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Effect of Different Twin Boundary Spacing on the Mechanical Properties of Truncated Octahedral Pt

Abstract: In this paper, molecular dynamics method is used to study the strengthening effect of different twin boundary spacing on 10 nm and 20 nm truncated octahedral Pt grains. This paper studies the mechanical properties and deformation mechanisms of the 0.67 nm, 1.55 nm, 2.43 nm, 3.31 nm, 4.19 nm twin boundary spacing during the 10 nm and 20 nm grain stretching process. The research results show that there is an optimal twin boundary spacing in both 10 nm grains and 20 nm grains, 1.55 nm and 2.43 nm, respectively, which have the best mechanical properties. At the same time, it is found that deformation mechanisms occur before and after the critical twin boundaries. The dislocation is transformed from sliding along the twin boundary to vertical twin boundary growth.

1. 引言

Lu [7] 等制备出具有高密度纳米尺寸孪晶的纯铜薄膜，其拉伸强度是普通纯铜的10倍以上，并表明孪晶界可以有效地阻碍位错的运动，实现金属强化。Lu [8] 等制备出高密度纳米尺度生长孪晶，将孪晶片层平均厚度从100 nm减小到约4 nm，实验发现减小孪晶片层厚度伴随着材料强度的增加，当孪晶片层厚度为15 nm时，材料强度达到最大值。Wei [9] 等报告了纳米孪晶铜的强度最大化以及其受晶粒尺寸的影响，研究发现强度最大的临界孪晶界厚度与d1/2成正比，最大强度与d−1/2成正比。Gao [10] 等通过分子动力学对不同温度和不同孪晶厚度的纳米孪晶Pd和Cu进行模拟，发现存在一个软化温度，在这个温度下，强度首先增加，然后减小，而在临界孪晶厚度表现出最大强度以及硬化到软化的转变。

Wang [11] 等对晶粒尺寸在4 nm到20 nm的纳米孪晶Pt进行拉伸，观察表明纳米孪晶Pt的变形孪生是通过由单一原子层分隔的两个堆叠断层的形成而开始的，并在这两个堆叠断层之间产生部分位错。Wang [12] 等对晶粒尺寸约25 nm的纳米孪晶Pt进行拉伸，观察表明对于高层错能金属，与共格孪晶界相交的完全和部分位错以及共格孪晶界和非共格孪晶界的迁移都有助于提高纳米孪晶金属的大应变能力。Wang [13] 等通过实验研究了晶粒尺寸和孪晶界间距对纳米孪晶Pt的影响，研究结果表明当晶粒尺寸减小到6 nm以下时，晶界主导了变形，并且发现晶界是纳米孪晶金属的一个重要的位错源，它可以提供足够的位错成核位点来适应较大的应变。目前，对于低层错能金属的研究较为深入，而对高层错能金属的研究较少，对于金属Pt，研究多集中于实验，分子动力学模拟较少，所以开展相关的研究很有必要。

2. 模拟计算方法

2.1. 模型构建

(a)(b)

Figure 1. (a) The initial configuration of the 10 nm grain twin boundary spacing. (b) Initial configuration of 20 nm grain twin boundary spacing

2.2. 拉伸模拟方法

${\text{π}}_{i}^{\alpha \beta }=\frac{1}{\omega }f\left(x\right)=\left\{-{m}_{i}{v}_{i}^{\alpha }{v}_{i}^{\beta }+\frac{1}{2}\underset{i\ne j}{\sum }\left(-\frac{1}{{r}_{ij}}\frac{\partial U}{\partial {r}_{ij}}\right){r}_{ij}^{\alpha }{r}_{ij}^{\beta }\right\}$ (1)

3. 结果与讨论

3.1. 力学行为

(a) (b)

Figure 2. (a) Stress-strain curves of 10 nm grains with different distances between twin boundaries during stretching. (b) The average flow stress of 10 nm grains with different twin boundary spacing during the stretching process

(a) (b)

Figure 3. (a) Stress-strain curves of 20 nm grains with different distances between twin boundaries during stretching. (b) The average flow stress of 20 nm grains with different twin boundary spacing during the stretching process

Table 1. Yield stress of 10 nm grains with different spacing between twin boundaries

Table 2. Yield stress of 20 nm grains with different spacing between twin boundaries

3.2. 微观变形机制

(a) (b) (c) (d) (e)

Figure 4. (a) Atomic snapshot of the (110) cross section at the yield strain of the 0.67 nm twin boundary spacing of 10 nm grain. (b) Atomic snapshot of the (110) cross section at the yield strain of the 1.55 nm twin boundary spacing of 10 nm grain. (c) Atomic snapshot of the (110) cross section at the yield strain of the 2.43 nm twin boundary spacing of 10 nm grain. (d) Atomic snapshot of the (110) cross section at the yield strain of the 3.31 nm twin boundary spacing of 10 nm grain. (e) Atomic snapshot of the (110) cross section at the yield strain of the 4.19 nm twin boundary spacing of 10 nm grain

(a) (b) (c) (d) (e)

Figure 5. (a) Atomic snapshot of the (110) cross section at the yield strain of the 0.67 nm twin boundary spacing of 20 nm grain. (b) Atomic snapshot of the (110) cross section at the yield strain of the 1.55 nm twin boundary spacing of 20 nm grain. (c) Atomic snapshot of the (110) cross section at the yield strain of the 2.43 nm twin boundary spacing of 20 nm grain. (d) Atomic snapshot of the (110) cross section at the yield strain of the 3.31 nm twin boundary spacing of 20 nm grain. (e) Atomic snapshot of the (110) cross section at the yield strain of the 4.19 nm twin boundary spacing of 20 nm grain

(a) (b) (c) (d) (e)

Figure 6. (a) Atomic snapshot of the (110) cross section of 10 nm grain with a 0.67 nm twin boundary spacing strain at 0.1. (b) Atomic snapshot of the (110) cross section of 10 nm grain with a 1.55 nm twin boundary spacing strain at 0.1. (c) Atomic snapshot of the (110) cross section of 10 nm grain with a 2.43 nm twin boundary spacing strain at 0.1. (d) Atomic snapshot of the (110) cross section of 10 nm grain with a 3.31 nm twin boundary spacing strain at 0.1. (e) Atomic snapshot of the (110) cross section of 10 nm grain with a 4.19 nm twin boundary spacing strain at 0.1

(a) (b) (c) (d) (e)

Figure 7. (a) Atomic snapshot of the (110) cross section of 20 nm grain with a 0.67 nm twin boundary spacing strain at 0.1. (b) Atomic snapshot of the (110) cross section of 20 nm grain with a 1.55 nm twin boundary spacing strain at 0.1. (c) Atomic snapshot of the (110) cross section of 20 nm grain with a 2.43 nm twin boundary spacing strain at 0.1. (d) Atomic snapshot of the (110) cross section of 20 nm grain with a 3.31 nm twin boundary spacing strain at 0.1. (e) Atomic snapshot of the (110) cross section of 20 nm grain with a 4.19 nm twin boundary spacing strain at 0.1

4. 结论

1) 通过应力应变曲线、屈服强度、平均流变应力，我们发现在10 nm晶粒和20 nm晶粒中均存在一个最优孪晶界间距，分别为1.55 nm和2.43 nm，拥有最好的力学性能。这对制备高性能的金属Pt材料提供了思路。

2) 通过分析10 nm晶粒和20 nm晶粒不同孪晶界间距的微观变形机制，我们发现在临界孪晶界前后会产生变形机制的转化，位错由沿着孪晶界滑移转变为垂直孪晶界生长，预测了金属Pt的临界孪晶界间距。

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