# Effect of Heat Treatment Process on Microstructure and Fatigue Behavior of a Nickel-Base Superalloy

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## Abstract

**:**

## 1. Introduction

## 2. Experimental Section

#### 2.1. Heat Treatment Procedure

- ProcedureI (Pro.I): 1050 °C/8 h, AC + 1000 °C/4 h, AC + 775 °C/16 h, AC + 700 °C/16 h, AC
- ProcedureII (Pro.II): 1100 °C/8 h, AC + 1000 °C/4 h, AC + 775 °C/16 h, AC + 700 °C/16 h, AC

#### 2.2. Fatigue Tests

## 3. Cyclic Stress Response Behavior and Deformation Mechanism

#### 3.1. Grain Morphology and Precipitates

**Figure 1.**The grain morphology of the nickel-base superalloy. (

**a**) the original grain; (

**b**) Pro.I; (

**c**) Pro.II.

**Figure 2.**The precipitates of the nickel-base superalloy after heat treatment. (

**a**) Pro.I; (

**b**) Pro.II.

#### 3.2. Fatigue Property and Fatigue Life Model

_{t}/2 is the total strain, Δε

_{e}/2 is the elastic strain, Δε

_{p}/2 is the peak plastic strain, and 2N

_{f}is the reverse number of fatigue cycles. It can be found that fatigue resistance significantly drops with the increase of strain amplitudes under the condition of the same heat treatment procedure. It can be observed that the fatigue life of Pro.I is higher than that of Pro.II at the same total strain amplitude, which indicates that the heat treatments play an important role in fatigue behavior and the fatigue resistance of Pro.I is better than that of Pro.II.

Heat Treatment | $\Delta $ ε_{t}/2 (%) | $\Delta $ ε_{e}/2 (%) | $\Delta $ ε_{p}/2 (%) | 2N_{f} |
---|---|---|---|---|

Pro.I | 0.3 | 0.297 | 0.003 | 68,468 |

0.4 | 0.378 | 0.022 | 8094 | |

0.5 | 0.413 | 0.087 | 1746 | |

0.6 | 0.473 | 0.127 | 768 | |

0.7 | 0.489 | 0.211 | 282 | |

0.8 | 0.51 | 0.29 | 174 | |

Pro.II | 0.3 | 0.297 | 0.003 | 52,032 |

0.4 | 0.368 | 0.032 | 4528 | |

0.5 | 0.414 | 0.086 | 1622 | |

0.6 | 0.466 | 0.134 | 648 | |

0.7 | 0.491 | 0.209 | 242 | |

0.8 | 0.508 | 0.292 | 148 |

_{f}is the fatigue strength coefficient; ε′

_{f}is the fatigue ductility coefficient; b is the fatigue strength exponent, c is the fatigue ductility exponent; E is Young’s modulus.

_{0}, m and c are material constants. Fatigue limits of alloys are considered in the relationship.

**Figure 3.**Total; elastic and plastic strain range vs. number of cycles to failure of the nickel-base alloy. (

**a**) Pro.I; (

**b**) Pro.II.

**Figure 4.**The reciprocal of cycles to failure versus the total strain ranges of the nickel-base alloy. (

**a**) Pro.I; (

**b**) Pro.II.

_{f}are shown in Figure 5. The predicted effect is perfect when the fatigue life predicted points fall on the red line. It can be found that fatigue life predicted points fall basically within two fold safety factor specified dispersing band. As for Manson-Coffin relationship, the predicted effect is in good agreement with total strain amplitudes from 0.5% to 0.8% and poor for total strain amplitudes from 0.3% to 0.4%. However, as for three-parameter power function, the predicted effect is in good agreement with total strain amplitudes from 0.3% to 0.6% and poor for total strain amplitudes from 0.7% to 0.8%. The conclusion that the predicted effect of Manson-Coffin relationship is more accurate than that of three-parameter power function for high total strain amplitude section can be drawn.On the contrary, the predicted effect of three-parameter power function is more accurate than that of Manson-Coffin relationship for low total strain amplitude section. The relevance of experimental values and predicted values is used to evaluate the predicted effect of fatigue life model in the engineering. In order to evaluate fatigue life prediction method for above two kinds of fatigue life models, an error analysis method is adopted. The parameters of error analysis method are as follows [15]:

Heat Treatment | Manson-Coffin | Three-Parameter Power Function |
---|---|---|

Pro.I | 2.83 | 2.79 |

Pro. II | 2.65 | 6.61 |

#### 3.3. Cyclic Stress Response Behavior

#### 3.4. Fatigue Fracture Morphology and Fracture Mechanism

**Figure 7.**Fractographs of Pro.I after LCF at 650 °C. (

**a**,

**b**) Δε

_{t}/2 = 0.4%; (

**c**,

**d**) Δε

_{t}/2 = 0.6%; (

**e**,

**f**) Δε

_{t}/2 = 0.8%.

**Figure 8.**Fractographs of Pro.II after LCF at 650 °C. (

**a**,

**b**) Δε

_{t}/2 = 0.4%; (

**c**,

**d**) Δε

_{t}/2 = 0.6%; (

**e**,

**f**) Δε

_{t}/2 = 0.8%.

#### 3.5. Dislocation Structures and Deformation Mechanism

**Figure 9.**Typical dislocation structures of Pro.I after LCF tests at 650 °C (bright field). (

**a**) Δε

_{t}/2 = 0.4%, pcum = 3.562; (

**b**) Δε

_{t}/2 = 0.6%, pcum = 1.952; (

**c**) Δε

_{t}/2 = 0.8%, pcum = 1.012.

**Figure 10.**Typical dislocation structures of Pro.II after LCF tests at 650 °C (bright field). (

**a**) Δε

_{t}/2 = 0.4%, pcum = 2.898; (

**b**) Δε

_{t}/2 = 0.6%, pcum = 1.738; (

**c**) Δε

_{t}/2 = 0.8%, pcum = 0.867.

## 4. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**MDPI and ACS Style**

Zhang, P.; Zhu, Q.; Chen, G.; Qin, H.; Wang, C.
Effect of Heat Treatment Process on Microstructure and Fatigue Behavior of a Nickel-Base Superalloy. *Materials* **2015**, *8*, 6179-6194.
https://doi.org/10.3390/ma8095299

**AMA Style**

Zhang P, Zhu Q, Chen G, Qin H, Wang C.
Effect of Heat Treatment Process on Microstructure and Fatigue Behavior of a Nickel-Base Superalloy. *Materials*. 2015; 8(9):6179-6194.
https://doi.org/10.3390/ma8095299

**Chicago/Turabian Style**

Zhang, Peng, Qiang Zhu, Gang Chen, Heyong Qin, and Chuanjie Wang.
2015. "Effect of Heat Treatment Process on Microstructure and Fatigue Behavior of a Nickel-Base Superalloy" *Materials* 8, no. 9: 6179-6194.
https://doi.org/10.3390/ma8095299