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Influence of shot peening on surface-layer characteristics of a monocrystalline nickel-based superalloy

Published online by Cambridge University Press:  29 February 2012

Y. H. Chen ,
C. H. Jiang ,
Z. Wang  and
K. Zhan
Y. H. Chen
Affiliation:
School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China and School of Physical Science and Technology, Xinjiang University, Urumqi 830046, China
C. H. Jiang*
Affiliation:
School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
Z. Wang
Affiliation:
School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
K. Zhan
Affiliation:
School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
*
a)Author to whom correspondence should be addressed. Electronic mail: chenyanhua97@163.com
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Abstract

Shot peening was conducted on [100]- and [111]-oriented monocrystalline nickel-based superalloy samples to study the effect of crystal orientation on the distributions of the residual stress and evolution of microstructures in the deformation layers on the sample surfaces as a function of the coverage up to 400%. The XRD results show that the orientation randomizations and the values of compressive residual stress in the [111]-oriented samples are relatively higher than those in the [001]-oriented samples. Moreover, the residual-stress distribution in each sample is anisotropic, and the residual stress is maximum along the 〈110〉 direction. This phenomenon can be explained by the anisotropic properties of a single-crystal alloy and mechanism of the dislocation slip in the plastic deformation layers. Line profile analysis was also used to obtain microstructural information of the samples.

Keywords

shot peeningnickel alloyresidual stressmicrostructureline profile analysisX-ray powder diffraction
Type
Technical Articles
Information
Powder Diffraction , Volume 25 , Issue 4 , December 2010 , pp. 355 - 358
Copyright
Copyright © Cambridge University Press 2010

I. INTRODUCTION

Monocrystalline nickel-based superalloys, having a face-centered-cubic (fcc) crystal structure, are widely used in aircraft, power-generation turbines, rocket engines, and other challenging environments, including nuclear power and chemical processing plants (Pollock and Tin, Reference Pollock and Tin2006; Leidermark et al., Reference Leidermark, Moverare, Simonsson, Sjöström and Johansson2009). These alloys have superior thermal, fatigue, and creep properties compared to conventional cast alloys because grain boundaries have been eliminated (Huang et al., Reference Huang, Gibson, Zhang and Neu2009; Prasad et al., Reference Prasad, Rao and Rajagopal2005). During the production route of turbine blades and services, the surface is treated mechanically, which resulted in a plastic deformation layer on the surface, and thus directly affects several important properties and characteristics of the machine (Pollock and Tin, Reference Pollock and Tin2006; Webster and Ezeilo, Reference Webster and Ezeilo2001).

In order to improve the performance of engineering components, shot peening (SP), a mechanical surface treatment, has been used widely (Boeckels and Wagner, Reference Boeckels, Wagner, Schulze and Niku-Lari2005; Harada et al., Reference Harada, Fukaura and Haga2007). In this process, in-plane compressive residual stress (CRS) and work hardening are introduced into the surface layers of metallic components simultaneously by the impingement of a stream of shot directed at the metal surface at high velocity and controlled conditions (Tian et al., Reference Tian, Dai, Villegas, Shaw, Klarstrom and Ortiz2008; Hammersley et al., Reference Hammersley, Hackel and Harris2000; Kim et al., Reference Kim, Evans, Shackleton, Bruno, Preuss and Withers2005). These factors together with the refined grain size could distinctly affect the mechanical behavior of the material. It has been shown that, when properly controlled, SP can indeed enhance the tensile properties of the C-2000 nickel-based alloy (Kim et al., Reference Kim, Evans, Shackleton, Bruno, Preuss and Withers2005). However, up to now, studies of nickel-based alloys focus mainly on nanocrystallization (Villegas et al., Reference Villegas, Dai, Shaw and Liaw2005; Villegas and Shaw, Reference Villegas and Shaw2009) and rafting microstructure (Pollock and Tin, Reference Pollock and Tin2006; Touratier et al., Reference Touratier, Andrieu, Poquillon and Viguier2009) in the deformation layer induced by the shot peening process. Up to now the residual-stress distribution and the effect of crystallographic orientation on the surface-layer characteristics during SP have been rarely reported.

In this work, a SP process was applied to bulk samples of DD3, a nickel-based superalloy. The influence of crystallographic orientation on the residual-stress distribution of the residual stress and evolution of microstructure in the deformation layers after SP with various degrees of peening coverage have been investigated by X-ray diffraction (XRD). Line profile analysis (LPA) was also used to obtain microstructural information on the samples.

