Effects of secondary ageing on precipitate distribution and mechanical properties of magnesium alloy WE54

Magnesium alloys containing rare-earth elements are widely used in modern industry. Some of their advanced properties, such as low density, relatively high tensile strength and good creep resistance, make them attractive as structural materials in applications where weight saving is of great importance. Most of these properties are related to the formation of intermediate precipitate phases in the magnesium grains. Among most commercial Mg-R.E.(rare-earth) alloys, alloys based on the Mg-Y-Nd system have been the most successful competitors in the market. Alloy WE54 (Mg-5.25wt.%Y-3.5wt.% Heavy R.E. (1.5-2wt.%Nd)-0.45wt.%Zr), developed by the Magnesium Elektron Ltd. Company, is the backbone of this alloy system. In industrial applications, components that are made of this alloy are typically aged at 250°C to peak hardness (T6 condition). However, it has been found that the tensile elongation of the components gradually decrease when they are exposed to temperatures around 150°C; and the ductility becomes unacceptable after long term exposure. The embrittlement problem is slightly controlled by reducing the Y concentration in the alloy WE43 (Mg-4wt.%Y-2.25wt.%Nd-1wt.% Heavy R.E. -0.4wt.%Zr), but the tensile strength is compromised. The early studies on the embrittled WE54 alloy reported that the problem is caused by the formation of β” precipitate particles with the D0₁₉ structure. However, there remains some controversies over the structure of the β” precipitate; the previous results obtained by TEM are insufficient to support the proposed structure. Additionally, many mechanical properties of the embrittled WE54 alloy, such as fracture toughness and Charpy impact energy, were not reported in detail due to commercial confidentiality reasons. Therefore, the major aim of this project is to provide a comprehensive understanding of the effects of secondary ageing treatment on the microstructure and mechanical properties of magnesium alloy WE54. In the present study, a secondary ageing treatment at 150°C after the primary T6 heat treatment at 250°C is used to simulate the environment causing embrittlement. The microstructure before secondary ageing (T6 condition) has been identified as a distribution of three major primary precipitates, i.e. β’, β₁ and β phases. The multi-particle configurations between primary precipitates have been investigated. For the β₁ plates with ends attached to β’ particles, a β₁ plate and an attached β’ particle on one end can be regarded as a β’/β₁ couple. Three types of β’/β₁ couples have been identified. The β’/β₁ interfaces are found to be fully coherent for all the three β’/β₁ couples. In order to minimise the interfacial energy and shear strain energy associated with the β₁ precipitate, the lattice of β’ particles adjacent to the β’/β₁ interface is slightly rotated by 0.5° with respect to the β₁ lattice. Configurations of multiple β₁ precipitate plates have also been found, i.e. V-shape configurations between two β₁ plates and triadic arrangement of three β₁ plates. The three β₁ precipitate plates in a triad are three β₁ variants with the same sense of shear strain. The triangular Mg region isolated by these three β₁ plates is always rotated by ~10.5° with respect to the Mg matrix outside this region. Depending on the type of β₁ variants isolating the triangular magnesium phase, a counter clockwise/clockwise rotation of ~10.5° about [0001]α of the isolated Mg phase with respect to the matrix outside the region has been observed. In the samples after secondary ageing at 150°C for 4 weeks, a dense distribution of secondary precipitates has been observed. The G.P.1 and G.P.2 zones (single-layer and double-layer plate precipitates) are the dominant secondary precipitates. The G.P.1 zone is a plate shape precipitate consisting of a single {02 ̅20}α layer of rare-earth atoms. The habit planes of G.P.1 zones are parallel to the {01 ̅10}α planes. The G.P.2 zone is composed of two parallel G.P.1 zone plates which are separated by three layers of {02 ̅20}α Mg atoms. With increasing secondary ageing time (secondary aged at 150°C for 12 weeks), the G.P.1 and G.P.2 zones are transformed to nanometre-sized β’ particles which are composed of a minimum of three G.P.1 zone building blocks. The embrittlement problem caused by secondary ageing treatment has been confirmed by tensile tests, Charpy impact tests and chevron-notched fracture toughness measurement. The investigations on the distribution of precipitates and fracture mode of samples indicate that the embrittlement problem is related to the reduced widths of grain boundary precipitate free zone and increased localised slip, which are both caused by the formation of secondary precipitates. By performing a re-ageing treatment on the embrittled samples at 250°C for 10min, the secondary precipitates are fully dissolved. Therefore, the tensile properties and fracture toughness of the samples are restored to T6 condition.