Advanced Materials Research Vols. 15-17

Abstract: The superiority of creep in Ti-48at%Al alloy with fully transformed lamellar structure to that in Ti-50at%Al alloy with γ single phase is characterized by the extension of transient stage. This extension of the transient stage derives by the retarding effect of α2 plate on the onset of the accelerating stage, through suppressing the dynamically recrystallization which is the main reason of the accelerating stage. This superiority in Ti-48at%Al alloy will become more clear by investigating the creep of the single crystal designated as the PST crystal, because of removing the grain boundaries which is the formation site of dynamic recrystallization. By using the PST crystal, the predominant deformation using primary slip plane of γ plate will continue, because the α2 plate restricts the operation of other slip planes. In PST crystals with the angle between the stress axis and the lamellar plates, designated as φ, less than 45°, the uniform deformation will proceed, because of the decrease in creep rate due to the decreasing in Schmid factor through the monotonous decrease in φ. But these suppositions have not confirmed. In this study, the deformation manner of the PST crystals with φ of less than 45° is investigated by the analyzing of creep curve, macrostructure and inverse pole figure of the PST crystals interrupted the creep tests at 1148K/68.6MPa at the strains of 0.20 and 0.65. Inverse pole figures of PST crystal are obtained using SEM-EBSD method. By accepting the creep deformation, the stress axes of the PST crystals move for [001]-[1, – 11] line with slip system of (111)<1, – 01>, and after reaching at this line, the stress axis turn to [1, – 11] pole position with (111)<1, – 10> slip system. The change in stress axis is not homogeneous in gauge portion accepting small strain, by subjecting the further creep deformation up to the onset of the accelerating stage, this heterogeneous deformation in gauge portion disappeared.

Abstract: The creep deformations of γ-single phase Ni-20mass%Cr single crystals with stress axes within standard stereographic triangle and at the three pole positions have been investigated. The most of the creep life is occupied by the transient stage, which consists of Stage I and Stage II. In Stage I, the creep rate just after loading remains constant. In Stage II, the creep rate decreases continuously. Except for the single crystals with stress axes of [001] and [1,–11] poles, the single crystals make the creep deformation using the primary slip plane of (111). As a result, the cross section of the specimens turns from circular to elliptical in shape. However, there are marked difference in deformation manner among single crystals with the stress axes within standard stereographic triangle. The single crystals whose angle between stress axis and primary slip plane of (111), θ. is more than 45° shows the heterogeneous deformation during creep. While, the homogeneous deformation will be expected in the single crystals with θ less than 45°. In this study, by using the four single crystals with θ less than 45°, the change in the stress axis with the creep deformation at 1173K-29.4MPa, is investigated and the deformation manner due to the primary slip plane of (111) is estimated by conducting the creep interrupting tests. In the two single crystals with stress axes in the standard stereographic triangle where the moving range of θ is narrow, comparing to the others, the spot of the stress axis in the inverse pole figure moves for <1,– 01> direction by using (111)<1,–01> slip system, and after arriving at the [001]-[1,–11] line, the spot turns to its direction for [1,–11] pole using (111)<1,–10> slip system. While, in the other two single crystals whose stress axes located in the area with wider moving range of θ, the spot of stress axis only move for <1,–01> direction. And, the widely spread spot of the stress axis is confirmed after subjecting the small strain.

Abstract: The features of the creep deformation of γ-single phase single crystals with the composition of Ni-20mass%Cr are characterized by the extended transient stage, which consists of Stage I and Stage II. In the Stage I, the creep rate just after loading remains unchanged, while the creep rate decreases continuously in Stage II. In the single crystals except for the single crystals with the stress axis of [001] and [1, – 11], the predominant creep deformation using the primary slip plane continues. By this deformation, the cross section of specimen turns to elliptical in shape. However, in the single crystals with the angle between stress axis and primary slip plane (111) is more than 45°, the deformation using the primary slip plane does not continue, as a result, the duration of Stage II turn to shorter one. The single crystal with the stress axis of [011] has the largest angle of 55°. In this study, the deformation manner during transient stage of single crystal with the stress axis of [011] orientation is investigated from the two viewpoints. The first one is to clarify the change in deformation manner with decreasing the stress. As a result, with decreasing the stress, the Stage I become clear and strain during Stage I and Stage II become small, furthermore, the decreasing ratio of creep rate with definite strain becomes larger. While, the second viewpoint is to investigate the change in crystallographic orientation of the [011] single crystals with creep deformation using the inverse pole figure obtained by the EBSD method. As a result, at the stress of 29.4 MPa, the spot of stress axis turns from the [011]-[1, – 11] line to the <1, – 01> direction. While, at the stress of 19.6 MPa, the stress axis moves for the [1, – 11] pole along the [011]-[1, – 11] line from the [011] pole. And, it is noteworthy that the spot widely spread from the [011] pole during transient stage. This indicates the large distortion in the primary slip plane and the evidence of heterogeneous deformation.