II. EXPERIMENTAL

The material examined in this paper is the monocrystalline Ni-based superalloy, DD3 (Ni, 9.5 Cr, 5.9 Al, 2.2 Ti, 5.2 W, 5 Co, 3.8 Mo, in wt %). The material was solution heat treated for 4 h at 1250 °C and subsequently air cooled to room temperature (RT), followed by a stage ageing process with 32 h at 870 °C. Two sets of samples were machined from cast bars: one with its longitudinal axis runs parallel to the [001] direction and the other parallel to the [111] direction. The deviation from the ideal orientation of each sample was less than 5°. Before SP, all samples were polished electrolytically. The gauge of each sample was shot peened at RT according to the conditions of 0.3 MPa jet pressure, 0.2 mm average diameter of ceramic beads, 100 mm distance between nozzle and specimen, and 0.6 mm N Almen peening intensity. The degrees of peening coverage, defined as ratio of the dimpled surface to the total surface after SP, for each set of the samples were 80%, 200%, and 400%.

The residual stresses were determined with Cr K α radiation using the (220) diffraction peak. The analysis was based on the sin2ψ method. Young’s modulus of E=115 GPa and Poisson’s ratio of ν=0.3 were used to calculate the residual stresses from the observed lattice strains, without considering the elastic anisotropy. X-ray powder diffraction patterns were recorded using a Dmax/rc X-ray diffractometer with Cu K α radiation. Other experimental conditions are 40 kV, 100 mA, continuous scan, 30°–100° 2θ, and 0.02° 2θ step size. The domain size and microstrain in the deformation layer of each sample were estimated from the XRD pattern with the Voigt method (Langford, Reference Langford1978). The relationship of integral breadths is given by

where subscript L and G denote the Lorentzian and Gaussian components and h, f, and g denote the measured line profile, the structurally broadened profile, and the instrumental profile, respectively. The as-received (or annealed) sample was used as the standard. For all profiles, the backgrounds were subtracted. Using Eq. (1), β f L and the β f G can be calculated, and then the microstructure was obtained. Using the Voigt method, with an assumption of the Lorentzian “size” and the Gaussian “microstrain” profiles for the domain size and microstrain, respectively, are given by

The broadening for low-angle reflections mainly relates to the domain size, and the broadening at high-angle reflections is predominantly determined by the internal strain (Villegas et al., Reference Villegas, Dai, Shaw and Liaw2005; Shaw et al., Reference Shaw, Luo, Villegas and Miracle2003). In this study, Ni (111) was used to calculate the domain size D, while Ni (311) was employed to estimate the microstrain ε. The dislocation density was calculated by the modified Warren-Averbach (MWA) method (Ungár and Borbély, Reference Ungár and Borbély1996; Ungár et al., Reference Ungár, Gubicza, Ribárik and Borbély2001).

III. RESULTS AND DISCUSSION

A. Microstructure

Figure 1 shows the XRD patterns of the [001]- and [111]-oriented samples with the values of SP coverage of 80%, 200%, and 400%. Line broadening of the diffraction peaks was detected at all values of coverage (Figure 1), indicative of grain refinement and presence of lattice microstrain at and near the surface. Table I shows the values of grain size and microstrain estimated from Eq.(2) as well as the dislocation density calculated using the MWA method for these samples. Clearly, nanograins have formed at the surface region even at 80% coverage. With increasing peening coverage, a continuous broadening of the diffraction peaks and decreasing diffraction intensities were observed. These observations indicate an increase in the degree of lattice disorder and a decrease in the grain size. In other words, SP can randomize preexisting texture and the randomization effect increases with increasing coverage.

A decrease in domain size and increases in microstrain and dislocation density with increasing coverage are observed. It should also be noted that the domain size of each oriented samples was found to be about 30 nm, while the microstrains in the [111]-oriented samples are slightly larger than those in the [001]-oriented samples. This indicates that the crystallographic orientation of a sample has little effect on the domain-size distribution. Comparing the XRD patterns,

Figure 1. (Color online) Evolution of the XRD patterns for the DD3 alloy with coverage during SP: (a) [001]-oriented sample and (b) [111]-oriented sample.

the orientation randomizations in the [111]-oriented samples are much higher than those in the [001] oriented samples, especially the samples with 80% coverage. This implies that there is more extensive plastic deformation in the [111]-oriented samples. This difference comes from the fact that the elastic modulus in the 〈111〉 directions of Ni is considerably larger, and hence a smaller displacement is required in those directions to trigger plastic deformation (Huang et al., Reference Huang, Gibson, Zhang and Neu2009). Therefore, plastic deformation is more extensive in the 〈111〉 directions, which results in higher orientation randomization and lower texture intensity.

