Molecular Dynamics Simulation of Microscopic Crack Initiation and Extension Mechanism in 8Cr4Mo4V Bearing Steels

doi:10.3969/j.issn.1004-132X.2025.10.003

Abstract

Abstract:

To investigate the influences of cementite on the mechanics properties of the matrix and the initiation and propagation of microcracks in 8Cr4Mo4V bearing steels， molecular dynamics models were used to systematically analyze the effects of cementite's geometric parameters （such as shape， size， and position） on crack initiation and extension mechanism. And combined with cohesive force theory， the characteristics of interface crack propagation were studied. The results indicate that cementite significantly enhances the mechanics properties of the bcc-Fe matrix， with smaller cementite particles providing a more pronounced strengthening effectiveness. While the shape and position of cementite exert a relatively minor impact on overall mechanics performance， sharper inclusions accelerate crack propagation， and the position of inclusions determines the crack propagation path. Furthermore， interfaces between the bcc-Fe matrix and cementite， as well as twin boundaries with larger misorientation angles， exhibit increased resistance to crack initiation and propagation.

Key words: 8Cr4Mo4V bearing steel, cementite, molecular dynamics, crack extension, cohesive parameter

摘要：

为研究8Cr4Mo4V轴承钢中渗碳体对基体力学性能及微裂纹萌生与扩展的影响，采用分子动力学方法系统分析了渗碳体的几何参数（如形状、尺寸、位置）对裂纹萌生和扩展的影响机制，并结合内聚力理论研究了界面裂纹扩展特性。研究结果表明：渗碳体显著提高了bcc-Fe基体的力学性能，渗碳体尺寸越小，它对基体力学性能的增强效果越显著；渗碳体的形状和位置对力学性能影响较小，但尖锐的夹杂加速了裂纹扩展，且夹杂的位置决定了裂纹的扩展路径；在bcc-Fe基体与渗碳体间的界面和错向角较大的孪晶界面，裂纹更难萌生。

关键词: 8Cr4Mo4V轴承钢, 渗碳体, 分子动力学, 裂纹扩展, 内聚力参数

CLC Number:

V252.1

Tianyu MA, Gu GONG, Hongrui CAO, Jianghai SHI, Xunkai WEI, Lijun ZHANG. Molecular Dynamics Simulation of Microscopic Crack Initiation and Extension Mechanism in 8Cr4Mo4V Bearing Steels[J]. China Mechanical Engineering, 2025, 36(10): 2179-2189.

马天宇, 巩固, 曹宏瑞, 史江海, 尉询楷, 章立军. 8Cr4Mo4V轴承钢微观裂纹萌生与扩展机制的分子动力学模拟[J]. 中国机械工程, 2025, 36(10): 2179-2189.

Figures/Tables 21

Fig.1 Tensile model of 8Cr4Mo4V bearing steel material

Tab.1 Comparison of bcc-Fe elastic constant calculation and experimental values

弹性常数	势函数计算值/GPa	实验值^［19］/GPa	相对误差
C₁₁	244.3	230.0	6.2%
C₁₂	145.3	135.0	7.6%
C₄₄	116.3	117.0	$-$ 0.60%

Tab.1 Comparison of bcc-Fe elastic constant calculation and experimental values

弹性常数	势函数计算值/GPa	实验值^［19］/GPa	相对误差
C₁₁	244.3	230.0	6.2%
C₁₂	145.3	135.0	7.6%
C₄₄	116.3	117.0	$-$ 0.60%

Tab.2 Comparison between calculated elastic constants of cementite and DFT values

弹性常数	势函数计算值/GPa	DFT值^［20］/GPa	相对误差
C₁₁	383.4	388	$-$ 1.2%
C₂₂	349.5	345	1.3%
C₃₃	304.5	322	$-$ 5.4%
C₄₄	45.3	15	$-$
C₅₅	125.8	134	$-$ 6.1%
C₆₆	118.6	134	$-$ 11.5%
C₁₂	129.0	156	$-$ 17.3%
C₁₃	155.1	164	$-$ 5.4%
C₂₃	156.2	162	$-$ 3.6%

Tab.2 Comparison between calculated elastic constants of cementite and DFT values

弹性常数	势函数计算值/GPa	DFT值^［20］/GPa	相对误差
C₁₁	383.4	388	$-$ 1.2%
C₂₂	349.5	345	1.3%
C₃₃	304.5	322	$-$ 5.4%
C₄₄	45.3	15	$-$
C₅₅	125.8	134	$-$ 6.1%
C₆₆	118.6	134	$-$ 11.5%
C₁₂	129.0	156	$-$ 17.3%
C₁₃	155.1	164	$-$ 5.4%
C₂₃	156.2	162	$-$ 3.6%

Fig.2 Stress strain curve of bcc-Fe matrix model with crack at edge

Fig.3 Configuration evolution and stress cloud map of bcc-Fe matrix model with crack at edge

Fig.4 Stress strain curves of four models containing carbide inclusions

Fig.5 Configuration evolution diagrams and strain cloud maps of four models containing carbide inclusions

Fig.6 Atomic stress cloud maps of four models during the initiation of pores or microcracks

Fig.7 Schematic diagram of lattice transformation

Fig.8 Comparison of stress-strain curves for five models

Fig.9 Stress strain curves of carbide inclusion models of different sizes

Fig.10 Configuration evolution of inclusion models with different sizes when voids appear in the inclusion area

Fig.11 Stress strain curves of cementite models at different positions

Fig.12 Configuration evolution of different position models during crack propagation

Fig.13 Configuration evolution of three twin crystal models during crack propagation

Fig.14 Average atomic tensile stress distribution of three twin crystal models during crack propagation

Fig.15 Cohesion-displacement curve

Fig.16 Cohesion-displacement curves of three twin crystal models

Tab.3 Cohesive parameters of three twin crystal models

模型

损伤应力/

GPa

损伤位移/

临界断裂能/

（N·m^-1）

初始损伤刚度/

（N·μm^-3）

Fig.17 Cohesion-displacement curves of different twin grain boundary misorientation angle models

Tab.4 Cohesive parameters of three twin crystal models

晶界错向角 α/（°）	损伤应力 T_max/GPa	临界断裂能 G_IC/（N·m^-1）
10	10.3133	6.464
15	10.3974	9.632
20	9.258 55	8.299
25	10.4863	9.100
30	10.2065	9.384
35	11.0262	9.914
40	11.5953	10.851

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