Role of liquid Zn and α-Fe(Zn) on liquid metal embrittlement of medium Mn steel: An ex-situ microstructural analysis of galvannealed coating during high temperature tensile test

Abstract Zn assisted liquid metal embrittlement (LME) has characteristics that differ from the general characteristics of other LME systems: the formation of Fe-Zn intermetallic compounds and the high solubility of Zn in Fe. Consequently, there is a need to investigate the influence of the Fe-Zn interaction and resulting phase composition on LME behavior. However, little research has been conducted to reveal such relationships. This paper explores the influence of high temperature phases in galvannealed coating, specifically liquid Zn and α-Fe(Zn) on the LME sensitivity of medium Mn steel using a high temperature tensile test with eight combinations of temperatures (700 °C to 900 °C) and hold times (1 s, 50 s, and 100 s). The test results are categorized as LME, microcrack and LME free cases in terms of LME behavior. The study reveals that the LME sensitivity is governed by the fraction of effective liquid Zn, which is a consequence of isothermal solidification owing to the thermodynamic stability of liquid Zn, diffusivity of Zn in Fe and the partitioning coefficient of liquid Zn. Moreover, LME is influenced by the breaking of α-Fe(Zn) layer, which directs the liquid Zn into the substrate steel. Finally, a water-tank analogy is proposed to demonstrate the effect of the fraction of liquid Zn and the breaking of α-Fe(Zn) layer on LME.