M.C. Chaturvedi

A differential scanning calorimetric investigation of precipitation kinetics in the Al-1.53 wt% Cu-0.79 wt% Mg alloy

Abstract Heat effects characteristic of the precipitation process in the as-quenched Al-1.53 wt% Cu-0.79 wt% Mg alloy were calculated from DSC thermograms obtained from suitably designed experiments. DSC peaks due to precipitation of GPB zones, dissolution of GPB zones, dissolution of GPB zones-dislocation complexes, formation of the S′ phase and dissolution of the S′ phase were identified. Values for the fractional transformation, Y, and the rate of transformation, (dY/dt), were calculated using relations specially developed for each process. Expressions for the kinetics of the processes were formulated which enabled the calculation of activation energies, Q∗, frequency factors, k0, and the function ƒ(Y) of the kinetic equation ( d Y/ d t) = ƒ(Y)k 0 exp (−Q∗/RT ). The measured kinetic parameters were consistent with the available data and expected mechanisms of some of the transformations.

Microstructural study of transient liquid phase bonded cast INCONEL 738LC superalloy

The microstructure of the transient liquid phase (TLP) bonded joint of a cast INCONEL 738LC superalloy, made with a commercial Ni-Cr-B filler alloy, Nicrobraz 150, was examined by analytical scanning and transmission electron microscopic techniques. Due to incomplete isothermal solidification at the bonding temperature, the residual liquid interlayer transformed to nonequilibrium eutectic microconstituents consisting of chromium-rich M5B3, nickel-rich M23B6, and nickel-based γ solid solution phases. Also, a significant volume fraction of complex fcc Cr-Mo-W rich carboborides was observed in the joint/base alloy interface region. This is contrary to the predictions of the currently available TLP models that predict a precipitate-free joint/base alloy interface. It is suggested that solid-state diffusion of boron prior to completion of equilibration process induced the formation of carbo-boride phase, which needs to be adequately considered to develop an optimum postbraze heattreatment process to produce a joint with optimum properties.

Effect of homogenization heat treatment on the microstructure and heat-affected zone microfissuring in welded cast alloy 718

The effect of homogenization temperature on microfissuring in the heat-affected zones of electronwelded cast INCONEL 718 has been studied. The material was homogenized at various temperatures in the range of 1037 ° to 1163 ° and air-cooled. The homogenized material was then electron-beam welded by the bead-on-plate welding technique. The microstructures and microfissuring in the heat-affected zone (HAZ) were evaluated by analytical scanning electron microscopy (SEM). The grain boundary segregation of various elements was evaluated by secondary ion mass spectroscopy (SIMS). It was observed that the total crack length (TCL) of microfissures first decreases with homogenization temperature and then increases, with a minimum occurring in the specimen heat treated at 1163 °. This trend coincides with the variation in segregation of B at grain boundaries with homogenization temperature and has been explained by equilibrium and nonequilibrium segregation of B to grain boundaries during the homogenization heat treatment. No other element was observed to segregate at the grain boundaries. The variation in volume fraction of phases like δ-Ni3Nb, MC carbide, and Laves phases does not follow the same trend as that observed for TCL and B segregation at the grain boundaries. Therefore, microfissuring in HAZ of welded cast INCONEL 718 is attributed to the segregation of B at the grain boundaries.

Isothermal solidification during transient liquid phase bonding of Inconel 738 superalloy

Abstract An investigation was carried out on transient liquid phase brazing of IN-738 superalloy with Ni–Cr–B filler. Conventional models used to model the Transient Liquid Phase (TLP) bonding process assume that boride precipitation does not occur during the joining process. However, a significant deviation from this assumption was observed in all brazed samples as extensive intragranular and intergranular precipitation of chromium and boron rich particles was observed to occur within the base alloy in regions beside the braze/ base metal interface. Application of two diffusion models to the isothermal solidification stage was successful in reasonably predicting the time required to achieve complete isothermal solidification of the liquid insert, which is vital in preventing the formation of deleterious centreline eutectic. Diffusion analysis showed that the boron rich particles formed as a result of substantial solute diffusion into the base metal before equilibration of the liquid–solid phases.

