Weihong Zhang

Effect of B on microstructure and properties of low thermal expansion superalloy

Abstract The effect of B on microstructure and various properties including coefficient of thermal expansion (CTE), HV hardness, and both smooth and notch stress rupture properties of modified Thermo-Span alloy was studied. The results show that B hardly dissolves in matrix. Increasing B content constrains the formation of Laves phase and grain boundary (GB) precipitation of Laves and G phases, but promotes the formation of M(Co, Fe)NbB boride. In low B doped alloy, its intrinsic high susceptibility to intergranular cracks leads to reduced rupture life and notch sensitivity. Increasing B improves grain boundary cohesion, tying up vacancies and reducing GB diffusion, which constrains the nucleation and propagation of intergranular microcracks, prolongs the rupture life and eliminates the notch sensitivity in the new alloy. Compared with conventional Thermo-Span alloy, the B doped modified alloy shows lower CTE and improved notch sensitivity.

Hot Working Behavior of Casting Thermo-Span Alloy

Thermo-Span alloy is an oxidation resistant, low thermal expansion superalloy with good mechanical properties at the service temperature. This paper investigated the hot working behavior of casting Thermo-Span alloy deformed at 900~1150°C, with strains of 20%, 40% and 60% at strain rates of 1 and 10 s-1. Dynamic recrystallization (DRX) grains were formed at 1110°C with a strain of 20%, at 1050°C with a strain of 40%. Increasing the deformation rate and strain can promote the DRX. However, when the strain exceeded 60% or the deformation temperature was above 1150°C with 40% strain, the surface cracks occurred, indicating that the alloy should be deformed in one heat no larger than 60%. By forging and rolling at 1050°C, Thermo-Span alloy with good surface quality and homogeneous grains was produced, and the tensile properties were still acceptable.

Re-recognition of the Effects of Phosphorus and Boron on the γ″ and γ′ Phases in IN718 Alloy

The effects of P and B on the matrix strength and precipitations of γ′ and γ″ phases in the grain interior were re-recognized in this study. The combination addition of P and B markedly accelerated the precipitations of γ′ and γ″ phases and strengthened the matrix of IN718 alloy when air-cooled from high temperature, while made no difference when water-quenched from high temperature. The effect of single addition of P on the precipitations of γ″ and γ′ phases was the same with that of the combination addition of P and B, while the single addition of B had no effect on the precipitations of the two phases. Therefore, it was P rather than B which accelerated the precipitations of γ′ and γ″ phases. P could take part in the precipitations of γ′ and γ″ phases, which was revealed by electrochemical extraction and quantitative analysis of chemical composition. It also revealed that P atoms were dissolved in the γ matrix to a relatively high degree at the temperature that γ′ and γ″ phases began to precipitate, and consequently the precipitations of γ′ and γ″ phases were accelerated. The first-principle calculation indicated that P decreased the formation enthalpies of γ′ and γ″ phases when it occupied the Ni lattice sites in the two phases, which explained the effect of P on the γ′ and γ″ phases.

Distribution of Phosphorus and Its Effects on Precipitation Behaviors and Tensile Properties of IN718C Cast Superalloy

The effect of phosphorus on the precipitations of γ″, γ′ and δ phases and associated tensile properties in IN718C alloy are investigated in this study. It is revealed that P atoms are dissolved in the grain interior to a relatively high degree and hence influence the precipitation behaviors in the grain interior and improve the tensile strength of IN718C alloy. γ″ and γ′ phases did not precipitate in the alloy without P addition during air cooling, while γ″ and γ′ phases precipitated in the grain interior during air cooling in the alloys with P addition, and the amounts of γ″ and γ′ phases increased with increasing P content. Therefore, the Vickers micro-hardness in the as-cast state increased gradually with increasing P content. In double-aging state, the sizes of γ″ and γ′ phases in the alloys with P addition were larger than that in the alloy without P addition, while the sizes were invariable when the P content (wt%) was higher than 0.015. Therefore, the micro-hardness and tensile strength of IN718C alloy treated by double aging increased first and then kept invariable with increasing P content. The precipitations of δ phases both in the grain interior and on grain boundaries were inhibited by P markedly. The inhibitory effect of P on δ phase enhanced gradually with increasing content of P, but the plasticity increased first and then decreased. What is more, the crack tended to propagate into the matrix around the particles (Laves phases and NbC carbides) in the alloys without P addition at the beginning of the tensile fracture, while it tended to propagate along the interfaces between the matrix and those particles in the alloys with P addition, which resulted from the synthetical effect of P on γ″, γ′ and δ phases.

