Effects of temperature and strain range on fatigue cracking behavior in Hastelloy X

The low cycle fatigue tests of Ni-base superalloy Hastelloy X have been carried out in the temperature range of 650 - 870 °C with various total strain ranges. A change of slope in the Coffin-Manson (C-M) plot was found at 870 °C: the fatigue life significantly decreased at the total strain range less than 0.6%. The fatigue cracks initiated at the surface of the specimens and propagated transgranularly, regardless of test condition. However, the fatigue crack initiation site on the surface shifted from grain interior to grain boundary (GB) predominantly when the discontinuity of slope in the C-M plot began to occur. The fatigue crack tended to initiate preferentially at the oxidized GB rather than grain interior at 870 °C with total strain range below 0.6%. Under this condition, cyclic stability was pronounced, while cyclic hardening occurred at the rest of test conditions. The dislocation structures responsible for the cyclic stress response may partly account for the determination of major fatigue crack initiation site.  

Investigation of EDM characteristics of nickel-based heat resistant alloy

The EDM processing characteristics of one of the nickel-based heat resistant alloys, Hast-elloy-X, were investigated under the various EDM conditions and analyzed in terms of surface integrity. This alloy is commonly used as a material for the hot gas path component of gas turbines and it is difficult to machine by conventional machining methods. The primary EDM parameter which was varied in this study were the pulse-on time. Since the pulse-on time is one of the main factors that determines the intensity of the electrical discharge energy, it was expected that the machining ratio and the surface integrity of the specimens would be proportionally dependent on the pulse-on duration. However, experimental results showed that MRR (material removal rate) and EWR (electrode wear rate) behaved nonlinearly with respect to the pulse duration, whereas the morphological and metallurgical features showed rather a constant trend of change by the pulse duration. In addition the heat treating process affected the recast layer and HAZ to be recrystallized but softening occurred in recast layer only. A metallurgical evaluation of the microstructure for the altered material zone was also conducted.  

The Effect of an Applied External Tensile Stress on the Oxidation Behavior of a Nickel-Base Alloy with a Re-base Diffusion-barrier-coating

A diffusion-barrier-coating system with a duplex layer structure comprised of an inner Re-base alloy layer and an outer β-NiAl layer was formed on the Ni–Mo alloy, Hastelloy-X. Alloy specimens with and without the coating were oxidized at 970 °C in air for up to 200 h with an imposed tensile stress of 22.5 MPa. The oxidation behavior under the stress-free condition was also investigated for comparison purposes. Strain rates of the specimens with a diffusion-barrier-coating system decreased rapidly for about 5 h, followed by a slow creep-deformation with a strain of 3.5% and strain rates of (0.7–0.2) × 10⁻⁷/s for 200 h. There was little change in both the coating structure and the composition (at%) of the inner Re-base alloy layer. Considering the creep behavior of the uncoated alloy, as well as the fact that there were few cracks and flaws in the Re-base alloy layer, it was concluded that this inner layer was subject to creep-deformation along with the alloy substrate. The external scale on the coated alloy consisted mainly of θ-Al₂O₃ at the early stage of the oxidation/deformation, and with further oxidation the surface scale formed a duplex layer structure consisting of outer plate-like θ-Al₂O₃ and inner equi-axed Al₂O₃. There was exfoliation of the outer θ-Al₂O₃ scale during the creep deformation. After the 200 h oxidation the outer β-NiAl contained (40–50)% Al, while the alloy substrate near the inner layer had less than 1 at% Al. It was found that the Re-base alloy layer acted an effective barrier against inward Al diffusion and outward diffusion of alloying elements.  

