Abstract Scope |
Liquid-metal-embrittlement (LME), also known as liquid-metal-induced-embrittlement (LMIE) or liquid-metal-assist-cracking (LMAC) reveals a remarkable reduction of ductility if a polycrystalline metal is exposed to tensile stress while in direct contact with liquid metal. The first report of LME traces back to 1874 when a galvanized iron wire lost its ductility remarkably when exposed to an elevated temperature. Since then, LME has been reported by numerous studies in some specific systems such as aluminum-gallium (Al/Ga), nickel-bismuth (Ni/Bi), copper-bismuth (Cu/Bi), iron-lead (Fe/Pb) and iron-zinc (Fe/Zn). Among them, Fe/Zncouple, as the main focus of the present study, have been considerably investigated during the last decade due to the development of zinc-coated advanced high strength steels (AHSS) in the automotive industry. LME occurs by concurrent action of three elements: (i) presence of an aggressive environment such as liquid metals, (ii) a susceptible polycrystalline metal, and (iii) tensile loading. Thus far, several models have been proposed to elucidate atomic events leading to LME in different solid/liquid systems. Among them, grain boundary diffusion model has been shown to have the best agreement with experimental observations in Fe/Zn system. Based on this model, it is assumed that crack nucleation happens through the diffusion of embrittler atoms into a substrate grain boundary. It has been reported that diffusion of Zn atoms first occurs in austenite grain boundary due to higher Zn diffusivity compared to the ferrite grain boundary. Therefore, austenitic structures have the highest susceptibility to LME cracking. Some investigations also mentioned that the presence of austenite is an essential prerequisite for LME formation. It should be noted that there is a specific temperature range known as ‘ductility trough’ for the occurrence of LME-cracking. This temperature range varies between 700˚C-940˚C in different families of AHSS. Therefore, it is believed that austenite is formed by exposure of steel substrate at this temperature range and finally controls LME-crack formation behavior. In this regard, no study has been conducted yet to investigate LME-cracking in a fully ferritic structure. The present work is the first research to study the occurrence of LME-crack susceptibility of a fully ferritic microstructure.
The as-received material was 0.03C-1.00Mn-1.00Si-0.20 Ti-(17-19)Cr (wt.%) electro-galvanized 439-type ferritic stainless steel with a nominal thickness of 1.0±0.1 mm. LME was induced in this study by subjecting ferritic stainless steel to a thermomechanical cycle from resistance spot welding using electrode force of 4 kN, welding time of 100-250 ms, and holding time of 167 ms. Cu-Cr dome radius type electrodes with a 6 mm tip diameter with a constant cooling water rate of 6 L min-1 was used. All welds were made at the expulsion current (9 kA), which is defined as the minimum current where three consecutive welds show expulsion. LME severity was quantified from crack measurements of the welded samples using mean crack number, mean crack length, and crack index as described in previous work. LME crack characterization was conducted by scanning electron microscope (SEM, JSM7001F), energy-dispersive spectroscopy (EDS), electron probe micro-analyzer (EPMA), and electron back-scattered diffraction (EBSD) methods. Dilatatometry was used at relevant heating and cooling rates to determine the non-equilibrium austenite formation start and finish temperatures during heating and cooling.
The present study showed that a fully ferritic structure is prone to the LME phenomenon and has a high susceptibility to LME-cracking which makes it a novel observation adding to a pool of knowledge regarding LME occurrence. The presence of fully ferritic microstructure in the vicinity of the LME-crack showed that cracking happened in a ferritic structure. Moreover, dilatometry analysis confirmed that the austenite to ferrite transformation starts at approximately 1000 ˚C, which is higher than the temperature range for occurrence of LME. Consequently, LME-cracking has occurred in a fully ferritic structure. The occurrence of grain dropout as well as a Zn-containing crack in grain boundary without any branches with other cracks showed that grain boundary sensitization has assisted LME-cracking. |