Ji-Won Lee

Composition and growth rate effects in directionally solidified TiAl alloys

Abstract Effects of alloy compositions and growth rates on the microstructure and the deformation behavior of fully lamellar TiAl alloys were systematically investigated by directional solidification (DS) techniques. Some β stabilizing elements, such as Mo, Nb and Cr, have been added in order to increase the β phase field in the binary system. It was found that the lamellar orientation was aligned nearly 0–45° to the growth direction in Ti–46Al–2Nb and Ti–46Al–2Cr(at.%) DS ingots grown at the growth rate of 90 mm h −1 . Therefore, it could be concluded that there is a good possibility to control the lamellar orientation by adding some β stabilizing elements, such as Mo, Nb and Cr, in the TiAl system. In the Ti–47.5Al–2.5Mo DS alloy, the lamellar boundary was nearly perpendicular to the growth direction at growth rates of 180 and 360 mm h −1 , however, it was nearly parallel to the growth direction at the growth rate of 90 mm h −1 . These results indicate that the lamellar boundary orientation of DS ingot has been affected by the growth rate as well as the alloy composition.

Microstructure control of TiAl alloys containing β stabilizers by directional solidification

Abstract Phase transformation at elevated temperatures in Ti-Al-X(X=Mo, Re, W) systems has been investigated by analyzing the dendrite morphology of arc-melted buttons. It was found that the addition of β stabilizers, such as Mo, Re and W, shifted the β phase field at liquid/solid temperatures to the high Al composition side. So, the β phase formed as a primary crystal even in higher Al compositions in Ti-Al-X ternary systems compared with Ti-Al binary. The addition of W was found to be the most effective β (bcc) stabilizer among Mo, Re, W, and the Ti-47Al-2W composition was selected for β solidification. In the directional solidification of Ti-47Al-2W alloy at the growth rate of 90 mm/h using the Bridgman type DS apparatus, it was found that the lamellar orientation in the columnar grains was parallel or inclined with the angle of 45° to the growth direction. This means that the β phase forms at the solid/liquid interface in the Ti-47Al-2W alloy. The tensile elongation of DS alloys was clearly improved compared with that of the polycrystalline alloy with a fully lamellar microstructure, while maintaining the same yield stress level.

Effects of phosphorus on the δ-Ni3Nb phase precipitation and the stress rupture properties in alloy 718

The effects of phosphorus on the phase transformation and stress rupture properties of alloy 718 were investigated. The nucleation of δ-phase, which does not contain phosphorus, was suppressed by the enrichment of phosphorus at grain boundaries. A low level of phosphorus resulted in the formation of faults-containing film-like δ-phase along the grain boundaries, while a higher level of phosphorus favored the long lath-like δ-phase precipitation. Phosphorus greatly prolonged the stress rupture life of the alloy in the range of 0.0008–0.013 wt.%, while it reduced the stress rupture life in the range of 0.013–0.049 wt.%. The effect of phosphorus on the stress rupture properties was closely related to its interaction with oxygen. Phosphorus atoms, in the range of 0.0008–0.013 wt.%, enhanced the resistance to oxygen intrusion along the grain boundaries, protected the grain boundaries from decohesion by oxygen atoms and oxidation, and subsequently prolonged the rupture life of the alloy. The protection effect of P is clearly demonstrated by the phenomenon that the crack initiation site was shifted from the surface to the center in the stress-ruptured samples with increasing addition of P. Over 0.013 wt.%, the protection effect of phosphorus is excessive and phosphorus began to display its inherent effect of damaging the grain boundary strength; the stress rupture life of the alloy was reduced accordingly. Maximum stress rupture life was thus obtained at ≈0.013 wt.% P.

The eutectic characteristic of MC-type carbide precipitation in a DS nickel-base superalloy

Abstract The precipitation of MC-type carbide in a DS nickel-base superalloy has been studied. The nucleating temperature of the MC carbide was determined by analysis. The MC carbide and γ phase were complementary in composition, and the eutectic reaction of (γ+MC) took place in different forms at varied solidification rate. The (γ+MC) eutectic reaction was an important factor of determining the carbide morphology. At an extremely high solidification rate, such as quenching, the compositional ranges for the (γ+MC) eutectic reaction was broadened and the carbide precipitated through the quasi-eutectic reaction of (γ+MC). As a result, the carbide appearing frequency was increased noticeably. The dendrites of the quasi-eutectic carbide formed in quenching paralleled to each other closely for facilitating diffusion between the two eutectic components of carbide and γ phase. The script carbide precipitated at a relatively high drawing rate around 50 μm/s. At this rate, diffusion was insufficient and the eutectic reaction of (γ+MC) was suppressed at most. The carbide grew quickly along the interdendritic liquid passages due to the high degree of segregation of carbide forming elements there, and hence formed a very complicated dendritic structure. The faceted carbides precipitated when the drawing rate was lower than 5 μm/s, and some of them contained a γ phase core. The incorporation of γ phase into the carbide body was caused by the eutectic reaction of (γ+MC) in the midst of the carbide precipitation. Two forms of the (γ+MC) eutectic reaction were favored by sufficient diffusion at 0.5 μm/s drawing rate. The (γ+MC) eutectic reaction took place at the final stage of the carbide precipitation in an isolated liquid pool and formed small ridges distributed uniformly on the carbide surface. Diffusion operated sufficiently throughout the whole residual liquid passage guaranteed the continual eutectic reaction by a sandwich like mode of γ/MC/γ, and the extremely long carbide was formed along the grain boundaries as a result.

