Study on staged work hardening mechanism of nickel-based single crystal alloy during atomic and close-to-atomic scale cutting

Highlights

• New Molecular dynamics simulation models have been established.

• The staged work hardening mechanism of ACS cutting process is analyzed.

• The reason of work hardening of nickel-based single crystal alloy during ACS cutting and nano compression was studied.

Abstract

Nickel based single crystal alloys have excellent properties such as heat resistance, corrosion resistance and creep resistance, which are widely used in aerospace and other national defense fields. Severe work hardening occurs in the process of cutting nickel based single crystal alloy. How to improve the machining quality and grasp the cutting deformation mechanism has become the research focus. In this paper, the effect of work hardening on the surface of workpiece during the atomic and close-to-atomic scale (ACS) cutting process is studied. The model of $S{i}_3{N}_4$ ceramic tool cutting the nickel based single crystal alloy was established, and the ACS cutting process was simulated by the molecular dynamics method. The existing strain rate conversion model was modified to make it suitable for the process of ACS cutting into nano compression with the same strain rate. The results show that the dislocation density of Ni-based single crystal alloy workpiece changes greatly with the change of cutting distance. According to the change of microstructure in the workpiece, a new staged work hardening mechanism is proposed. The development of work hardening in the cutting process is divided into three stages, and the transition node of each hardening stage is defined. An important sign of the transformation from the first stage to the second stage of work hardening is the occurrence of a large number of dislocation pile-up group, dislocation tangles and the appearance of non-basal slip lines. The distinctive feature of the transformation from the second stage to the third stage of work hardening is that a large number of screw dislocations are cross-slip and the dislocation pile-up group is destroyed. At the same time, the different hardening mechanisms in each stage and the reasons for the change of work hardening mechanism in different stages are summarized. The research content is believed to be helpful to understand the mechanism of significant work hardening effect in nickel-based alloys.

Introduction

Ni-based single crystal alloy is a kind of alloy with high strength and certain anti-oxidation and corrosion resistance at 650–1000 °C. Because it can still maintain high corrosion resistance, thermal fatigue and thermal shock resistance at high temperature, and effectively inhibit creep and oxidation [1,2], it is commonly used to manufacture high-precision components of aeroengine blades and rocket engines, nuclear reactors and energy conversion equipment [3]. Nickel-based single crystal alloy is typically hard to machine material. In the traditional cutting process, the solid solution structure is easy to produce a rapid work hardening effect. Due to the low thermal conductivity and a large number of hard spots in the nickel-based alloy workpiece, serious work hardening phenomenon occurs during processing, and surface integrity problems appear (such as subsurface defects, surface roughness increase, microhardness increase) [[4], [5], [6], [7]]. These defects, work hardening effect and microstructure change have an important influence on the fatigue life of these mission-critical structure components. The work hardening and microstructure changes of nickel based superalloy surface layer fundamentally determine the final performance of aeroengine structural components [8]. In the actual cutting process, under the action of large strain load, the work hardening trend of nickel based alloy workpiece is more serious, resulting in the subsurface hardening layer, which makes the subsequent processing difficult. It is found that the work hardening behavior of Ni-based alloy is the most reliable factor to evaluate the surface integrity in machining [9].

In order to explore the mechanism of work hardening effect in the cutting process, the surface microstructure changes of the workpiece after cutting are studied. Ren [8,9] and others carried out turning experiments with nickel-based alloy workpieces, and studied the relationship between work hardening and cutting parameters (cutting speed and feed rate). It is found that the degree of work hardening increases with the increase of cutting speed. The gradient microstructure and work hardening can be optimized by controlling the cutting parameters. As shown in Fig. 1, grain refinement, dislocation density and increase in the number of micro twins in the workpiece after turning are the main causes of subsurface work hardening. A large number of cutting experiments show that the main feature of the plastic deformation of the workpiece is the work hardening behavior [2]. The plastic deformation makes the machined surface layer harden, resulting in a work hardening effect [10]. Huang et al. [11,12] found that interstitial carbon atoms affect the dislocation density evolution of TWIP steel, which is the key reason for the existence of a high work hardening rate. They introduce high density dislocation and lamellar structure into the steel. After many times of plastic deformation, the material has a high dislocation density. A large number of dislocations pile up at the grain boundary and interact with each other during deformation, which improves the yield strength of the material.

Nickel based alloy belongs to austenitic alloy. Many studies have been carried out on the evolution of dislocation structure during the machining of austenitic alloy. Michel et al. [13] introduced the evolution process of dislocation tangle arrangement into the cell structure, and pointed out that different structures may be related to the work hardening stage, but these stages are not quantitatively characterized. Recently, in order to study the staged hardening characteristics of austenitic alloys, Christopher and Choudhary [[14], [15], [16], [17]] used the relationship of $d\sigma /d\epsilon$ versus at different temperatures to draw experimental curves. According to the shape of the curve, there are three stages. According to the characteristics of the third stage, Christopher and Choudhary [14] defined it as the inverted " parabolic " hardening under higher stresses. They did not describe in detail the characteristics of the second and third stages of work hardening. When plastic deformation occurs in austenite alloy, it is found that the evolution of the dislocation structure is closely related to the dynamic strain aging(DSA)and the staged characteristics of work hardening. In the first stage of work hardening, when the plastic deformation of the workpiece reaches 22%, the incipient distribution cells begin to form. In the transition between the first stage and the second stage, the dislocation density of dynamic strain aging reaches the maximum [18]. The hardening rate of the second stage of work hardening decreases. The proposed mechanisms of this stage are based on the dynamic recovery of dislocation annihilations and a large number of stacking faults, resulting in polygonization, which promotes the formation of subgrain [19]. In addition, the interaction between solute atoms and dislocations lead to DSA [14], which may also affect the evolution of the dislocation structure.