B. Residual stress

Considering the anisotropic properties of the single-crystal alloys, the residual stresses at various φ angles were measured. The residual-stress distributions of the 80% and the 400% coverage samples are depicted in Figure 2. For both [001]- and [111]-oriented samples, the residual stresses at the surfaces are compressive. Moreover, the profiles and characters of CRS are similar. The value of CRS increases with increasing coverage. It is well known that the amount of

TABLE I. Estimated domain size, microstrain, and dislocation density in SP-processed DD3 specimens from XRD experiments.

plastic deformation is responsible for the magnitude of surface residual stress (Harada and Mori, Reference Harada and Mori2005). The degree of plastic deformation also increases with increasing coverage. Hence, the value of CRS is larger at a higher coverage. It should also be noted that the value of CRS in the [111]-oriented sample is relatively higher than that in the [001]-oriented sample at same coverage. Because during plastic deformation, the anisotropy in elastic modulus (Li et al., Reference Li, Ellison and Smith1994) leads to yield strength anisotropy along different crystallographic directions, the yield strength along the [111] direction is generally higher than that along the [001] direction. On the other hand, the mechanical properties of the shot-peened samples have an important influence in their CRS values, and there is a close linear relationship between surface residual stress (σ s r s) and the yield strength (σ 0.2).

Figure 2. (Color online) Distributions of the residual stress of the DD3 alloy at 80% and 400% coverages. The illustration in top-left corner of the pattern shows the definition of corresponding sample orientation, where the X and Y axes correspond to φ=0° and 90°, respectively: (a) [001]-oriented sample and (b) [111]-oriented sample.

Hence, the CRS value is larger in a sample with higher yield strength, processed under the same condition (Gao et al., Reference Gao, Yao and Li2002; Wang et al., Reference Wang, Li and Wang1998).

Moreover, for each sample, the distribution of the residual stresses is anisotropic and significantly lower along the 〈110〉 directions. In the [001]-oriented sample, the maximum CRS appears along the 〈100〉 directions (φ=0° and 90°), where σ s r s〈110〉<σ s r s〈010〉. For the [111]-oriented sample, the CRS value was maximum along the 〈112〉 direction (φ=0°), where σ s r s〈110〉<σ s r s〈112〉. This is because the primary slip systems in a fcc material are the {111} planes in the 〈110〉 directions. During plastic deformation, dislocations slip along the 〈110〉 directions, which lead to a smaller CRS value. It is also been observed that, at a higher coverage, the anisotropic distribution of residual stress was weakened distinctly. This is attributed to larger plastic deformation, which can significantly induce grain-orientation randomization. Consequently, the influence of the preexisting texture on the distribution of residual stress diminishes. This result is consistent with the conclusion drawn from the XRD analysis discussed above.

IV. CONCLUSION

The microstructure and distribution of the residual stresses of DD3 nickel-based alloy subjected to a SP treatment have been investigated. The results show that crystallographic orientation plays an important role in the evolution of microstructure and the distribution of the residual stress. The following conclusion may be drawn:

  1. (i) The SP processing can weaken the preexisting texture, and the randomization effect can be improved at a high coverage.

  2. (ii) XRD analysis reveals that the orientation randomization in the [111]-oriented sample is relatively higher than that in the [001]-oriented sample, which is attributed to a higher elastic modulus and consequently extensive plastic deformation in the [111] direction.

  3. (iii) LPA shows that the domain size appears to be ∼30 nm, while the microstrain in the [111]-oriented sample is slightly larger, implying that the crystallographic orientation has little effect on the grain-size distribution.

  4. (iv) Compared to the [001]-oriented sample, the value of CRS in the [111]-oriented sample is much larger, which can be explained by the linear relationship between σ s r s and σ 0.2, and σ 0.2(111)>σ 0.2(100).

  5. (v) The distribution of residual stresses is anisotropic, being significantly lower along the 〈110〉 directions, which is attributed to the 〈110〉 directions being the main slip directions in plastic deformation.

  6. (vi) The anisotropic distribution of residual stress can be weakened distinctly at a high coverage. This is attributed to larger plastic deformation, which can significantly induce grain-orientation randomization.

ACKNOWLEDGMENT

Financial support from the National Natural Science Foundation of China under Project No. 50771066 is acknowledged.

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Figure 0

Figure 1. (Color online) Evolution of the XRD patterns for the DD3 alloy with coverage during SP: (a) [001]-oriented sample and (b) [111]-oriented sample.

Figure 1

TABLE I. Estimated domain size, microstrain, and dislocation density in SP-processed DD3 specimens from XRD experiments.

Figure 2

Figure 2. (Color online) Distributions of the residual stress of the DD3 alloy at 80% and 400% coverages. The illustration in top-left corner of the pattern shows the definition of corresponding sample orientation, where the X and Y axes correspond to φ=0° and 90°, respectively: (a) [001]-oriented sample and (b) [111]-oriented sample.