Research article
Solid solution softening of Ti2AlC induced by alloying of boron

https://doi.org/10.1016/j.jallcom.2022.166712Get rights and content

Highlights

  • SSS occurs in Ti2AlC by alloying with boron.

  • SSS led to the great enhancement of ductility and plasticity in Ti2AlC.

  • SSS in Ti2AlC can be explained by the reduction of Peierls stress and GSFEs caused by alloying with boron.

Abstract

Solid solution softening is an unusual adjustment behavior that is attractive for material strength. In this paper, the effect of boron alloying on the strength of Ti2AlC is investigated by first-principles calculations. Our results show that the elastic modulus and ideal strength of Ti2AlC decrease overall due to alloying with boron, which indicates that solid solution softening occurs. Conversely, ductility and plasticity of Ti2AlC are greatly enhanced by alloying with boron. This phenomenon in Ti2AlC is mainly related to the reduction of Peierls stress and generalized stacking fault energies caused by alloying with boron. Our work provides an ideal candidate platform to study the solid solution softening effect in ceramics and provides theoretical guidance for engineering applications of MAX phase materials.

Introduction

Solid solution effect is one of the common mechanisms to adjust the mechanical performance of materials [1], [2]. In general, alloying of solute atoms often leads to the solid solution hardening (SSH), which is due to solute atoms impeding the movement of dislocations [3]. However, in some special cases, the solute atoms can cause a decrease in strength to occur solid solution softening (SSS) [4]. SSS allows an easier plastic deformation process in materials, thereby prevent the devices from cracking and mechanical failures [5]. While SSS has been an attractive behavior of intense investigations, the underlying mechanisms of SSS are still being explored, and it is not clear for those cases where hardening or softening occurs exactly.

For alloys, SSS can occur at low solute concentration and low temperatures, which has been observed in metals with body-centered cubic (BCC) structure [6], [7], [8], magnesium alloys with hexagonal close-packed structure [9], [10], high-entropy alloys (HEAs) with face-centered cubic (FCC) structure [11], and MoSi2 [12]. Two theories have been proposed to explain the underlying mechanisms governing SSS, including scavenging theory [1] and the double kink nucleation enhancement theory [7]. Scavenging theory mainly describes the capture of interstitial atoms and the suppression of dislocation and impurity segregation at grain boundaries. The double kink nucleation enhancement theory was connected with a lowering of the Peierls barrier due to the weakening of interatomic bonds near the solute. Although both theories provide a good explanation for SSS in some alloys, the applicability of above theories to systems other than alloys remains unclear. In fact, SSS has been observed not only in alloys, but also in ceramics [13]. To date, only few candidates are available for the study of SSS in ceramics, such as MgO-Al2O3 [13]. Furthermore, ceramics possess different dislocation slip modes with metals [14], which leads to the direct applicability of the above SSS mechanisms to ceramics being questionable. Therefore, it is desirable to explore more ceramics with SSS, which is significant to investigate the underlying mechanisms of SSS in ceramics as well as to improve ceramic plasticity and engineering applications.

In this paper, we find that the SSS occurs when boron alloying is performed in Ti2AlC by using first-principles calculations. Compared to Ti2AlC, Ti2AlB0.5C0.5 and Ti2AlB have an overall decrease in elastic modulus, ideal tensile and shear strength. The SSS in Ti2AlC is mainly related to the reduction of Peierls stress and generalized stacking fault energies (GSFEs) caused by alloying with boron. We expect our study to provide a potential platform to help reveal the underlying mechanisms of SSS in ceramics.

Section snippets

Computational methodology

The Vienna Ab-initio simulation package (VASP) [15], [16], [17] is used for our density functional theory (DFT) [18], [19] calculations. The projector augmented wave (PAW) [20] method is considered for evaluating the electron-ion interactions. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) [21] functional is chosen to describe the exchange-correlation function. All calculations are set with a single cut-off energy parameter of 500 eV for the plane wave

Structural properties and stability

Generally, mechanical properties of materials can be determined by their crystal structures. Herein the progressive alloying of C in Ti2AlC by B leads to the formation of two different types of borides [23], Ti2AlB0.5C0.5 and Ti2AlB, respectively, as shown in Fig. 1. It can be seen that there are nano-laminated atomic arrangements in all of the structures, which are characterized by edge-sharing Ti6C or Ti6B octahedra forming alternating two-dimensional layers perpendicular to the z axis

Conclusions

The SSS behavior induced by boron alloying in Ti2AlC was systematically investigated by first-principles calculations. According to the formation energy, phonon spectrum and elastic constant, all of these compounds have good stability. With increasing of B alloying, the values of elastic modulus, ideal tensile and shear strength of the three compounds are decreased, which indicates an SSS in Ti2AlC. In contrast, with increasing of B alloying, the value of B/G increases, indicating that the

CRediT authorship contribution statement

Xing Feng performed the calculations and wrote the paper. Linggang Zhu, Jian Zhou and Zhimei Sun helped with the analysis and provided constructive discussions.

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 is financially supported by the National Natural Science Foundation of China (Grant No. 51871009, and No. 51872017).

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