Investigation of Work Hardening Behavior of Inconel X-750 Alloy

The constant strain rate uniaxial compression tests were conducted in this paper for studying the work hardening behavior and revealing the underlying microstructure evolution involved in the plastic response of the nickel-based Inconel X-750 alloy. The work hardening rate versus true strain plots of Inconel X-750 alloy resembled that of low-stacking-fault energy (SFE) alloys with distinct four stages. The dislocations were found in the planar arrangements at a strain of 0.1 located at the onset of stage II, and the dislocation density was increased and the planar arrangement configuration was partially destroyed at a strain of 0.36 located in stage III. It was unexpected that deformation twins were observed at a strain of 0.69 located in stage IV although the alloy has been classified into materials with a higher SFE value. The result is different with a similar study, in which the deformation twins were absent in Ni–Cr-based alloy Inconel 625 even when the strain was as high as 0.65. It was deemed that the low level of solution strengthening favored the deformation of matrix and the activation of slip system for twining in Inconel X-750 alloy. Unlike the low-SFE alloys that the twins were always formed at the end of stage I, the higher SFE delayed the twin formation to stage IV for Inconel X-750 alloy. The well-developed planar dislocation configuration gave rise to the stage II with a slightly decreasing rate, the collapse of planar dislocation arrangements caused the occurrence of stage III with an accelerated decreasing rate, and the twin formation led to the stage IV with a nearly constant work hardening rate.

Effects of Co on the Solidification and Precipitation Behaviors of IN 718 Alloy

The effects of Co from 0 to 11.60 % (in mass fraction) on the solidification and precipitation behaviors of IN 718 alloy had been investigated. The results showed that the volume fraction of the dendrite core increased with the addition of Co. In the alloys with 0-5.84 %Co, the addition of Co could restrain the precipitation of blocky Laves phase and promoted the formation of eutectic Laves phase. In the alloys with 9.00-11.60 % Co, the eutectic gray phase and small blocky Laves phase precipitated in the interdendritic region. The eutectic gray phase increased and small blocky Laves phase decreased with increasing Co. The parallel lath-like δ-Ni3Nb phase was observed to precipitate in some interdendritic region without the formation of gray phase and Laves phase in the 9.00-11.60 % Co alloys. Further research found that Co slightly segregated in the dendrite core and markedly raised the solubility of element Mo in the dendrite core which resulted in reduced Mo in the residual liquid, and consequently, restrained Laves phase while promoted the precipitation of Mo-depleted gray phase and δ-Ni3Nb phase. Furthermore, Co was seemed to elevate the solidification point of the γ matrix while decrease that of the Laves phase.

The role of primary Laves phase on the crack initiation and propagation in Thermo-Span alloy

Abstract Thermo-Span alloy is an oxidation resistant low thermal expansion superalloy and shows good mechanical properties, which has been used to fabricate components in gas turbine engine to control the clearance between static and rotating part, and thus improve the efficiency. Owing to the insufficient Cr, it was not applicable for this alloy to employ long homogenisation treatment at high temperature, and consequently, the primary Laves phase formed during the solidification process might be remained in this alloy. However, few studies were carried out on studying the role of primary Laves phase in this alloy. This paper studied the effects of primary Laves phase particles on the crack initiation and propagation during tensile test at room temperature, 650 and 750°C of the rolled bar made from casting Thermo-Span alloy. It was found that at room temperature, cracks were mainly related with the Laves phase larger than 5 μm and the Laves clusters. At 650°C, cracks were formed preferentially at large Laves particles, but they would propagate faster at grain boundaries. At 750°C, as the hardening effect of γ’ phase was not effective, and the stress concentrated at large Laves phase would be released by grain deformation, so that large cracks were no longer widely spread and more intergranular cracks especially at surface of the specimen were found.

Uniaxial Compression Test and FEM Simulation for GH4145 Superalloy at Room Temperature

In this paper, uniaxial compression testsfor GH4145 superalloywere conducted on Gleeble-3800 at room temperature and 60°C with strain rates ranging from 0.5s-1 to 10s-1. No cracks were observed in all the specimens deformed from 10% to 70%, indicating that the GH4145 superalloy possesses great ductility under room temperature. The flow stress of GH4145 superalloywas not sensitive to strain rate deformed around room temperature and it decreased slightly with the temperature increasing. Dynamic recovery during deformation will start to balance the effect of work hardening when the true strain was arisen up to 0.4. Good agreement between the calculated and simulated temperature and strain shows that calculation method is accurate enough for the prediction.

Precipitation and Dissolution Behavior of δ Phase in Alloy GH4169G

The precipitation and dissolution behavior of δ phase at varied states including as cast, heat treated and thermal processed of GH4169G alloy, have been investigated. The results indicated that δ phase existed in the vicinity of Laves phase and carbides with mainly the rod-like, needle-like morphology, and lath-like morphology because of the segregation of Nb in the interdendritic area. The amount of the δ phase decreased with the increase of the annealing temperature, and the dissolution rate increased with the increase of the annealing temperature. After initial holding of 2 h at 1020°C, the residual δ phase fully dissolved at 1140°C for 20min for the as-cast ingot. The minimum holding time for δ phase dissolution at 1020°C was 2h.