Prakt. Met. Sonderband 41 (2009) 93
A metallographic study of the influence of isothermal age hardening
: on micro specimens cast from a Au-Ag-Cu ternary alloy
ungen massiv
ikroskopische
der Probe zu A. Reif, B. Okolo, E. Kerscher*, A. Wanner
kop kaum zu Institut fiir Werkstoffkunde I, Universitit Karlsruhe (TH), D-76128 Karlsruhe
nuell polierte * now: Lehrstuhl fiir Werkstoffkunde, Technische Universität Kaiserslautern, D-67653 Kaiserslautern
cheinen (z.B.
bnisse liefern,
pographie der
"Az 1 Introduction
\bschattungen
vermeintliche With the advent of microsystem technologies (MST), new challenges have arisen in materials test-
ing and characterization. Due to their small sizes, it is difficult to handle MST components within a
nden werden. testing frame in a precise manner. Conventional testing methods are designed for considerably lar-
nn eine lange ger components. Thus MST materials research requires that either test rigs are modified to accom-
nn schon die modate the geometries of micro-components or entirely new testing methods are developed specifi-
oberflächlich cally for micro-components [1]. Challenges due to miniaturization exist also in materialography.
zeigten, nicht For instance sample preparation steps often require grinding and polishing to a certain surface qual-
ity. These mechanical processes are characterized by materials removal and the development of
je und genaue scratches of various length-scales at the worked surface. Due to these factors, acquiring useful im-
häufig wegen ages from metallographically prepared micro sections of small size specimens can be a daunting
‚en kann aber task. This is because as the dimensions (length, width and depth) of the mechanically induced
zur einzigen scratches on the sample surface approaches that of the sample it then becomes difficult to resolve
and discriminate between useful details on the sample and details from preparation artefacts. Soft
och über den ductile materials are especially prone to these problems.
Klaren sein. In the present study a precipitation-hardening gold-based alloy with chemical composition 62.2 wt.-
% Au, 25.0 wt.-% Ag, and 12.8 wt.-% Cu was investigated in the as-cast and heat treated states.
The aim of the heat treatment was to increase the hardness state in the material using isothermal ag-
ing methods. By this the physical properties of the material will ensure low contact wear making
this material fit for technologies where contacting surfaces in relative motion is essential. Literature
ation of reported on the most suitable age hardening conditions for this gold alloy show that in the hardened
state, values of up to 200 HV [2] can be realized. The hardness increase associated with isothermal
aging of this class of gold alloys is characterized by the formation of ordered crystallographic re-
gions and the decomposition of the single phase alloy into Ag-rich and Cu-rich phases [3]. These
literature reports are based on investigations performed on relatively bulky samples. It is also
known that gold alloys are chemically inert and exhibit good corrosion resistance properties [4].
ANT, Mid- These attributes make them attractive materials in the area of MST sensor applications and me-
chanical systems where aqueous media and moderately high temperature conditions are a concern.
Microstructure plays a decisive role in these property states. The temperature gradients imposed on
cast materials during the casting process significantly influence their solidification behaviour and
consequently the development of their microstructure. Hence information on the microstructure
helps assess the quality of the casting process and also the suitability of the cast parts for specific
applications. Pores, cavities, undesirable segregations at the grain boundaries and intragranular de-
fects can make cast parts unfit for use under load bearing conditions. With these considerations in
