TEM Microstructural Evolution and Formation Mechanism of Reaction Layer for 22MnB5 Steel Hot-Dipped in Al–10% Si

Abstract

Microstructural evolution and formation mechanism of reaction layer for 22MnB5 steel hot-dipped in Al–10Si (in wt %) alloy was investigated. The microstructural identification of the reaction layer was characterized via transmission electron microscopy and electron backscatter diffraction. In addition, the formation mechanisms of the phases were discussed with vertical section (isopleth) of the (Al–Si–Fe) ternary system. The solidified Al–Si coating layer consisted of three phases of Al, Si, and τ5 (Al8Fe2Si). The reaction layer on the Al–Si coating layer side is a fine τ5 phase (Al8Fe2Si) of 5 μm thickness. The layer on the steel side consisted of an η phase (Fe2Al5) of thickness of 500 nm or less. τ1 (Al2Fe3Si3, triclinic) phase of 200-nm-thickness was formed in the η phase, and κ phase (Fe3AlC) of 40–50 nm thickness was formed between η phase and steel. The τ5 phase was formed by isothermal solidification at 690 °C in the liquid Al–10 wt % Si when 3.73–29.0 wt % of Fe was dissolved from the boron steel into the Al–Si liquid bath. It was considered that the η phase was formed by the diffusion reaction of Al, Si, and Fe between τ5 and ferrite steel. κ (Fe3AlC) phase was formed by the reaction of the carbon, which is barely employed in η and τ phases, and diffused Al.

Phase Composition of Al-Si Coating from the Initial State to the Hot-Stamped Condition

“…The irregular shape of the τ 5 /Al-Si coating interface can be explained by the solidification in the Al-Si melt [ 6 , 26 ]. These findings are in good agreement with the literature [ 4 , 6 , 18 , 20 , 26 ]. The purpose of adding Si to the hot-dipping bath is to form this ternary τ 5 inhibition layer, which impedes the further diffusion of Fe into the coating, and Al in the opposite direction, thus forming a thick layer of intermetallics, especially the Fe 2 Al 5 brittle intermetallic phase.…”

“…According to the chemical composition determined by SEM-EDS ( Table 2 ), this ternary phase can be identified as Fe 2 Al 8 Si (also denoted as τ 5 ) with a hexagonal structure and the P63/mmc space group [ 18 ]. The presence of the τ 5 phase was also confirmed by XRD and micro-XRD analysis ( Figure 3 , JCPDS card no 00-020-0030).…”

“…According to the chemical composition (point 5, Figure 1 b), this sublayer was identified as η-Fe 2 Al 5 with an orthorhombic structure and the Cmcm space group, which has also been mentioned in Refs. [ 4 , 5 , 6 , 18 , 20 , 27 , 28 ]. The content of Si ( Table 2 ) in this phase corresponded to that observed by Windmann et al [ 6 ], who reported an even slightly higher content of Si in the η-Fe 2 Al 5 phase, reaching up to 6.1 at.%.…”

“…Note, however, that FeAl 3 was found after hot-dipping in a bath containing a significantly lower Si content (up to 2%) for an immersion time that was considerably longer than the typical operating time. Shin et al [ 18 ] studied the microstructural evolution of hot-dipped Al-Si coating on 22MnB5 boron steel, and stated that the intermetallic sublayer at the steel substrate interface consisted of Fe 2 Al 5 and Fe 3 Al 2 Si 3 when the Si content exceeded 5 wt.%. The authors also provided a TEM analysis of the phase evolution during hot-dip aluminizing.…”

“…PHS is commonly aluminized by hot-dipping into a bath containing 7–11 wt.% Si, up to 3 wt.% Fe and Al bulk at temperatures of approximately 670–700 °C [ 4 ]. Si is added to the bath to form an inhibition layer of the Fe 2 Al 8 Si (τ 5 ) phase (also referred to in the literature as Fe 2 Al 7 Si and Fe 2 Al 7.4 Si), retarding the formation of the very brittle Fe 2 Al 5 phase while significantly influencing the formation of a layered structure of the coating [ 15 , 16 , 17 , 18 ]. However, the melting temperature of the Al-Si coating with a typical chemical composition of Al-Si10 is approximately 600 °C, which is substantially lower than the austenitization temperature (~920 °C) [ 2 , 4 ].…”