Dan Jia

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.

Effect of Mo Addition on the Grain Growth of IN718 Alloy

The effect of Mo addition on the grain growth of Inconel 718 alloy has been investigated. It was revealed that δ phase was precipitated at the grain boundaries when the Mo addition is in the range of 2.80% - 4.00%, while the Mo-bearing phase was precipitated at the grain boundaries when the Mo addition is in the range of 5.50% - 7.50%. By pinning the grain boundary, the grain boundary precipitate can effectively prevent the grain growths. So the grains grow rapidly with increase the solution temperature when the grain boundary precipitate is absent. The activation energy value for grain growth of IN718 alloys with different Mo addition is 210 - 255 kJ/mol and the grain growth mechanism is controlled by the self-diffusion. The grain growth exponent reduces with increasing Mo addition.

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.

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.

Effect of Mo on As-Cast Microstructure of a Modified 718 Alloy

The microstructure of as-cast 718 alloy after modified with Mo has been investigated by means of optical microscope (OM), scanning electron microscope (SEM) and X-ray diffraction (XRD). It was found that the dendrite arm space was widened and the interdendritic area was reduced by increasing Mo addition. And no new phase was precipitated in the modified alloys by Mo addition in the range of the present test. The addition of Mo promoted the precipitated of Laves phase, restrained the formation of δ phase, and elevated melting point of the Laves phase.

Nitrogen-Bearing Carbides in K417G Alloy Doped with Different Nitrogen Contents

By adding FeNCr6-A, K417G alloys with different nitrogen content from 4 to 32ppm were prepared by using the vacuum induction melting (VIM). The precipitation of the carbonitrides in the virgin alloys was studied. The results show that N mainly exists in Ti (N, C) phase which sometimes served as heterogeneous cores for the carbides in the alloy. Besides, a few isolated regular square blocky Ti (N,C) particles were also found in alloy with 10ppm N, and these isolated particles became larger in the alloy with 21ppm N. Moreover, as N level reaching 32ppm, the Ti (N,C) phase did not grown but these particles usually gathered together and formed many clusters. For the alloy with 4ppm N, the content of N in Ti (N,C) was lower, but for the alloys not less than 10ppm N, the N content of Ti (N,C) was higher and relatively stable.