Pathway to understand liquid metal embrittlement (LME) in Fe-Zn couple: From fundamentals toward application

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Abstract

Liquid metal embrittlement (LME) has been reported in many structural materials including steel, aluminum, brass, and nickel during hot-working processes e.g. welding, hot-deformation, brazing, heat-treatment, or in-service time. In many of the applications, such as automotive, aerospace, nuclear industries, LME is considered as a serious safety concern. Over the last decades, research activities have grown considerably striving to understand the abnormal LME phenomenon. However, a fundamental understanding of LME has not yet been unraveled due to the diverse, contradicting proposed mechanisms in different processing routes and materials. In the present overview, first various proposed mechanisms are integrated into a systematic manner. Revisiting the proposed mechanisms based on the most-recent experimental discoveries reveals that the stress-assisted grain boundary diffusion mechanism is a viable LME mechanism. The connection between sub-atomic (electronic) structure modification and the embrittlement at grain boundaries provides insights into the grain boundary decohesion during LME. In a bottom-up approach, then this overview selects Zn-induced embrittlement during assembly welding as an example case of LME. Explanations are provided to show why LME cracks are both seen to degrade and be innocuous to material’s performance. Finally, this review outlines recommended directions for future research steps to overcome LME-cracking risk, both from material and process aspects.

Introduction

Liquid metal embrittlement (LME) is characterized as an abnormal phenomenon in which intimate contact of certain solid-liquid metals results in the penetration of liquid into the contacted solid metal and rupture (Fig. 1). LME affects various industrially important materials, including steels, brass, aluminum, nickel, in a wide range of applications and scenarios [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. These scenarios can be classified as: (a) processing failures: hot-dip galvanizing and hot-stamping [11], [12], [14]; (b) fabrication failures: soldering, brazing, and welding [15], [16], [17], [18]; (c) operation failures: nuclear industries [19], [20], [21]; and (d) secondary failures, in which LME occurs after an initial bolt, bearing, or pressure vessel failure occurs [22], [23], [24]. Due to this broad range of failure risks, LME is recognized as a serious safety issue in many applications across aerospace, nuclear, and automotive industries.

It is known that LME occurs due to concurrent action of 3 factors (Fig. 2): (1) the presence of an aggressive liquid metal (hereafter called embrittler) such as Bi, Ga, Zn; where all the embrittlers have relatively low melting points (ranging from room temperature to 419 °C), (2) a susceptible material into which the embrittler may penetrate (LME has been more frequently reported in FCC metals), and (3) an external tensile loading or internal (residual) tensile stresses which is normally below the material’s yield stress [5], [25], [26], [27], [28], [29].

LME has been reported in various solid-liquid metals couples (so-called LME couples), including Al-Ga, Ni-Bi, Cu-Bi, Cu-Hg, Fe-Pb, Fe-Zn, etc. [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72], [73], [74], [75], [76], [77], [78], [79], [80], [81], [82], [83], [84], [85], [86], [87], [88], [89], [90], [91], [92], [93], [94], [95] (Fig. 3). Among the LME couples, Al-Ga system is the most deeply studied system and known as the LME model system due to high LME susceptibility and structural simplicity for atomistic modeling [1], [2], [5], [6], [90], [91], [92]. The Ni-Bi and Cu-Bi couples are also frequently reported in the literature where ab-intio modeling and high-resolution transmission electron microscopy (TEM) investigation provided understanding of the grain boundaries role in LME [3], [4]. However, since 2000, there has been a large research effort to understanding LME in the Fe-Zn couple due to the introduction of advanced high strength steels (AHSS). These high strength-high alloy steels were seen to be more susceptible to LME than lower strength steels. These observations resulted many studies to understand the mechanisms controlling LME in the Fe-Zn couple [11], [12], [13], [29], [30], [89], [93], [94], [95]. Despite this large body of research, LME is still associated with unexplained results, controversies, and a lack of fundamental understanding. The diverse and contradicting proposed LME mechanisms in different processing routes and LME couples is puzzling the research community. Thus, the present manuscript reviews the advances in understanding of LME: Section 2 focuses on the systematical summary of the proposed LME mechanisms. The details of the stress-assisted grain boundary diffusion model, as a viable LME mechanism, is reviewed in this section. Section 3 describes the micro-events leading to LME cracking from the crack initiation and progression viewpoints. The role of grain boundary network in LME and details of grain boundary decohesion are discussed in Section 4. This will be preceded by a description of LME in a macro process scale: how process variables influence LME (Section 5), LME occurrence during welding and what potential mitigation methods exist to minimize LME (Section 6), and how LME affects material performance (Section 7). Lastly, Section 8 summarizes the main conclusions of the present critical review and possible future directions and perspectives in the field.