Abstract: Non-destructive evaluation (NDE) techniques offer the potential advantage of achieving fast throughput microstructural characterization as well as monitoring that could be extremely valuable in assessing the mechanical integrity of turbine engine components. It is well known that the γ′ precipitate size and distribution can often determine the mechanical strength of nickel base alloys. In this investigation, Waspaloy, an age-hardening superalloy, was chosen as the candidate to produce varied microstructures, which were subsequently characterized via the DC four-probe resistivity method. Specimens with average grain sizes of 13, 52 and 89 μm were obtained upon solutionizing at 1045°C, 1090°C and 1145°C respectively. The specimens were annealed at 1045°C to stabilize the vacancy concentration prior to aging at 800°C for times ranging from 0.1 to 100 hours. Sub-grain microstructures examined in the SEM showed progressive growth of γ′ precipitates with increased aging duration in all cases. The measurements of DC four-probe resistivity showed a consistent drop in the resistivity with increased aging time, which was concurrent with the growth of the precipitates.

Abstract: Mathematical model, based on Fick’s second law of diffusion, was used to predict the time required to complete isothermal solidification and to determine the effect of process variables during the transient liquid phase bonding of Inconel 625 and 718 superalloys with nickel based brazing filler alloy BNi-2. Experimental investigations were carried out in the range of 1325 – 1394K to verify the model and the predicted times were in excellent agreement with the experimentally determined values. The obtained activation energies for diffusion of boron were very close to the ones reported for other nickel base polycrystalline superalloys; however, it was observed that the time required for complete isothermal solidification are significantly less than that of other nickel based superalloys with different nickel based brazing filler alloys. Because of this advantage, these combinations of base and filler alloys are expected to replace other currently used ones. Further, significant reduction of holding time was observed with increasing brazing temperature and with decreasing joint gap. The composition of the joints at the end of holding period, when the holding time was not sufficient to complete isothermal soldification, has been determined in order to predict the amount of brittle eutectic phases in the final joint microstructures.

Abstract: One method for lining the inner surface of a steel cylinder with copper alloy is to pour molten copper alloy into a heated cylinder, which has been previously filled with borax anhydride. This process replaces the molten borax anhydride with molten copper alloy. After the cylinder is cooled, the embedded copper alloy is drilled along its center axis so that a prescribed thickness of the copper alloy may remain. However, when the cylinder is made of cast iron including high concentration of carbon, the copper alloy does not bond to the inner surface of the cylinder. To solve this problem, we investigated to utilize the decarburization phenomenon. Two methods were investigated. In one method, the cast iron cylinder filled with FeO powder is heated at a high temperature so that the carbon precipitates in the cast iron may get out through reaction with O2 formed by decomposition of FeO. In the other method, the cast iron cylinder is only heated in air. A decarburized layer is formed beneath an oxide layer. In both methods, the lining of cast iron with copper alloy was attained.

Abstract: The effect of a mixed powder on the wide gap transient liquid phase diffusion bonding of a directionally solidified Ni base superalloy, GTD-111 was investigated. The mixed powder consisted of a mixture of a powdered Ni base filler (GNi-3) and powdered base metal (GTD-111). The range of the base metal powder was 40 to 70wt%. Bonding was performed at a temperature of 1463K, using various holding time. In the case of a lower 50wt%, the base metal powders completely melted and base metal mating at the interface dissolved at an early time, and extent of dissolution of base metal decreased with increasing mixing ratio. Liquid was eliminated by isothermal solidification, which was controlled by the diffusion of B into the base metal. The solids in the bonded interlayer grew epitaxially from the mating base metal inward from the insert metal and the number of grain boundaries formed at the bonded interlayer corresponded with those of the base metal. The finishing time for isothermal solidification was about 74ks. In the case 60wt% and higher, the base metal powders partially melted and remained in the vicinity of bonded interlayer. The solid was formed from the remaining powder and base metal mating at the interface. Finally, the bonded interlayer underwent the poly-crystallization when isothermal solidification was complete. The contents of Al and Ti in the bonded interlayer with a holding of 74ks were equal to that of the base metal.

Abstract: Multiaxial compression (MAC) is a severe plastic deformation (SPD) method that allows sequential uniaxial compression of prismatic samples to relatively large cumulative strains. The technique involves a change in loading direction (x to y to z to x…) between successive compression passes. A high-purity α-iron containing 60 mass ppm C was thus strained using passes of ε ∼ 0.4 at room temperature (0.16 Tm) and 450 °C (0.40 Tm) to total ε ranging from 1.4 to 2.9. Both optical and electron microscopy were used to characterise the deformed microstructures. Fragmentation of the initial grain structure occurs mainly in the form of a dense, homogeneous network of low angle boundaries (LAB) delimiting subgrains of about 1 3m. The original grains are easily distinguishable and maintain a relatively equiaxed appearance even at larger strains. At room temperature, high angle boundaries (HAB) are observed within some of the initial grains, and not necessarily close to the grain boundaries. These HAB may be open or closed, and tend to align themselves at approximately 45° to the orthogonal axes, suggesting the presence of microshear bands and thus a heterogeneous deformation. Such bands of localised strain criss-cross as a result of different slip systems being activated from one pass to another. When the temperature is increased to 450 °C, grain boundary migration becomes significant owing to the lack of impurities that could otherwise provide a pinning effect. The resultant subgrain structure is coarsened to about 4 3m. Besides, the enhancement of recovery at higher temperatures also appears to discourage the generation of HAB by dislocation accumulation processes.