Papers by Author: Qiang Feng

Abstract: Since gas turbine blades in engines suffer centrifugal stress and gas corrosion during service, a good creep resistance in hot corrosion environment is one of the important considerations to evaluate service performance of the blade materials. In this work, the creep rupture behaviors of the directionally solidified superalloy DZ466 with and without salt deposition at 760°C/765MPa, 850°C/500MPa and 950°C/220MPa are preliminarily investigated based on the creep properties measurement and microstructure observations. The effects of hot corrosion on the creep properties and fracture mode are examined. The results show that the creep-rupture life in salt-deposit environment is lower than that in air-exposure environment at different temperatures. The creep-rupture life reduction caused by hot corrosion is increased with increase of the creep temperature. The fracture mode is exhibited by transgranular fracture in all crepted specimens. The propagation directions of all the secondary cracks are almost perpendicular to the crept specimen surface or the stress axis. The surface cracks are mainly produced in air-exposure environment at low temperature or in salt-deposited environment at high temperature. Induced by the stress concentration, the internal cracks are initiated surrounding the carbides in both air-exposure and salt-deposit environments. The creep-rupture life is dependent on the crack initiation at low temperature but on the crack propagation at high temperature. The reduction of the active load bearing area in transversal direction is the main reason why the creep-rupture life is decreased at 950°C.

Abstract: The phase equilibria of Co-base trial alloys of Co-9Al-7.5W, Co-5.5Al-10W and Co-9Al-3W at 1300°C and 900°C were determined in the present investigation. The experimental alloys were γ single-phase alloys at 1300°C, although the existence of the γ' phase and the ε phase was identified due to cooling effect. The nearly cuboidal γ' phase and the plate-like DO₁₉ phase in alloys 9Al-7.5W and 5.5Al-10W were coarsened after long-term annealing at 900°C. It is suggested that the alloy 9Al-7.5W was in γ+γ' two-phase region, while the alloys 5.5Al-10W and 9Al-3W were in γ+DO₁₉ two-phase region and γ singe-phase region at 900°C, respectively.

Abstract: The as-cast and heat-treated microstructure has been investigated in Co-Al-W-based superalloys with additions of 2 and 4 at.% Mo. The results revealed that Mo promoted the formation of (μ+γ) eutectic structure in the as-cast alloys and μ-phase precipitation after solution treatment. In addition, after aging treatment at 900oC, an extensive network of DO19 precipitates was observed, besides the γ+γ′ primary phases and the μ phase inherited from the as-cast and solution-treated conditions. Meanwhile, a solid-state transformation from the μ phase to the DO19 phase occurred; and long-term aging enhanced this transformation. In addition, high levels of Mo promoted the DO19-phase precipitation.

Abstract: The femtosecond laser ablation has been investigated on second generation single crystal superalloy CMSX-4 using a commercial titanium:sapphire laser system (λ = 780nm, τ = 120 fs). The ablation thresholds of bulk alloy CMSX-4 have been determined as a function of the pulse number (1, 10, 100, 1000) in air, argon gas and vacuum. The results indicate that the multiple-pulse threshold of this material decreased with increasing the pulse number in all the cases. For the same pulse number, the threshold changed in the descending order: air, vacuum and argon gas. The incubation coefficient in air has been determined: ξ = 0.86 ± 0.03. The preliminary results showed the better quality of femtosecond laser machining of CMSX-4 in vacuum and argon gas than in air.

Abstract: The effects of ruthenium and chromium contents have been investigated on phase transformation temperatures and the morphology of γ’ precipitates as well as microstructural stability in high refractory Ni-base superalloys. The solidus and liquidus temperatures were determined by differential scanning calorimetry (DSC), suggesting that the addition of Cr resulted in a decrease in solidus/liquidus temperatures while the Ru addition (3.5 at.%) had the neutral effect. The morphology of γ’ precipitates in the heat-treated microstructure was changed under the influence of the Cr and Ru additions, suggesting that Ru and Cr contents affected the γ−γ’ lattice misfit through changes in the associated partitioning to the constituent phases. The microstructural instability has been investigated at 1000 oC. High levels of Cr addition (8 at.%) strongly promoted the formation of TCP phases, while Ru improved the microstructural stability to some extent.