Microstructure control in two-phase (B2+L12) Ni–Al–Fe alloys by addition of carbon

Abstract Carbon and titanium were added in two-phase (B2+L1 2 ) Ni–Al–Fe alloys for carbides precipitation, and the additions of 0.2 wt.% C and 3 at.% Ti have been found to produce the most adequate high temperature structural material. Carbon doped alloys showed more refined microstructure than carbon free alloys. The carbides, which were formed in the as-cast microstructure showing equiaxed dendrites, appeared to play a role in suppressing grain growth and coarsening of the second phases. The NAF29-10 alloy, including no carbides, showed the lamellar type microstructure, while the NAF29-10-C alloy, including carbides, showed a much more refined mesh type microstructure. When the carbon free alloys were quenched into water, cracks occurred at the grain boundaries probably due to martensitic transformation, however, these cracks were not observed in the carbon doped alloys. In order to suppress quenching cracks in the carbon free alloys, the solutionizing treatment time was reduced. However, cracks still formed as a severe intergranular mode at an early stage of elastic deformation. The refined microstructure of carbon doped alloys could compensate for elongation loss due to solution hardening and precipitation hardening. As a result, the carbon doped alloys showed good room temperature ductility and also showed much higher yield strength than other alloys including no carbides over the entire temperature range.

Spatio-temporal microstructure evolution in directional solidification processes

Experimental studies in Al–4.0u2009wt%u2009Cu alloys were conducted in samples of different sizes in which the effect of convection increases with increasing sample diameter. Detailed measurements of dendrite tip radii and the primary arm spacing have been carried out under both diffusive and convective growth conditions. The experimental results on tip radius and primary spacing show a good agreement with the microsolvability theory and Hunt–Lu model, respectively, for diffusive growth processes. In larger samples where convection effects are present, an inhomogeneous microstructure develops. A localized growth model is proposed to explain the spatio-temporal microstructure formation under convective growth conditions. The key parameter in this model is the effective diffusion coefficient which increases with the sample diameter and plays the decisive role in selecting the length scales of the solidification microstructures under convective growth conditions.

Influence of solidification rate on precipitation and microstructure of directional solidification IN792 + Hf superalloy

The effect of solidification rate on the precipitation and microstructure of directional solidification IN792 + I-If alloy was studied. The solidification sequence and the initial precipitation temperature of different phases were determined by the observation of the quenched microstructure combined with the differential thermal analysis measurement. The script carbide was turned into faceted carbide with the drop of solidification rate. It was concluded by microstructure analysis that the faceted carbide was pushed by the gamma solid front before it was captured. The incorporation of gamma phase into the faceted carbide was due to the dendrite growth of the carbide toward one point and the mergence of the dendrites. Some long carbide bars were formed along the grain boundaries by continual reaction of eutectic (gamma + MC carbide) at a solidification rate of 0.5 mu m/s. Two zones, the gamma forming elements enriched zone and depleted zone, were found in the residual liquid area. Eutectic gamma/gamma nucleated in the gamma forming elements enriched zone. The eta-phase precipitation was controlled by the ratio of (Ti + Hf + Ta + W)/Al in the residual liquid. The growth of eutectic gamma/gamma increased the ratio and induced the eta-phase precipitation. A lower solidification rate decreased the ratio by sufficient diffusion and hence efficiently suppressed the eta-phase precipitation.

First evidence of grain boundary serration in a specifically heat treated wrought Alloy 625 Ni-based superalloy

Abstract Grain boundary serration is an effective way to increase the high temperature resistance of superalloys and steels. The popular Alloy 625 Ni-based superalloy was until now believed not to form serrated grain boundaries based on previous considerations of serrability criteria. Following the recent strain-induced serration mechanism, a special heat treatment involving continuous slow cooling between the solution and aging temperature was designed. As a result, significant serration was observed for the first time for Alloy 625 promoted by slow cooling. Grain boundary M23C6 carbides were systematically detected from either degenerescence of solidification MC carbides or heterogeneous nucleation. Upon aging, serration amplitude increased and precipitation of the δ phase proliferated.