Zhixun Wen

The tension/compression asymmetry of a high γ′ volume fraction Nickel-based single-crystal superalloy

A high γ′ volume fraction single-crystal superalloy specimens with various crystallographic orientations were tested under uniaxial tension/compression. The results show that there is apparent tension/compression asymmetry at the initial yielding (plastic strain < 0.2%, where no Kear-Wilsdorf lock and superlattice stacking fault generates during the deformation, so the existing models such as LCP and PPV cannot apply to two-phase nickel-based single-crystal superalloys). In this paper, the effect of elastic deformation on tension/compression asymmetry is discussed. A new constitutive model of two-phase nickel-based single-crystal superalloys is proposed to simulate the tension/compression asymmetry of yield strength. The model could well explain tension/compression asymmetry of two-phase alloy in [0 0 1] and [0 1 1] orientations, however it is not suitable for [1 1 1] orientation mainly due to the different slip mode

Thickness Influence on the Creep Response of DD6 Ni-Based Single-Crystal Superalloy

Abstract The effect of specimen thickness on the creep response of Ni-based single-crystal superalloy DD6 was investigated. With the thickness of 0.3 mm, 0.6 mm and 1.2 mm, a series of thin-wall specimens were tested in this paper respectively at 760℃, 980℃ and 1,100℃. Under the conditions of lower temperatures and higher stresses, the creep life of thin-wall specimens increases with the increase of δ, but it is almost equal under higher temperatures and lower stresses conditions. Compared with the standard specimens, an obvious reduction (about 60%) of creep life of the thin-wall specimens was found at 760℃, whereas it is almost the same at 980℃ and 1,100℃. Therefore, obvious thickness effect is prone to lower temperature and higher stress. The thickness effect is a comprehensive effect, which is caused by fracture mode, the degree of necking, the shape and quantity of creep cavities, oxide thickness, etc. Under each condition, an increased thickness resulted in increased creep strain to rupture.

The Influence of Crystal Orientation on Vibration Characteristics of DD6 Nickel-Base Single Crystal superalloy Turbine Blade

Modeling of a nickel-base single crystal superalloy turbine blade which accounts for material orthotropy is carried out to investigate the influence of axial direction and randomness crystallographic orientation on the dynamic natural frequency. In this paper, the natural frequency of the blade is calculated by the finite element method based on the commercial software ABAQUS. The results show that the deviation of the axial directions has a significant effect on the lower frequency as well as on the higher frequency, while the randomness in crystallographic orientations has a great impact only on higher order frequency. It is necessary to take the crystal orientation into account as an optimized factor and the variation of the vibration characteristics can be predicted.

Crystallographic life model for single crystal turbine blade and validation by the miniature specimens cut from the turbine blades

Purpose – The purpose of this paper is to found a life model for the single crystal (SC) turbine blade based on the rate‐dependent crystallographic plasticity theory.Design/methodology/approach – This life model has taken into consideration the creep and fatigue damages by the linear accumulation theory. A SC blade was taken from an aero‐engine, which had worked for 1,000 hours, as the illustration to validate the life model.Findings – The crystallographic life model has a good prediction to the life and damage of the SC turbine blade. In the mean time, the micro damage study of the miniature specimens showed that creep damage has more serious influence on the material performance in the blade body but it is fatigue damage in the blade rabbet.Originality/value – The life model can reflect the crystalline slip and deformation and crystallographic orientation of nickel‐based SC superalloys.

Effects of Blade Curvature on Fatigue Life of Nickel-Based Single Crystal Structures with Film-Cooling Holes

The curved thin-walled structures of multi film-cooling holes with different curvatures were adopted to simulate film-cooling turbine blades. The low cycle fatigue (LCF) characteristic was studied based on the theory of crystallographic slip damage. Results show that there is obvious stress interference among cooling holes. Two slip bands around the holes were found linear at approximately 45° and 135° to the loading axis. The maximum resolved shear stress reduces with the increase of curvature radius. Meanwhile, the LCF life positively increases with the changing of curvature radius. When curvature radius is less than 13 mm, it makes a remarkable effect on the resolved shear stress and LCF life; however, when the curvature radius exceeds 13 mm, it can be replaced by the plate structure. Furthermore, an exponential curve is found fitting for the relation between the curvature radius and the logarithmic fatigue life.

The Effect of Multi-inclined Holes on the Creep Properties of Nickel-Based Superalloy

Abstract The creep properties of GH3536 nickel-based superalloy plate specimens without/with multi-inclined holes were studied under applied stress 80/90/100u2006MPa at 850u2006℃, respectively. Interesting finding is focused on the inflection point, that is, both the fraction elongation and creep strain achieve the maximum value under 90u2006MPa. Further study is carried out by two methods: the finite element analysis (FEA) calculation and scanning electron microscope (SEM). The FEA results show that the dangerous areas appear in the regions near the end of ellipse axis along the inclined angle orientation, which is similar to the actual fracture appearances. What is more, the tiny holes and dimples are the main characters of creep fracture for multi-inclined hole specimens, whereas the creep fracture of specimens without holes is the result of growth and coalescence of voids. In addition, based on creep performance, laser drilling is better than that of the electric spark drilling, which provides a proof that the creep performance of specimens with multi-inclined holes will be better with the improvement of the drilling process.

The Influence of Dwell Time on Low Cycle Fatigue Behavior of Ni-base Superalloy IC10

Abstract Low cycle fatigue and creep-fatigue experiments of IC10 Ni-base superalloy plate specimens with multiple holes were performed below 1,000u2006°C. The average fatigue life is 105.4 cycles, while the creep-fatigue life is 103.4 cycles, which shows that the life of creep-fatigue is reduced 1–2 times compared with low cycle fatigue life. After tests, the detailed fracture and microscopic structure evolution were observed by scanning electron microscopy (SEM); meanwhile, the constitutive model based on crystal plasticity theory was established and the fracture mechanism was analyzed. Three conclusions have been obtained: First, the load during dwell time leads to the damage accumulation caused by deformation and the interaction of fatigue and creep shortens the service life of materials seriously. Second, in order to maintain the macroscopic deformation, a new slip plane starts to makes the dislocation slide in reverse direction, which leads to fatigue damage and initial cracks. Third, the inner free surface creates opportunities for escape of the dislocation line, which is caused by the cavity. What’s more, the cure dislocation generated by cyclic loading contributes to the formation and growth of cavities.

The Effect of Post-heat Treatment on the Microstructures of Single Crystal DD6 Superalloy

Abstract Various thermal cycles at the end of solution heat treatment and their influences on microstructure of single crystal superalloy DD6 were studied by experiments. During various thermal cycles, the qualitative and quantitative microstructure of samples quenched of the transformations is microscopically characterized. This completely includes the large changes in volume fraction, size distribution and morphology of gamma prime precipitate experienced in the upper temperature transformation. Noticeable deviation from the equilibrium volume fraction of γ’ phase is detected in both the dissolution and precipitation processes above 1,120°C for both moderate cooling and heating rate; differences were mainly attributed to the unsteady nature of the turbulent flow. The growth and alignment of the γ’ precipitates are deeply influenced by several factors, e.g. ageing time, cooling rate and quenching temperature. In addition, interesting findings such as “labyrinth” and “cluster” morphologies were observed by scanning electron microscope. During precipitation processes, the complicated microstructure evolution is illustrated by considering the consecutive equilibrium shapes of a coherent precipitate, which grows under the interaction with its neighbors and the coherency of the precipitates improves their potential to resist dissolution.