The Main Factors Affecting the Aging of Magnesium-Zinc Alloys

Abstract

Hardness measurements and electron microscopic observations were made to study factors affecting the aging of Mg-Zn alloy and Mg-Zn-(Ag) alloys. In regard to the homogeneous and heterogeneous nucleation in these alloys, direct aging, twostep aging and strain aging were carried out, with the results summarized as follows:the nucleation rate largely depends on the quenching rate, the concentration of zinc and the small amount of silver (0.04wt%).(2) In the C-curves which were obtained by plotting the incubation time against the reciprocal of the absolute aging temperature, the nose or the lower part of it gives an optimum condition for the pre-aging. As regards the direct aging and the observations show that these phases preferentially precipitate on dislocations and fine structures such as twin boundary and sub-boundary.

Natural Aging in Mg-Zn(-Cu) Alloys

“…Addition of Cu also increases the eutectic temperature and permits the use of higher solution treatment temperatures. [2,3] The precipitation sequence in the Cu-containing Mg-Zn alloys above 150°C [4] has been reported to be the same as the most probable aging sequence for a binary Mg-Zn alloy: [5][6][7][8][9][10][11][12] SSSS ! solute clusters !…”

“…[14] The precipitation of GP zones was also confirmed by the effect that low-temperature preaging had on hardness and microstructure evolution during subsequent artificial aging. [7,8,15] A more recent study showed that the precipitation in Mg-Zn alloy commences by clustering of Zn atoms, while during aging at intermediate temperatures (up to about 100°C), four to six different types of precipitates were simultaneously present in the microstructure leading to hardening of a significant magnitude. [16] Age hardening at ambient temperature after quenching from the solution heat treatment temperature is common to age-hardenable aluminum alloys; however, this was not observed in age-hardenable magnesium alloys in the past.…”

Precipitation and Hardening in Magnesium Alloys

“…For Mg-Zn-Al alloys with compositions lying in the (a-Mg + /) two-phase field and produced by high-pressure die casting or permanent mold casting, recent studies using TEM and convergent-beam electron diffraction patterns [107,108] indicate that primary intermetallic particles in the as-cast condition have a quasi-crystalline structure (point group of m " 3 " 5, quasi-lattice parameter~0.515 nm) and a composition of approximately Mg 55 [109] and Mg 32 (Al,Zn,Cu) 49 [110] intermetallic alloys, but it is smaller than the that (0.528 nm) reported for the icosahedral Mg 38.5 Al 52 Zn 9.5 phase in Al-Mg-Zn alloys. [111] Even though the equilibrium T phase is often regarded as the crystalline approximant of the icosahedral phase, [112] the metastable quasi-crystalline phase does not transform to the T phase after prolonged heating at elevated temperatures such as 598 K (325°C).…”

“…[52][53][54][55][56] The aging curves of two binary Mg-Zn alloys are provided in Figure 5(a). Depending on the alloy composition and aging temperatures, it is commonly accepted in the literature [6,8,30] that the decomposition of the supersaturated solid-solution matrix phase reportedly involves the formation of G.P.…”

Materials Processing for Structural Stability in a ZK60 Magnesium Alloy

“…[10][11][12][13][14][15][16][17] For example, Mg-5 mass%Zn alloy exhibits age hardening by the precipitation of needle shaped MgZn phase after prolonged aging at $422 K. 10) To date, however, TMT to attain microstructural stability has not been investigated in magnesium alloys.…”