Elsevier

Materialia

Volume 11, June 2020, 100668
Materialia

Suppression of liquid-metal-embrittlement by twin-induced grain boundary engineering approach

https://doi.org/10.1016/j.mtla.2020.100668Get rights and content

Abstract

Grain boundary engineering (GBE) has lately been recognized as a viable approach to manipulate grain boundary characteristics to improve resistance against intergranular degradation. One of the most challenging intergranular degradation phenomena is liquid-metal-embrittlement (LME), where a reactive liquid metal penetrates along random grain boundary network. In contrast to random grain boundaries, special coincidence site lattice (CSL) boundaries has been shown to be resistant against the embrittlement. The present study investigates the feasibility of using a GBE technique to arrest LME in a Fe (FCC)-Zn couple. Two sets of low-strain-heat-treatment processing routes were used to optimize grain boundary characteristics based on: (a) Σ3 and Σ3n boundaries frequency, and (b) the material's texture and grain size evolution. The optimum characteristics resulted in significantly improved resistance against LME. The mechanism of the embrittlement arrest is discussed based on the random-grain boundary network continuity and the grain boundary triple junctions distributione.

Introduction

In polycrystalline metals, grain boundaries (GBs) play a dual role in the material's overall performance: (a) they interact with dislocations, and therefore, strengthen the material, and (b) act as inherent failure locations in embrittlement [1], [2], [3], [4], creep [5,6], and corrosion [7,8]. Recently, the grain boundary engineering (GBE) concept has been introduced as a viable method to mitigate the undesirable aspects of GBs [9], [10], [11], [12], [13]. The various GBE techniques attempt to eliminate the “weak” high-energy GBs and instead induce the “stable” low-energy GBs [14,15]. Generally, the stable GBs refer to low-Σ (Σ≤29) coincidence site lattice (CSL) boundaries which contrast to the high-misorientation angle random-GBs or high-Σ (Σ>29) CSL counterparts (Σ is the density of common sites between grains sharing a boundary) [14,16,17]. Despite a wide range of proposed thermo-mechanical treatments [18], [19], [20], [21], [22], the twin-induced GBE which induces a high frequency of low-Σ CSL boundaries is the most common GBE approach. This method can be further sub-divided into strain-recrystallization and strain-annealing based approaches [10,12,[23], [24], [25].

A material's ability to generate numerous CSL boundaries is the main precondition of twin-induced GBE, which has been reported in low stacking fault energy (SFE) FCC metals such as nickel-alloys [24,26,27], copper-alloys [25,28], and austenitic steels [10], [11], [12], [13],18,21]. In the thermo-mechanical treatment, the migration of random boundaries and emission of Σ boundaries from the existing GBsregenerates low-Σ boundaries. The Σ-boundary regeneration process occurs according to (Σ3n+ Σ3n+1 → Σ3) [14,18,23], where a junction of two Σ3 boundaries generates a Σ9 boundary. Then, a Σ9 runs into a Σ3 boundary and regenerate a new Σ3 in the structure. This example of the regeneration process explains how the significant alteration of grain boundary character distribution (GBCD) and fragmentation of random-GB network take place [14,18,23].

Recent studies have demonstrated examples of successful application of GBE in suppression of GB-based deterioration phenomena. The study by Shimada et al. [13] has shown that a slight pre-strain and annealing resulted in improved intergranular corrosion resistance of the austenitic stainless steel. Kokawa et al. [10] have confirmed that a high frequency of uniform CSL boundaries produced by the twin-induced GBE process significantly suppressed intergranular corrosion of type 304 austenitic steel. The GBE treatment generated 82% of CSL boundaries which efficiently disrupted the connection of the random grain boundary network (GBN), and hence, arrested the decay of GBs [10]. Bi et al. [15] showed chromium depletion was controlled through the promotion of low-energy special boundaries over the random (high-energy) GBs. Increasing twin-based special boundaries also reduced the intergranular heat-affected-zone (HAZ) liquation-cracking in a nickel-based superalloy [29]. This literature survey shows that GBE improves material performance: (a) directly through the generation of a significant frequency of low-energy GBs, and (b) indirectly by the special-random GB interactions which creates structural barriers against the degradation of the continuous randomGBN.

In liquid metal embrittlement (LME), penetration of a reactive liquid metal along GBs causes an abrupt rupture. It is reported that in some LME-sensitive solid-liquid couples e.g. Al–Ga [30,31], and austenitic steel-Zn [32], [33], [34], [35], LME occurs as a results of the spontaneous replacement of GBs by an aggressive liquid-metal even in the absence of an applied stress [30,31]. This stress-free LME mechanism is referred as grain boundary wetting (GBW) and occurs at T>Tw, where Tw is the GBW transition temperature [31]. Austenitic stainless steels are known to be LME sensitive when exposed to liquid sodium [36], lithium [37], mercury [38], cadmium [39], and zinc [32], [33], [34], [35]. Pancikiewicz et al. [34] reported Zn-induced LME susceptibility of 304 stainless steel during welding, leading to cracks up to ~3 mm deep in the HAZ. Bruscato [33] showed severe LME-cracking during dissimilar welding of austenitic stainless steel to galvanized carbon steel. The LME-cracking is reported at temperatures above 750°C, where the cracks propagate along the GBs [33]. This study recommended that Zn-coating must be completely removed prior to any high-temperature processing [33]. The direct correlation between the degree of Zn contamination and LME-crack length in the HAZ of stainless steel (304 and 316L) pipe weldments is confirmed by Shinohara and Matsumoto [32]. The present work investigates the feasibility of LME-cracking suppression by inducing a considerable fraction of special boundaries into the microstructure, and modification of the GBN. Moreover, the effects of different GB triple junction types on the LME behavior of the austenitic structure are studied.

Section snippets

Material and experimental procedure

In the present study, a commercial 304-type austenitic stainless steel with a nominal thickness of 0.9 mm was used. The as-received samples were machined to a modified dogbane shape [40], solution heat-treated at 1050 °C for 0.5 h under Ar-atmosphere and quenched. It has been shown that the combination of low pre-strain of 5% and subsequent heat-treatment results in the maximum generation of CSL boundaries [13]. Hence, the thermo-mechanical treatment to manipulate the GBN was started with

Special GB Frequency

The initial microstructure of the austenitic (FCC) steel contained ~77% high-angle (θ>15°) random GBs and 23% of special GBs as shown in Fig. 1. Most of the special GBs were Σ3 (21%) whereas only a minor fraction of higher order Σ-boundaries such as Σ5, Σ7, and Σ9 (total ~2%) was observed. The previous studies [10,13] showed that a thermo-mechanical GBE treatment of 5% tensile deformation followed by annealing at 900 °C and 1000 °C can effectively modify the initial GBN. It has also been shown

Conclusions

The present study investigated the feasibility of the twin-based grain-boundary-engineering approach to suppress liquid-metal-embrittlement in an austenitic steel. The grain boundary character distribution can be modified through the single-step thermo-mechanical treatment consisted of a slight cold deformation (5%) and subsequent annealing process at 900 and 1000 °C. The applied grain-boundary-engineering treatment promoted stable low-Σ special boundary frequency from ~20% in the initial state

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.

Acknowledgments

Authors would like to acknowledge the International Zinc Association in Durham, NC, USA, the Natural Sciences and Engineering Research Council (NSERC) of Canada, Canada Research Charis (CRC) Program, and ArcelorMittal Dofasco G.P. in Hamilton, Canada for providing the support to carry out this work.

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