Section snippets

Proposed mechanisms of LME

Thus far, several mechanisms have been proposed to describe micro-events leading to LME-cracking. Despite these efforts, there is still no universal agreement about the LME mechanism. This is due to the general complexity of the phenomenon, lack of experimental/modeling evidence, and differences of the responsible mechanisms for the various couples. Below is a summary of the major LME mechanisms, their breakthrough contribution in better understanding of LME, and the associated drawbacks.

LME micro-events: Crack initiation and progression

Included in an LME mechanism must be a description for the continuous initiation and propagation of the crack. Within the grain boundary-based model this is described in a multistep process of LME-cracking where a set of “micro-events” leads to cracking. Although deconvolution of these micro-events encounters several experimental difficulties due the rapid propagation rate of LME-cracks, the Gordon-An scenario [28] describes separate stages for crack initiation and propagation, both of which

Role of grain boundaries

Although there is a wide range of interpretation of the metallurgical and mechanical micro-events leading to LME, the role of grain boundary characteristics such as misorientation angle, grain boundary energy, and grain boundary microchemistry in LME is not understood yet. It is accepted now that the grain boundaries are not just amorphous interfaces, but are structures with energetic aspects that are relatively ordered [125], [126].

Effects of temperature and strain rate on LME-cracking

Strain rate, temperature, and time are known as the major process parameters affecting LME occurrence. Beal et al. [139] extensively studied the influence of the process parameters on Zn-induced LME in AHSS using thermomechanical Gleeble testing. A ‘ductility trough’ temperature range of 700–900 °C was identified and it was concluded that LME occurs in this range. However, it was also noted that the minimum LME temperature lowers with strain rate. Additionally, Kang et al. [94] examined TWIP,

LME-cracking during welding and reduction methods

With an increase in the use of Zn coated AHSS in automotive construction, LME during resistance spot welding (RSW) has become a more pressing concern. During the welding process, liquid Zn is in contact with solid steel both at joint faying surface and at the electrode/sheet interface. Contact occurs on the steel surfaces when the temperature is above the melting point of the Zn (419 °C), but below the melting point of the steel substrate (~1475 °C). During welding, liquid Zn penetrates into

Impact of LME-Cracking on mechanical performance

The initial consensus in the literature was that LME cracks had no effect on mechanical weld performance [145], [158], [159], which was widely accepted by industry and academic researchers. However, when examining their methodology, it can be seen these studies did not properly isolated the impact of LME cracks. Work by Kim et al. [145] compared a high current (cracked) case to a low current (crack-free) case but did not control for the effect of nugget size on weld strength (Fig. 29). As the

Conclusions

Liquid metal embrittlement (LME) occurs in various liquid-solid metal couples, leading to a rapid, uncontrollable crack growth, and catastrophic failure. There has been much research into understanding the mechanisms controlling this phenomenon, of which many of the proposed models contract each other. An extensive review of the literature compared the various models controlling LME to the available experimental data from both the perspectives of crack formation at the grain boundary scale and

Disclaimer

The Auto/Steel Partnership, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. The views and opinions of authors expressed herein do not necessarily state or reflect those of the Auto/Steel Partnership. Such support does not constitute an endorsement by

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

The authors would like to acknowledge the Natural Sciences and Engineering Research Council (NSERC) of Canada, Canada Research Charis (CRC) Program, and the Auto/Steel Partnership for providing the support to carry out this work.

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