Prakt. Met. Sonderband 41 (2009) 277
Characterisation of the metastable microstructure of Au-La alloy
den beim stmu- T. Zupancic-Hartner'?, R. Rudolf’? A.C. Kneissl®, I. Anzel
ignetischen Flus-
ge der HACOM-
rieblich erzeugte Faculty of Mechanical Engineering, University of Maribor, Slovenia
deren Wirembe- ?Zlatarna Celje d.d., Department for Technological Development, Slovenia
r HACOM- und ‘Department Physical Metallurgy and Materials Testing, University of Leoben, Austria
läufe gemeinsam
nte eine Gefüge- Abstract
durch eine Fin-
sröffnete sich so- The influence of lanthanum addition and solidification conditions on the microstructure and
properties of gold have been studied. Rapidly solidified ribbons of Au-0.5 wt.% La alloy have been
zontaler Sehnen- prepared by melt spinning. An overall assessment of the ribbons has shown that up to three
esssignalen unter microstructural regions are distinguished: fine equiaxed grains, zone with a columnar structure and
mmung nach der coarse equiaxed grains. Their number, type and height depend strongly on the ribbon thickness
unterschiedlichen (average cooling rate). Very fine precipitates inside the grains, the eutectic microsegregations at the
elegt werden. grain boundaries and some other features have been observed in the metastable rapidly solidified
sser nach ASTM microstructure. The strengthening effect of microstructural refinement and La addition in gold are
on mechanischen discussed.
ckgrenze und auf
. 1. Introduction
1en langeren Zeit-
© Online- Daten- It is known that pure gold is relatively soft and ductile with a low yield point and must be
e Korngréfienbe- ; . i : }
strengthened to be applicable in various commercial products. The improvements in strength and
hardness in gold are mostly achieved with alloying additions. The conventional alloying elements
typically used include silver, copper, nickel, platinum, palladium, manganese and chromium. Of
these, silver and copper are the main so called substitutional solid solution strengthening elements.
Silver atoms are a little larger in size than gold atoms whereas copper atoms are much smaller.
Their inclusion in the gold crystal lattice leads to distortion of the lattice and consequently to
ste, B. Heutling, increased force necessary for sliding of dislocations through the lattice.
uktiven Bestim- In recent years, a number of strengthened gold alloys, with alloying additions of only 0.5wt.% or
ften, Technische less, have been developed [1, 2]. Such small addition of appropriate alloying elements, which can
be described as microalloying, can result in the microstructure with the presence of small second
ng Company, phase particles at gold grain boundaries or even with fine distribution of submicron particles within
the grains. These particles act as barriers to dislocation motion under conditions where thermally
1996. activated barriers such as solute atoms, precipitates and grain or subgrain boundaries become less
ngland, 2003. effective. This mode of strengthening is called dispersion hardening. As dispersoids may be used
ten von IF- the oxides of microalloying elements (ME) or intermetallic compounds formed in the selected Au-
ME alloying system. In various studies [3, 4] the oxide phases were identified as one of the best
candidates for dispersion strengthening of metallic materials. For dispersion strengthening of Au the
oxides of rare earth elements (RE) are also very attractive. They are thermodynamically stable at
high temperatures, chemically compatible with Au and have a fluorite-related structure that leads to
a large lattice misfit at the interface with the Au matrix. Additionally, the low solubility and
diffusivity of the RE in Au enhance the morphological stability of their oxide. The intermetallic
compounds that can be formed in the microalloyed Au-RE alloys are AugRE (for La, Ce, Pr, Nd.
Pm, Sm. Gd. Tb. Dy, Ho). AusRE (for Eu) and Au4RE (for Er, Tm, Yb, Lu, Sc).
