PM Nickel Research Articles

Effect of Ta Content on High Temperature Creep Deformation Behaviors and Properties of PM Nickel Base Superalloys

Effect of Ta Content on High Temperature Creep Deformation Behaviors and Properties of PM Nickel Base Superalloys

Competing mechanisms of particle fracture, decohesion and slip-driven fatigue crack nucleation in a PM nickel superalloy

Fatigue cracks may initiate around non-metallic inclusions via particle fracture, particle decohesion and slip-driven nucleation. Cohesive zone techniques within microstructurally faithful crystal plasticity modelling validated by micromechanical experiments (HR-DIC and HR-EBSD) are employed to investigate these nucleation phenomena. Particle fracture and decohesion lead to stress redistribution which influences subsequent energy storage driving slip-driven fatigue crack nucleation. Particle fracture and decohesion strengths were determined and using a stored energy criterion, the number of cycles to initiation of the fatigue microcrack was predicted. A threshold applied stress below which decohesion and fracture do not occur was obtained, thus modestly increasing fatigue life.

Novel powder packing theory with bimodal particle size distribution-application in superalloy

Powder packing behavior plays an important role in determining sintering ability of powder and the resultant performance of materials. In this study, a novel powder packing theory with bimodal particle size distribution is proposed by considering the loosening effect, wall effect and wedging effect. This theory is applied in PM nickel base superalloy by using mixture of coarse particles and fine particles. Microstructures of alloy sintered by vacuum hot pressing (HP) are observed by optical microscope (OM) and electron backscatter diffraction (EBSD). The prediction result by this theory is in good agreement with the experimental results. The enhanced sintering ability of powder containing appropriate fractions of coarse particle and fine particle is ascribed to the filling of fine particles to the voids between coarse particles, which enhanced the density of sample after sintering. Tensile behavior and the fracture morphology of alloys with various particle distributions are analyzed in details, suggesting the higher reliability of the present theory.

Effects of microstructure on high temperature dwell fatigue crack growth in a coarse grain PM nickel based superalloy

The influence of microstructure on the dwell fatigue crack growth behaviour of an advanced nickel-based superalloy was investigated at a temperature of 700°C. Microstructural variations were induced by heat treatment variables: different cooling rates of quenching from super-solvus solution heat treatment, 0.7 and 1.8°Cs−1, and an addition of a high temperature stabilisation heat treatment (2h at 857°C) between the solution treatment and the final ageing treatment. With a one hour dwell introduced at the peak load of the fatigue cycle, such different microstructural conditions can lead to a difference of up to two orders of magnitude in crack growth rates in air, when compared to those obtained under baseline fatigue loading. By performing such dwell fatigue and baseline fatigue tests in vacuum, it is confirmed that such increases in crack growth rates under dwell fatigue loading in air are mainly environmentally related. Transmission electron microscopy (TEM) was utilised to analyse both crack tip oxides and associated deformation mechanisms in the matrix. A novel mechanism taking into account competing interactions of crack tip oxidation (leading to increases in crack growth rates) and stress relaxation (leading to decreases in crack growth rates) is outlined.

Creep, fatigue and oxidation in crack growth in advanced nickel base superalloys

This paper presents some of our recent results from an ongoing collaborative research programme on creep-fatigue behaviour of two advanced nickel base superalloys for turbine disc applications. The role of creep, fatigue and oxidation in crack growth has been investigated at 650°C under typical loading waveforms at selected loading frequencies. Load-line deflections were monitored in selected tests under static and long dwell loading conditions. Scanning electron microscopy was adopted to identify the fracture mode and to facilitate the evaluation of oxidation. The results show that mixed time and cycle dependent crack growth seems to be the predominant crack growth mode in the two PM nickel alloys studied. Whilst limited creep may be present at the crack tip, particularly under static and long dwell loading conditions, oxidation appears to be the predominant mechanism for crack growth under the test conditions examined.

Discs of medium alloyed PM nickel base alloy : G.Genrikh et al. (All-Russia Inst. of Light Alloys (VILS), Moscow, Russia.)

Discs of medium alloyed PM nickel base alloy : G.Genrikh et al. (All-Russia Inst. of Light Alloys (VILS), Moscow, Russia.)

Effect of temperature on small fatigue crack growth in nickel-base superalloys : Okazaki, M., Nohmi, S. and Yamada, H. J. Soc. Mater. Sci., Jpn. (1995) 44 (498), 348–354

This paper presents some of our recent results from an ongoing collaborative research programme on creep-fatigue behaviour of two advanced nickel base superalloys for turbine disc applications. The role of creep, fatigue and oxidation in crack growth has been investigated at 650°C under typical loading waveforms at selected loading frequencies. Load-line deflections were monitored in selected tests under static and long dwell loading conditions. Scanning electron microscopy was adopted to identify the fracture mode and to facilitate the evaluation of oxidation.The results show that mixed time and cycle dependent crack growth seems to be the predominant crack growth mode in the two PM nickel alloys studied. Whilst limited creep may be present at the crack tip, particularly under static and long dwell loading conditions, oxidation appears to be the predominant mechanism for crack growth under the test conditions examined.

Model of viscoplastic behaviour of a PM nickel base superalloy at 650 °c : V. Aubourg et al. (University of Dayton, Dayton, Ohio, USA)

Model of viscoplastic behaviour of a PM nickel base superalloy at 650 °c : V. Aubourg et al. (University of Dayton, Dayton, Ohio, USA)

Effect of Boron on the Structure and Properties of a PM Nickel Base Superalloy

Effect of Boron on the Structure and Properties of a PM Nickel Base Superalloy

Effect of Minor Chemistry Variations on the Structural and Mechanical Behaviour of a PM Nickel Base Superalloy

Addition of 0.7% Hf to a heavily alloyed low carbon Ni base PM superalloy has been found to significantly reduce the alloy solidus in contrast to another chemisty variant free of Hf but with increased alloying. The Hf-bearing alloy showed major microstructural changes with increase in HIPing temperature. Reduction in C to < 0.02% severely affected its elevated temperature strength. The Hf-free alloy containing 0.03% C offered a good combination of tensile and stress rupture properties after a suitable heat treatment.