# Study on Transformation Mechanism and Kinetics of α’ Martensite in TC4 Alloy Isothermal Aging Process

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

_{2}polishing agent → Kroll reagent corrosion. The ratio of the corrosion reagent was HF:HNO

_{3}:H

_{2}O = 2:4:94.

## 3. Results and Analysis

#### 3.1. Solid Solution Preparation

#### 3.2. Phase Transformation Kinetic during Isothermal Aging

_{max}increased to −1.68 μm, −1.95 μm, −2.69 μm, −3.15 μm and −3.21 μm with the increase of aging temperature, and the equilibrium state time is reduced from 25,100 s (400−550 °C) to 15,000 s (600 °C), indicating that the increase of aging temperature can promote the decomposition degree of α′ martensite.

_{max}is the maximum expansion amount during aging isothermal transformation (t = 0 s); ΔL

_{t}is the amount of expansion at time t during aging; ΔL

_{min}is the expansion amount at the end of the aging isothermal transformation (t = end (s)), and the expansion amount will not decrease after that. It should be noted that when formula (1) is used to calculate the transformed volume fraction f, the expansion curve data is selected in the aging phase transformation stage (aging time from t = 0 (s) to t = end (s), (0 ≤ t ≤ t

_{end})). (The following three situations cannot predict and represent the overall change trend of expansion amount and transformed volume fraction f during aging phase transformation: (1) Before the aging phase transformation ends (0 ≤ t < t

_{end}); (2) Part of the aging phase transformation stage (0 < t < t

_{end}); (3) After the aging phase transformation (t > t

_{end})). The decomposition kinetics curve of α′ martensite during the aging at 400–600 °C could be obtained by formula (1) (Figure 5). The transformed volume fraction and the expansion amount at different aging temperatures had the same change trend, which further verifies the proportional relationship between the transformation of martensite and the expansion amount during aging.

_{1}was 1.01395 in the first stage, and n

_{2}was 0.56667 in the second stage (Figure 6d).

_{1}(n

_{1}is 0.946~1.013) at the initial stage of aging under different temperatures, indicating that this stage has the same nucleation and growth mechanism. A large number of dislocation defects in water-quenched martensite would promote the diffusion of solid solution elements, which in turn leads to local segregation of elements [25]. Therefore, it was preferred to nucleate on the dislocation line inside the α′ martensite, and the nucleation rate increases rapidly. At this time, the grain boundary had little influence on the nucleation of the precipitated phase, which is consistent with the conclusion that the proportion of time spent in the initial stage of aging is extremely small. The aging temperature was already close to the stress-relieving annealing temperature when the temperature rises to 600 °C. The higher temperature caused the dislocation defects inside the sample to decrease rapidly, which corresponds to the rapid decrease of the consumption time in the initial stage of aging. In the middle and late aging stage, the n

_{2}gradually decreased from 0.853 to 0.527 with the increase of the aging temperature. At this stage, the nucleation at the dislocation was saturated and begins to grow. It was hindered and formed an element enrichment zone when the element diffuses to the phase boundary, which leads to the nucleation site gradually transferring from the dislocation in the martensite to the martensite phase interface [26]. The degree of difficulty in phase transformation is generally judged by the activation energy Q [27], and k in Equation (2) can be expressed as:

_{0}is a constant, Q is the transformation activation energy of α′ → α + β, R is Molar gas constant (R = 8.314 J·mol

^{−1}·K

^{−1}). The scatter plot of ln(k) with 1/T at different aging temperatures could be obtained by taking the logarithm of both sides of Equation (4). Linear fitted to the scatter plot, the slope of the fitted line is −Q/R and the intercept is ln(k

_{0}) (Figure 7). According to the obtained slope −Q/R and intercept ln(k

_{0}), the transformation activation energy Q

_{1}(Q

_{1}= 41.8 kJ/mol) and k

_{1}(k

_{1}= 0.8945) at the initial stage of aging, and the activation energy Q

_{2}(Q

_{2}= 116.8 kJ/mol) and k

_{2}(k

_{2}= 33,7611.1) at the middle and late stage of aging can be obtained. The above Q

_{1}and Q

_{2}further verified the conclusion that dislocations caused to a decrease in nucleation activation energy, moreover, promotes nucleation.

#### 3.3. Microstructure Evolution During Isothermal Aging

#### 3.4. The Isothermal Aging Transformation Curve of Martensite

_{1}and k

_{1}have good consistency with the experimental results in the initial stage of aging, and the calculation results obtained by n

_{2}and k

_{2}are in good agreement with the experimental results in the middle and late stage of aging. The above results verified the accuracy and feasibility of the model prediction results.

## 4. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 6.**The relationship curve of ln[−ln(1 − f)]~ln(t) at different aging temperatures and fitting value: (

**a**) 400 °C, (

**b**) 450 °C, (

**c**) 500 °C, (

**d**) 550 °C and (

**e**) 600 °C.

**Figure 7.**The linear relationship between 1/T and ln(k) at different stages of aging: (

**a**) the early stage of aging and (

**b**) the middle and late stage of aging.

**Figure 8.**Microscopic morphology of precipitate phase at different aging temperatures: (

**a**) 400 °C/7h/AC; (

**b**) 500 °C/7h/AC and (

**c**) 600 °C/7h/AC.

**Figure 9.**Structure of precipitae phase at different aging temperatures: (

**a**) 400 °C/7h/AC; (

**b**) 500 °/7h/AC and (

**c**) 600 °C/7h/AC.

**Figure 10.**TEM morphology and selected area diffraction spots of TC4 alloy after 400 °C + 7 h aging treatment: (

**a**) β phase precipitate and (

**b**) α phase precipitate.

**Figure 11.**Calculation results and experimental results of the transformed volume fraction at different temperatures: (

**a**) 400 °C, (

**b**) 450 °C, (

**c**) 500 °C, (

**d**) 550 °C and (

**e**) 600 °C.

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**MDPI and ACS Style**

Yu, H.; Li, W.; Li, S.; Zou, H.; Zhai, T.; Liu, L.
Study on Transformation Mechanism and Kinetics of α’ Martensite in TC4 Alloy Isothermal Aging Process. *Crystals* **2020**, *10*, 229.
https://doi.org/10.3390/cryst10030229

**AMA Style**

Yu H, Li W, Li S, Zou H, Zhai T, Liu L.
Study on Transformation Mechanism and Kinetics of α’ Martensite in TC4 Alloy Isothermal Aging Process. *Crystals*. 2020; 10(3):229.
https://doi.org/10.3390/cryst10030229

**Chicago/Turabian Style**

Yu, Hui, Wei Li, Songsong Li, Haibei Zou, Tongguang Zhai, and Ligang Liu.
2020. "Study on Transformation Mechanism and Kinetics of α’ Martensite in TC4 Alloy Isothermal Aging Process" *Crystals* 10, no. 3: 229.
https://doi.org/10.3390/cryst10030229