On the mechanism of serrated deformation in aged Inconel 718

Abstract The serrated deformation of Inconel 718 alloy aged at 725°C for various lengths of time has been studied in the temperature range from 200 to 575°C and at constant strain rates in the range of 10−2 to 5 × 10−6 s−1. It was observed that serrated flow in the alloy depends on the interaction mechanism of dislocations with precipitates. Normal serrated flow was observed when deformation occurred by dislocations cutting through precipitates. However, when the Orowan looping mechanism was operative in the overaged material, normal serrated flow was observed only when the strain rate was high or the test temperature was low. In addition, the critical strain to serration was observed to increase with an increase in inter-particle spacing. This observation is inconsistent with the models proposed by Hayes et al. and McCormick. Therefore, a new mechanism, which considers the specific interaction mechanism of a moving dislocation with precipitates, has been proposed.

Dependence of creep fracture of inconel 718 on grain boundary precipitates

Abstract 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.

Electron beam welding of a Ti-45Al-2Nb-2Mn + 0.8 vol.% TiB2 XD alloy

Abstract The feasibility of using conventional electron beam welding to join a Ti–45Al–2Nb–2Mn+0.8% TiB 2 XD alloy was assessed. The alloy showed no evidence of hot cracking. However, a susceptibility to solid-state cracking was observed when weldments cooled at high cooling rates. The microstructural features of weldments were found to be a function of welding speed, heat input, preheating temperature, and thus cooling rate. Effects of cooling rates on microstructural evolution was evaluated with thermal simulation in a Gleeble 1500 system and it was found that cracking occurred when high cooling rates resulted in suppression of the α → γ phase transformation. It was observed that sound welds could be made when welding conditions were chosen so that the cooling rate at the fusion zone boundary was 250 K s −1 or less.

Development of crack-free welds in a TiAl-based alloy

Abstract Weldability of a Ti–45Al–2Nb–2Mn+0.8TiB2 alloy using electron beam welding process was investigated. It was found that the susceptibility of the alloy to solid-state cracking was due to the suppression of decomposition of the α phase and thermally induced stresses. The crack-free weld could be made on small-size workpieces when the weld cooling rate was properly controlled to allow the α phase to fully decompose. But when welding was conducted on highly restrained workpieces, the role of phase transformation was overshadowed by the stresses induced by the constraint in welds and the alloy became extremely difficult to weld.

ACTIVATION ENERGY FOR SUPERPLASTIC DEFORMATION OF IN718 SUPERALLOY

Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg, Manitoba,Canada, R3T 5V6(Received March 16, 2000)(Accepted in revised form April 21, 2000)Keywords: Superplasticity; Strain rate sensitivity index; Activation energy; SuperalloyIntroductionIN718 superalloy is reported [1–5] to exhibit superplasticity in the temperature range of 1173–1273 K.Superplastic behavior is characterized by high strain rate sensitivity index (m

The influence of electron-beam welding parameters on heat-affected-zone microfissuring in INCOLOY 903

0.3) and low activationenergy (Q), which is generally comparable with that for grain boundary diffusion. However, the stress(s) 2 strain («) curves obtained under superplastic condition are known [6] to exhibit strain hardeningand/or strain softening, especially, in the early part of deformation. Under such non-steady state flowcondition, the values of m and Q can vary with strain [6–7], making them as the non-unique parametersof the constitutive relationship [8] for high temperature deformation. Fortunately, the s-« curves for alarge number of superplastic materials [7] exhibit steady state flow behavior subsequent to the largeinitial transient stage of strain hardening/softening and concomitant microstructural evolution. Hence,the stress (s)–strain rate («) data obtained [9] beyond a certain strain level represent a reasonable steadystate condition. Therefore, the aim of the present work was to evaluate the m and Q parameters, of theconstitutive relationship for superplastic deformation of IN718 superalloy after such prestraining.ExperimentalIN718 superalloy of superplastic forming (SPF) grade was obtained in the form of 1.3 mm thick sheet.The analyzed composition of the alloy is given in Table 1.Tensile specimens with gage length and width of 20 mm and 5.3 mm, respectively, were machinedand tensile tested by Instron Universal Testing Machine. Test temperatures were controlled to anaccuracy of 61 K, and the specimens were soaked at the test temperature for 30 min. prior todeformation. Metallographic samples were prepared by mechanical polishing followed by and electro-lytic etching with 10% oxalic acid in water. Grain size was measured by mean-linear intercept methodand the error in the grain size data reported here were within 65%.