Stress Corrosion Cracking And Hydrogen Embrittlement Of The Extruded AZ91 Magnesium Alloy Modified By Gd

As a new type of green environmental protection material,magnesium alloy has good engineering application prospects,but it is limited due to its poor corrosion resistance.Especially under the combined action of the service environment and the external load,magnesium alloys are prone to stress corrosion and hydrogen embrittlement,causing sudden fracture of structural parts.This not only limits the application of magnesium alloys and causes economic losses,but also poses great safety hazards.Therefore,improving the stress corrosion resistance and hydrogen embrittlement of magnesium alloys has become extremely important,and related research has also attracted much attention.In this paper,AZ91 and AZ91-xGd(x=0.5,1.0 and 1.5 wt.%)series of magnesium alloys had been designed and prepared.The effect of rare earth Gd on the microstructure of the as-cast and extruded magnesium alloys AZ91 and AZ91-xGd was revealed.The corrosion behavior of the extruded magnesium alloys AZ91 and AZ91-xGd was tested,and the improvement mechanism of the addition of rare earth Gd on the corrosion performance of the extruded magnesium alloy AZ91 was discussed in detail.Through the slow strain rate tensile test of the extruded magnesium alloys AZ91 and AZ91-xGd,the mechanism of Gd on stress corrosion cracking behavior of the extruded magnesium alloy AZ91 was investigated and the effect of different solutions on stress corrosion cracking behavior of the extruded magnesium alloy were analyzed and discussed.In addition,the mechanism of hydrogen concentration and rare earth Gd on the hydrogen embrittlement of the extruded magnesium alloy AZ91 was deeply analyzed.The microstructure of the as-cast magnesium alloy AZ91 was composed of three phases:?-Mg matrix phase,?-Mg₁₇Al₁₂ phase and Al-Mn phase.The coarse skeletal divorced eutectic?-Mg₁₇Al₁₂ phase was distributed around the?-Mg phase in the form of a discontinuous network,accompanied by a small amount of flaky eutectic?-Mg₁₇Al₁₂ phase.The addition of rare earth Gd not only made the coarse skeletal?-Mg₁₇Al₁₂ phase became massive and inhibited form of the lamellar eutectic?-Mg₁₇Al₁₂ phase,but was also accompanied by the appearance of a new phase Al₂Gd/Al-Mn-Gd.In extruded magnesium alloys AZ91 and AZ91-xGd,?-Mg₁₇Al₁₂phase formsed a banded structure between?-Mg phase in the form of strip and block.They were distributed parallel along the extrusion direction,and granular Al₂Gd/Al-Mn-Gd was distributed along them.The corrosion rate of the extruded magnesium alloys AZ91 and AZ91-xGd decreased gradually with the increased of time.The addition of rare earth Gd reduced the volume fraction of?-Mg₁₇Al₁₂ phase and improved the corrosion resistance of the extruded magnesium alloy AZ91,especially the occurrence of pitting corrosion.With the addition of Gd,the pitting breakdown potential shifted to the right and the corrosion current density decreased in the potentiodynamic polarization curves.At the same time,the high frequency impedance in the impedance spectrum increased,the medium frequency capacitive circuit appeared and the low frequency inductor circuit disappeared.The corrosion product film formed by extruded magnesium alloys AZ91-xGd had better protective effect than that formed on extruded magnesium alloy AZ91.In 3.5 wt.%NaCl solution saturated with Mg(OH)₂,the extruded magnesium alloys AZ91-xGd showed better stress corrosion cracking resistance than AZ91.The addition of rare earth Gd refined the second phase?-Mg₁₇Al₁₂ and improved the protection performance of the corrosion product film,thus improving the local corrosion resistance of magnesium alloy.With the addition of rare earth Gd,the fracture mode of the extruded magnesium alloy AZ91 in 3.5 wt.%NaCl solution saturated with Mg(OH)₂ changed from intergranular stress corrosion cracking to transgranular and intergranular mixed stress corrosion cracking.In distilled water,3.5wt.%NaCl solution saturated with Mg(OH)₂ and 3.5 wt.%Na₂SO₄ solution saturated with Mg(OH)₂,extruded magnesium alloy AZ91-1.0Gd showed the characteristics of stress corrosion cracking.Among them,the stress corrosion susceptibility was the highest in 3.5 wt.%Na₂SO₄ solution saturated with Mg(OH)₂.When the hydrogen charging current density increased,the hydrogen concentration of the extruded magnesium alloy AZ91 increased and the hydrogen embrittlement resistance decreased.With the addition of rare earth Gd in the extruded magnesium alloy AZ91,the hydrogen concentration in the magnesium alloy gradually decreased,and the hydrogen embrittlement resistance was improved.The mechanism of improving hydrogen-induced cracking of the extruded magnesium alloy AZ91 by rare earth Gd was that during hydrogen charging,rare earth Gd in extruded AZ91-xGd magnesium alloys inhibited the invasion of hydrogen atoms into magnesium matrix,reduced the hydrogen concentration in magnesium alloy,and then improved the hydrogen-induced cracking resistance of magnesium alloy.