Oxidation Behaviour of Microstructurally Highly Metastable Ag-La Alloy

Abstract

A new silver-based alloy with 2 wt.% of lanthanum (La) was studied as a potential candidate for electric contact material. The alloy was prepared by rapid solidification, performed by the melt spinning technique. Microstructural examination of the rapidly solidified ribbons revealed very fine grains of α_Ag and intermetallic Ag₅La particles, which appear in the volume of the grains, as well as on the grain boundaries. Rapid solidification enabled high microstructural refinement and provided a suitable starting microstructure for the subsequent internal oxidation, resulting in fine submicron-sized La₂O₃ oxide nanoparticle formation throughout the volume of the silver matrix (α_Ag). The resulting nanostructured Ag-La₂O₃ microstructure was characterised by high-resolution FESEM and STEM, both equipped with EDX. High-temperature internal oxidation of the rapidly solidified ribbons essentially changed the microstructure. Mostly homogeneously dispersed nano-sized La₂O₃ were formed within the grains, as well as on the grain boundaries. Three mechanisms of internal oxidation were identified: (i) the oxidation of La from the solid solution; (ii) partial dissolution of finer Ag₅La particles before the internal oxidation front and oxidation of La from the solid solution; and (iii) direct oxidation of coarser Ag₅La intermetallic particles.

Keywords: Ag-La alloy; characterisation; formation mechanism; internal oxidation; metastable microstructure; rapid solidification.

Figure 1

SEM analysis of a rapidly solidified ribbon: (a) lower side (wheel surface) of a ribbon with dimples in the surface; (b) upper side (free surface) of a ribbon.

Figure 2

Grain size analysis in a transverse cross-section of the rapidly solidified ribbons: (a) EBSD mapping; (b) graph of grain diameter vs. area fraction.

Figure 3

SEM micrograph of a rapidly solidified microstructure in the FEG zone: (a) overview of the microstructure; (b) detail of the FEG zone.

Figure 4

SEM micrograph of a rapidly solidified microstructure in the CEG zone: (a) overview of microstructure; (b) distribution of intermetallic particles inside the grains.

Figure 5

SEM micrograph of the internally oxidised microstructure from the bottom layer of the ribbon (FEG zone after RS): (a) overview of a bottom layer; (b) detail from the bottom layer with super fine oxide particles in the αAg grain.

Figure 6

SEM micrograph of the internally oxidised microstructure from the top layer of the ribbon (CEG zone after RS): (a) overview of the top layer; (b) detail from the top layer.

Figure 7

Grain size analysis of a transverse cross-section of the internally oxidised microstructure: (a) EBDS mapping at magnification 1500×; (b) graph of grains’ diameter vs. area fraction.

Figure 8

Line microchemical analysis of an internally oxidised ribbon.

Figure 9

Micrograph of the chemical mapping of an internally oxidised microstructure with detection of the elements Ag, O and La.

Figure 10

STEM analysis of the internally oxidised ribbons: (a) bright-field image of the interface be-tween the αAg and La₂O₃; (b) corresponding FFT pattern from the bright-field image of the La₂O₃ oxide.

Figure 11

Microstructure of the cast hypereutectic composition of Ag-14 wt.% La: (a) as cast; (b) after partial internal oxidation, with a visible Internal Oxidation Front (IOF).

Figure 12

Microstructure of the Ag–2 wt.% La alloy: (a) as-cast and (b) transition between the non-oxidised (lower side) and internally oxidised (upper side) cast microstructure.

Figure 13

SEM micrograph of a hypereutectic microstructure: (a) overview of the completely oxidised microstructure; (b) detailed micrograph from the oxidised dendrite; (c) detail from the oxidised lamellae in the eutectic.

Figure 14

The model of internal oxidation of a “big Ag₅La particle” in the hypereutectic microstructure with moving internal oxidation front: (a) beginning of oxidation with a concentration profile of Ag and La; (b) intermediate stage; (c) completed oxidation with a crescent on the last oxidised side.

Figure 15

The model of oxide thickening in the reaction zone of the “Big Ag₅La particles” at the internal oxidation front.

Figure 16

The oxidation model of “Small Ag₅La Particles” in the hypoeutectic microstructure with an upward moving internal oxidation front: (a) submicron intermetallic particle before oxidation; (b) re-arrangement of atoms during oxidation; (c) separated area with oxide and segregated Ag.