The Structures of Precipitates in an Mg-0.5 at%Nd Age-Hardened Alloy Studied by HAADF-STEM Technique

Abstract

The crystal structures and microstructures of precipitates formed in an Mg-0.5 at%Nd alloy aged at certain temperatures ranging between 170 C and 250 C are studied in detail by high-angle annular detector dark-field scanning transmission electron microscopy. The precipitation sequence can be presented as Mg-solution ! GP-zone ! 0 (orthorhombic) ! 1 (fcc). At the early stage of aging (170 C for 2 h), fine precipitates of planar GP-zones appear in parallel to (100) m planes, with a thickness of sub-nm and a length of 5-15 nm (the subscript letter of m denotes matrix). With an advance of aging, the GP-zones increasingly grow larger and combine with the neighbours, thus making themselves further prolonged along the directions addressed above. When reaching at the top-stage of aging (170 C for 100 h), the alloy additionally allows the 0 -phase to coexist, taking the form of lens-shape with a thickness of 2-5 nm and a diameter of 5-15 nm. The 0 -phase has an orthorhombic structure (Mg 7 Nd) with a ¼ 0:64 nm, b ¼ 1:1 nm, and c ¼ 0:52 nm, which is coherently connected to the matrix. At the stage of over-aging, both the GP-zones and the 0 -phase disappear and instead coarse precipitates of the stable 1 -phase (Mg 3 Nd; fcc) are formed with particular crystallographic relations of ½001 m == ½110 p and ½110 m == ½112 p (the subscript letter of p denotes precipitate).

Precipitation and Hardening in Magnesium Alloys

“…The HAADF-STEM work of Saito and Hiraga [152] also revealed the existence of a precipitate phase that has never been reported for the binary Mg-Nd alloys. This phase, designated b¢, has an orthorhombic structure (a = 0.64 nm, b = 1.14 nm, c = 0.52 nm), a Mg 7 Nd composition, and a lenticular morphology.…”

“…This phase, designated b¢, has an orthorhombic structure (a = 0.64 nm, b = 1.14 nm, c = 0.52 nm), a Mg 7 Nd composition, and a lenticular morphology. Although it is not mentioned by the authors, the following orientation relationship can be deduced from Figure 7 in their work [152] : (100) The b¢ phase mentioned in previous studies was reported to form as plates on . Pike and Noble [149] suggested that the b¢ phase had a hexagonal structure (a = 0.52 nm, c = 1.30 nm) and the following orientation relationship with respect to the matrix phases:…”

“…Some hexagonal rings, defined by six columns of Nd atoms, are also observed in this latest work. [152] The separation distance between two neighboring columns of Nd atoms within the hexagonal ring is again approximately 0.37 nm, and the prism plane of the hexagonal ring is parallel to 10 " 10 È É a . Each hexagonal ring can be regarded as being constructed by linking three variants of the V-shaped unit mentioned in the preceding paragraph.…”

“…pct Nd samples aged at 443 K (170°C), [152] some peculiar zigzag arrays of Nd atoms aligned approximately along 2 " 1 " 10 a are observed and regarded as G.P. zones.…”

The Microstructures and Tensile Properties of As-Extruded Mg–4Sm–xZn–0.5Zr (x = 0, 1, 2, 3, 4 wt %) Alloys

“…Moreover, the market price of Sm is much cheaper than that of Nd and Y [15]. It is, therefore, meaningful to produce low-price, heat-resistant Mg-Sm alloys with proper mechanical properties to compete with traditional Mg-RE alloys [16][17][18].…”