Modeling the precipitation processes and strengthening mechanisms in a Mg-Al-(Zn) AZ91 alloy

Abstract

A model is developed that couples a description of the continuous precipitation of ␤-Mg 17 Al 12 laths at elevated temperatures to a summation of the different contributions to the room-temperature 0.2 pct proof strength in the commercial alloy AZ91 (Mg-9Al-1Zn (wt pct)). The central assumptions are the diffusion-controlled formation of shear resistant ␤-Mg 17 Al 12 laths on the basal planes of the hcp matrix and basal slip as the dominant room-temperature deformation mode. The calculations show quantitative agreement with the evolution of both the reported precipitation kinetics and the strengthening response as a function of aging time at 200 °C. The magnitudes of the different contributions to the hardening are discussed and strategies for enhancing this response are suggested.

Precipitation and Hardening in Magnesium Alloys

“…Unfortunately, during the isothermal aging treatment in the temperature range 373 K to 573 K (100°C to 300°C), the precipitation process seems to involve solely the formation of the equilibrium b phase (Table I). Although the b precipitates are resistant to dislocation shearing, [15,16] their distribution is relatively coarse, presumably because of the relatively high diffusion rate of Al atoms in the solid matrix of magnesium and a possibly high concentration of vacancies in the aMg matrix. Consequently, the age-hardening response of Mg-Al alloys [15,[17][18][19][20] is not as appreciable as expected (Figure 1(a)).…”

“…Consequently, the age-hardening response of Mg-Al alloys [15,[17][18][19][20] is not as appreciable as expected (Figure 1(a)). [19,20] Previous studies [16,21] revealed that the precipitation of the equilibrium b phase occurs both discontinuously and continuously. The discontinuous precipitation is also known as cellular precipitation, and in this reaction, the supersaturated solid solution a¢ phase decomposes into the b phase and an a phase that is structurally identical to the a¢ phase but has a less saturated concentration of aluminum.…”

Critical Assessment 9: Wrought magnesium alloys

“…Meanwhile, the best age hardened magnesium alloys are only around 10 times the strength of pure magnesium. 73 Therefore, in strength critical applications, there is a strong motivation to develop magnesium alloys with an improved age hardening response. A desirable wrought magnesium alloy would combine the ability to be easily formed in a soft condition with a large hardening response after forming to provide both good formability and high strength.…”

Effects of Solute Segregation on Precipitation Phenomena and Age Hardening Response of High-Purity and Commercial AZ91 Magnesium Alloys

“…16,17) The discontinuous precipitation, which originates from the grain boundaries, involves cell formation with a lamellar-type structure of -phase and an aluminum-depleted -Mg matrix phase, while the continuous precipitation involves the formation of isolated particles of -phase within the matrix grains. 7) These precipitations were monitored at regular time intervals by optical microscopy. The micrographs in Fig.…”

“…6,7) It has been noted that non-uniform -Mg 17 Al 12 precipitation occurred within the grains during aging, causing densely populated areas and leaving large precipitate-free regions. It has been suggested that these geometric patterns are due to the occurrence of continuous precipitation in those areas of the grains that are richer in aluminum.…”