With the continuous development of computer technology, the accuracy and speed of efficient large-scale molecular/atomic parallel computing have been significantly improved. The macro cutting experiment needs a lot of processing steps to get the ideal sample, and then it needs tedious imaging by the observation instrument to get the experimental results. Simulations can show the evolution of dislocation movement and other defects in the cutting process at any time, which can greatly reduce the cost and obtain excellent cutting parameter range efficiently. Software simulation has become an effective research method to study the micro cutting process after the experimental method. It is found that the development of human processing technology has gone through three stages. At present, with the continuous improvement of the accuracy of processing equipment, ultra precision machining has raised the machining accuracy to sub nano level, and the cutting scale has gradually entered the atomic level and close to the atomic level [20].

Nanoscale machining has reached atomic level removal [21,22], and the research on atomic and close-to-atomic scale(ACS) cutting has become a very urgent matter [20]. The molecular dynamics (MD) has been widely used in the analysis of atomic and near-atomic material removal processes, and in the study of microstructure changes in nano workpieces [[23], [24], [25], [26], [27]]. Since the 1990s, Lawrence Livermore National Lab of the United States first used the MD simulation method to study the nano-cutting process, and MD simulation research has made great progress [28]. At present, the MD method has become an irreplaceable method in the field of precision cutting to study the nano cutting mechanism [[29], [30], [31], [32], [33]]. With the increase of the number of atoms in the simulation system, the MD simulation has been able to obtain similar results with the experiment. With the development of potential function calculation method, EAM potential function for simulating metal system [34] was developed. In order to modify EAM potential function, MEAM potential function [35] was developed, and Tersoff potential function was used for a non-metallic system with a covalent bond [36]. With the development of machine learning, the accuracy of the potential function is still improving. Fang et al. [37] established the cutting model of single crystal copper with diamond tools with different shapes by the MD method. The simulation results show that the dislocation density increases greatly and the work hardening phenomenon occurs due to dislocation pile-up. Huang et al. [38] studied the relationship between temperature and work hardening degree in the cutting process of Ni–Fe–Cr–Al–Co high entropy alloy by MD method, and found that with the increase of temperature, the work hardening degree of Ni–Fe–Cr–Al–Co workpiece decreased. Liu et al. [39] studied the evolution of stacking fault tetrahedra (SFT) in single crystal copper by the MD method and its effect on surface hardness. The results of nanoindentation simulation show that SFT blocks the slip of other dislocations and leads to work hardening. Wang et al. [40] reported that the MD method was used to study the nanoscale cutting of single crystal copper workpiece with diamond tools. It was found that the Stair-rod dislocation and Lomer-Cottrell dislocation configuration hindered the dislocation slip during the cutting process. Fan et al. [41] used the MD method to simulate the work hardening phenomenon in the process of nano cutting Ni–Fe–Cr series Ni-base alloy with CBN tool. It is reported that fixed dislocations, dislocation tangles and solute atoms hinder the movement of other dislocations during the cutting process, resulting in work hardening on the machined surface. The cross slip of screw dislocations and the change of hardening rate during cutting are not studied.

At present, the experimental method is mainly used to study the staged characteristics of work hardening. There are few studies on the staged characteristics of the internal work hardening of Ni-based single crystal superalloy by MD simulation of the compression process. There is little research on the evolution mechanism of staged work hardening in the ACS cutting process of workpiece surface area. As nickel-based single crystal superalloy has been widely used in the aerospace/nuclear industry in recent years, how to improve the surface quality of cutting has become the research focus of universities and enterprises. In this paper, the MD method is used to study the ACS cutting process of Ni-based single crystal alloy workpiece with ${\text{Si}}_3{\text{N}}_4$ceramics tool. Firstly, an excellent ACS cutting model is established, and the Sialon ceramic tool (silicon nitride) is selected to cut nickel-based single crystal alloy. Then we select and calculate the suitable potential function for the MD simulation. In the third section, we carry out the nano compression simulation test, and get the true stress-strain curve through calculation. According to the curve, the work hardening process of a single-crystal nickel-based alloy can be divided into three stages, and the characteristics of each stage are pointed out. In the fourth section, compared with nano compression simulation, the characteristics of working staged hardening of the ACS cutting process are analyzed. Firstly, the formation process of work hardening can be divided into three stages according to the change of dislocation density curve and microstructures. Then the characteristics of each stage are described, and the evolution of defects and dislocations in each stage is analyzed. The internal mechanism of the change of work hardening rate in each stage is discussed. In the fifth section, we summarize the research content.