Formation and Oxidation Behavior of a Diffusion-Barrier-Coating System on a Ni–Mo base Alloy at 1,373 K in Air

A Ni-aluminide/Re-base diffusion-barrier-coating system and, for comparison, a sole Ni-aluminide coating were formed on the Ni–Mo based alloy Hastelloy-X, and the oxidation behavior of the coated alloys was investigated in air at 1,100 °C for up to 400 h. After 100 h of oxidation the diffusion-barrier-coating system consisted of a duplex structure comprised of an inner σ-(Re, Mo, Cr, Ni) and an outer β-NiAl layers and covered by a protective Al₂O₃ scale. By contrast, the Ni-aluminide coating had degraded, forming a reaction diffusion zone, γ-Ni(Cr, Mo, Fe, Al), with voids, and the external scale exfoliated extensively. After 400 h of oxidation of the diffusion-barrier-coating system, an intermediate reaction-diffusion zone was present. The Mo of the alloy substrate was enriched in the inner σ-layer, changing it from a Re-base alloy to a Mo-base alloy. The Mo-base alloy appeared to enhance both inward-Al diffusion and outward diffusion of alloying elements such as Mo, Fe, and Cr to form the intermediate reaction-diffusion zone. The outer layer consisted mainly of β-NiAl with channels of γ-Ni(Cr, Mo, Fe, Al). It was concluded that the Re-base alloy such as a σ-phase in the Ni–Cr–Re system acts as an effective diffusion-barrier layer, but that this is not the case for the Mo-base alloy layer.  

Creep characterization of a Ni-based Hastelloy-X alloy by using theta projection method

The θ projection method was applied to characterize the creep behavior of the Hastelloy-X alloy at 950 °C. Four θ parameters were established by a nonlinear least square fitting (NLSF) to the creep curves. In the NLSF of the full creep curves, the θ₁ and θ₂ parameters were not defined with a large error, but the θ₃ and θ₄ parameters were defined well without an error. An optimum cutoff strain range for defining the four θ parameters was found to be a 3% strain. Four θ parameters revealed a good linearity as a function of stress. The predicted minimum creep rate showed a good agreement with the experimental data. At 950 °C of the Hastelloy-X alloy, the creep curves, the creep rate, and the time to reach a limiting strain were estimated with a wide range of stresses.  

Forming behavior and workability of Hastelloy X superalloy during hot deformation

The hot deformation behavior of Hastelloy X superalloy has been characterized using hot compression tests in the temperature range of 900–1150 °C and strain rates varying between 0.001 and 0.5 s⁻¹. The results showed that both kinds of softening mechanisms, dynamic recovery and dynamic recrystallization, occurred during hot working. Hot workability of this alloy has been analyzed by calculating flow localization parameter and construction of workability map for occurrence of shear band. In addition, on the basis of flow stress data obtained as a function of temperature and strain rate in compression, power dissipation map and instability map for hot working have been developed.  

Microstructural evolution mechanism during brazing of Hastelloy X superalloy using Ni–Si–B filler metal

The paper aims at understanding solidification phenomena and solid state precipitations during diffusion brazing of Hastelloy X nickel base superalloy using a ternary Ni–4.5Si–3.2 B (in wt-%) filler metal. The joint is featured by the formation of Ni-rich solid solution in an isothermal solidification zone, borides/silicide formation during eutectic-type solidification in an athermal solidification zone, on-cooling precipitation of fine nickel silicides in the joint centerline, in situ precipitation of Mo–Cr-rich borides in the diffusion affected zone and grain growth in the heat affected zone. The implication of the phase transformations on the joint properties is discussed. It is shown that Hastelloy X exhibited very fast isothermal solidification which can be attributed to its high Mo and Cr content that promotes in situ boride precipitation during brazing.  

热处理对哈氏合金X焊接接头组织与性能的影响

本实验研究不同热处理工艺对哈氏合金X焊接接头组织和力学性能的影响,分析焊后热处理对哈氏合金X焊接接头性能的影响规律。结果表明：焊缝中心区域组织主要以等轴晶为主,熔合区以枝晶为主。经1050℃热处理的焊缝未析出二次相,经1100℃处理的焊缝在晶界处与晶粒内析出大量二次相,经1150℃处理的焊缝析出物发生重溶。经1150℃热处理工艺后接头抗拉强度最高达773.49 MPa；经1100℃处理的焊接接头断裂形式为沿晶脆性断裂,其他焊缝接头的断裂形式为韧性断裂。