respectively. All the s–?curves, obtained at ? ·= 1 × 10-4s-1for the temperature range 1173–1273 K, and at T= 1198 K for the strain rate range 1 × 10-4–1 × 10-2s-1, exhibited initial flow hardening, followed by flow softening. The microstructures revealed dynamic recrystallisation, grain growth, cavitation, and a variation in the amount of second phase particles. Grain growth and cavitation were found to increase with temperature in region II. Excessive grain growth at 1273 K led to the elimination of region II. Grain growth and cavitation were both found to be less pronounced as the strain rate increased in region III.

Microfissuring in the heat affected zone (HAZ) around electron beam (EB) welds was studied in two Inconel 718 based superalloys containing the minimum possible concentrations of C, S and P, and 11 and 43 wt-ppm concentrations of B. The microfissuring susceptibility was evaluated by measuring the microcracks formed in the HAZs of EB welds and by examining the hot ductility behaviour of the material. Bead on plate type EB welding, and on cooling hot ductility tests in a Gleeble 1500 thermomechanical simulator were performed on the two alloys with different grain sizes. The results of the hot ductility tests and crack measurements on EB welded samples correlated very well with each other. Using both techniques it was observed that the HAZ microfissuring of Inconel 718 increased with an increase in B concentration and an increase in grain size. The effect of B was attributed to its segregation to the grain boundaries, which resulted in a reduction in the melting point of the grain boundary material. This rendered them susceptible to liquation in the HAZ, and formation of microfissures due to the introduction of stresses during the cooling portion of the welding thermal cycle.

Cold rolled boron doped Ni76Al24was partially recrystallised at 850, 875, and 900°C. The recrystallised volume fraction and the interfacial area between recrystallised and unrecrystallised parts were determined as functions of time. The analysis of results showed the growth rates of recrystallised grains to be dependent on temperature, but independent of time. The activation energy for the transformation was 168 kJ mol-1and that for growth was 87 kJ mol-1. The activation energy for growth was much less than that for diffusion and was dependent on the degree of cold working. This has been attributed to the strong interaction of boron with the structural defects produced by cold working.

Nickel aluminide containing 23·2 at.-%Al and 0·1 at.-%B was cold rolled and fully recrystallised at 825, 850, 875, 900, 925, 975, and 1000°C. Increase in grain size as a function of isothermal annealing time at these temperatures was measured. All the data showed excellent fit with the developed equations. The value of m, the exponent of the grain growth equation (Dm- Dm0= Kt), was 2·5. The activation energy for grain growth was 263 kJ g-atom-1. Small changes in the boron content had an appreciable effect on the kinetic parameters. This was attributed to the solute drag effect of boron.

This paper reports the influence of the amount and temperature of deformation on the mechanical properties, dislocation structure and coarsening of second-phase particles in Alloy 718. The material was deformed 25% and 50% at room temperature, 723, 998 and 1173 K temperatures and then aged at 998 K for various lengths of time. The mechanical properties of the thermomechanically treated material were evaluated by room temperature tensile tests and elevated temperature creep tests. The dislocation substructures and coarsening behaviour of γ″ and γ′ precipitates in both pre-deformed and non-predeformed material were studied by transmission electron microscopy. The experimental results show that the tensile yield strength and ultimate strength are greatly improved by cold and warm working prior to ageing but these increases are accompanied by a small loss in ductility. The results of creep tests in the temperature range 873–923 K, however, show that creep resistance is significantly improved by predeformation. A pre-ageing deformation of 25% at a temperature of 998 K followed by ageing for 15 h at 998 K is suggested to be the optimum thermomechanical treatment for Alloy 718.

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The morphology, chemical composition and crystal structure of the δ-NiMo phase in the binary Ni-Mo alloy system have been investigated by means of scanning electron microscopy (SEM), energy-dispersive spectrum analysis (EDS) and transmission electron microscopy (TEM). The δ-NiMo phase has a tetragonal lattice with primitive unit cell of a= b= 0.9108 nm, c= 0.8852 nm. Its space group was determined as P4/mmm by convergent-beam electron diffraction (CBED) technique.

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In this study, Inconel 718 was heat treated to obtain various materials with identical grain size and microstructures but with a different state of dispersion of δ precipitates at the GBs. The density of δ precipitates ρ in different materials was from 0% to about 70%. Creep tests were conducted on these materials at 795 MPa and 625°C. It was found that both the rupture time and total creep strain decreased with an increase in the value of ρ when its value was below about 45%. However, when the value of ρ was above 45%, both the rupture time and creep strain increased with an increase in ρ to a value much higher than that observed in the material without δ precipitates at GBs. SEM observations of the crept samples and their fracture surfaces showed that the presence of δ precipitates at GBs resulted in the formation of creep voids in all specimens. However, the effect of creep voids on the final fracture of the material was dependent upon the value of ρ. At lower values of ρ, creep voids were observed to be isolated, and the fracture may be due to the propagation of wedge cracks initiated at triple points of grain boundaries. At higher values of ρ, the probability of wedge crack formation might be reduced. The fracture would then be controlled by the growth of cavities coupled with grain boundary sliding, which was observed to be strongly influenced by precipitates at grain boundaries. © 1997 Acta Metallurgica Inc.

A further study to our previous work has been carried out in an attempt to understand the welding parameters affecting heat–affected zone micro–fissuring in Incoloy 903. Electron beam welding (EBW) parameters such as travel speed, beam current, focus current and beam oscillation were varied, while the accelerating voltage and working distance were kept constant. The six independent variables were formulated into a response surface statistical design in order to investigate the effect of these factors on the dependent variable. The results of the experimentation showed that the main factors affecting micro–fissuring were beam current, focus current, welding speed and oscillation across the welding direction. These factors were combined into a regression equation showing the effect of each independent variable on the micro–fissuring behaviour.

Optical, SEM/EDX and TEM/EDX techniques have been used to characterize the parent, weld and heat affected zone (HAZ) microstructures in electron beam welded (EBW) Incoloy 903. The as–forged microstructure showed a duplex grain size of large warm–worked grains decorated by finer grains. MX carbides, fine MC carbides and MNP phosphides were observed on grain boundaries. The weld metal solidified in a cellular–dendritic mode: with segregation of Nb and Ti to interdendritic regions; no micro–fissures were observed originating in the weld metal. The HAZ, however, showed extensive micro–fissuring, especially on the long warm–worked grain boundaries. The micro–fissuring was attributed to constitutional liquation of the MX carbides (partial), fine MC and MNP phosphides on the grain boundaries. Micro–fissuring was, however, absent on the fine grain boundaries, though the boundaries exhibited a thickened appearance attributed to liquid film migration (LFM). The LFM was found to influence the solidification mode of the grain boundary liquid phase. Liquid solidifying via a non–LFM mode, resulted in a terminal solidification products of γ + MC carbide. These carbides were in extensive sheets along the grain boundary. Areas associated with LFM solidified to a Nb rich austenite and exhibited no evidence of micro–fissuring. The LFM resulted in a smaller solidification temperature range for the grain boundary liquid and resulted in a less sensitive microstructure for the weld cycle thermal stress to act on. LFM was also concluded to be the reason for the beneficial effect of fine grains on minimizing HAZ micro–fissuring.