A molecular dynamics study of dislocation-interphase boundary interactions in FCC/BCC phase transformation system
Graphical abstract
Introduction
Many industrial alloys contain two solid solution of metals, with one phase precipitated from the other, such as α + β Ti alloys [1] and duplex stainless steels [2]. Developing a steel consisting of ferrite and considerable amount of austenite is also a current trend for steels of optimum properties [3], [4]. Since both phases are deformable, the barrier to the dislocation propagation exerted by the interfaces between the second phase and the matrix play a particularly important role in strengthening the alloys. Because the two phases are related by phase transformation, the interphase boundaries (IPBs) are often semicoherent. For a quantitative understanding of the strengthening mechanism, it is essential to clarify how and to what extent various types of IPBs affect dislocation motion. Interactions between dislocations and IPB have been investigated by previous atomistic simulations [5], [6], [7]. The results show that one source of the impeding effect stems from dislocation core spreading in the IPBs sheared by the stress field of the dislocation. As a result, “weak” IPBs with low shear strength have stronger impeding force on dislocations than that IPBs with high shear strength [5]. Besides, geometric relationship between IPB and slip systems in the two phases can also influence the dislocation/IPB interaction process, which has been verified experimentally by the observed anisotropic impeding effect [8], [9]. Additionally, slip transmission can occur only when the size of dislocation pile-up is sufficiently large [10], indicating resolved shear stress is another factor that can affect the dislocation/IPB interactions. Therefore, dislocation/IPB interaction is a complicated issue involving several influencing factors at the same time.
Compared to the vast number of studies on dislocation/grain boundary (GB) interactions [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], the study on dislocation/IPB interaction much less advanced. Though slip transmission through IPB can be directly observed experimentally [10], the extraction of the dislocation/IPB interaction details is a challenging work in experiment. Alternatively, atomistic simulation is a powerful tool to reveal the fine details and atomistic mechanism of this interaction process, which has been applied to study the dislocation/IPB interaction in Cu-Ni, Cu-Ag, Cu-Nb and Cu-Cr alloy systems [5], [7], [22], [23], [24], [25], [26], [27], [28], [29], [30]. In these works, two types of IPBs were considered, i.e., coherent IPBs in Cu-Ni and Cu-Ag systems [24], [25], [26], [27], [28], [29] and semicoherent IPBs in Cu-Nb and Cu-Cr systems [5], [7], [22], [23], [30]. For coherent IPBs, the coherency strain poses the dominant resistance to slip transfer across the IPB [22]; while for semicoherent IPBs, dislocation/IPB interaction is affected by shear strength of the IPB [30], local interaction between dislocation and IPB [5], [31] and geometry of slip systems [7]. These pioneering simulations gave clear atomistic pictures of dislocation/IPB interaction. However, faceted interfaces in many precipitation systems usually have irrational orientation [1], [32], [33], [34], [35], i.e., the IPB does not parallel to low-index crystal plane of either phase, which has not been considered in previous atomistic simulations on dislocation/IPB interaction. Besides, IPBs in Cu-Nb alloy show a large impeding force on matrix dislocation due to low shear strength of the interface, and thus no direct slip transmission observed in the simulation [5], [6], [7]. Therefore, the detailed interaction process regarding more interaction results is still in demand.
In the present work, molecular dynamics (MD) simulation was employed to systematically reveal various long- and short-range dislocation/IPB interactions, including image force, slip transmission, activation of partial dislocation slip system and completely impinged dislocation at the IPB. The rest of this paper is organized as follows: Section 2 provides the MD simulation details, including the setup of simulation cells, simulation conditions, the calculation of image force and an introduction to slip system continuity. Section 3 shows the four types of dislocation/IPB interaction results and summarizes a gross relationship between the interaction results and several influencing factors. Then, detailed discussions are presented in Section 4 about the atomistic mechanism corresponding to different dislocation/IPB interaction results and how and to what extent each factor affects the interaction result. Finally, the mainly conclusion are summarized in Section 5.
Section snippets
Simulation details
To study the interaction between dislocation and IPB via MD simulations, an FCC/BCC bi-crystal model was constructed as the initial configuration, with a dislocation inserted within the BCC matrix phase. The simulation cell is schematically shown in Fig. 1. To mimic the dislocation/IPB interaction in real situation, it is desirable to select the geometry of IPB based on the experimental observations. The habit planes of precipitates in an fcc/bcc system, or the major IPBs, are often
Long-range dislocation/IPB interaction
The long-range dislocation/IPB interaction mainly stems from the image force of the IPB on matrix dislocation, which is one of the driving forces for matrix dislocation motion. The magnitudes of for all dislocation/IPB pairs are listed in Table 3, together with the motion state of the matrix dislocation without external loading, i.e., the motion of matrix dislocation is driven only by . Note that all ’s have positive values, indicating the image force on matrix dislocations always
Discussion
Dislocation/IPB interaction is a complicated issue due to multiple influencing factors, such as image force, resolved shear stress, slip system continuity and local interactions between dislocation and IPB. In this section, all these influencing factors will be discussed individually to reveal how and to what extent these factors can affect the interaction results.
Conclusions
In the present work, molecular dynamics simulation was employed to investigate the atomistic process and mechanism of the long- and short-range interactions between dislocations and IPB. Matrix dislocations corresponding to all 12 slip systems in BCC phase and two different types of IPB (OIF and CPP) were considered in the simulation. Based on the simulation results, some universal rules are attained regarding how possible factors, including image force, resolved shear stress, continuity of
CRediT authorship contribution statement
Zhipeng Sun: Data curation, Formal analysis. Fuzhi Dai: Formal analysis. Wenzheng Zhang: Supervision, Resources.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Nos. 51871131 and 51671111) and the National Key Research and Development Program of China (No. 2016YFB0701304). We are grateful to Professor Jian Wang for detailed discussions on simulation techniques.
Data availability
All the simulation data and computer codes during the current study are available from the corresponding author on reasonable request.
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