Microdiffraction, EDS, and HREM investigation for phase identification with the electron microscope.
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1. Introduction The properties of heterogeneous materials like alloys and composites depend critically on
their structure and composition'. During the aging heat treatment, the properties are optimized by the tight control of the type, size and composition of the intermetallic precipitates which form. Therefore the formation of undesirable precipitates may be detrimental to the particular properties 2 . These precipitates are often of nanometric size and one of the most adequate method for their analysis is electron microscopy. In this work, two new phases have been identified: the urphase which is closely related to the 6 phase inside the duplex steel; and in the reinforced aluminium alloy, a Cu-rich precipitate in epitaxial relationship with the well known A13Zr dispersoid. 2. Experimental The duplex austenite-ferrite (5+y) specimens were solution treated at 1350'C for 20 min to achieve homogeneity, they were subsequently quenched in order to retain a supersaturated and fully 8 microstructure3 . Specimens were then annealed in a temperature range between 550' and 161 Mat. Res. Soc. Symp. Proc. Vol. 589 0 2001 Materials Research Society
650'C for various times up to 355 hours followed by water cooling. The heat treatments were carried out in electric muffle furnaces under vacuum in order to minimize oxidation. The Al-SiC composite whose composition was Al - 6%Cu - 2% Mn - 0.3% Ag - 0.3 %
Mg - 1% (Cr, Ti, V, Zr), was fabricated by the Ospray preform which consisted in mixing the atomized molted alloy with 13% SiC particles of diameter smaller than 15 pm and quenching onto a mandrel. The composite was then extruded at 490'C and solution treated at 520'C for 20 min, followed by water quenching (T4 temper). Specimens were aged at 190'C (T6 temper). The details of the fabrication can be found elsewhere4. We analysed samples after extrusion, T4 and T6 tempers. The TEM samples were prepared by the conventional methods of electropolishing for the
duplex steel and by mechanical grinding followed by ion milling for the Al composite. Microdiffraction analysis was carried out in a Philips CM12 electron microscope equipped with a + 600 double tilt stage and the patterns were obtained using a nearly parallel beam which was focussed on a very small area of the thin foil (few tens of rm). HREM was carried out on microscopes with a point to point resolution better than 2 nm. 3. Results 3.1 The r phase The isothermal heat treatment of the 8-ferrite between 550 and 650'C leads to the formation of intergranular precipitation of fine and heavily faulted particles which exhibited a needle-like morphology (fig. 1). Close examination involving various orientations of the 8 matrix grains established that the habit planes and growth direction of the particles are ( 110) and 8, respectively. Therefore, the number of variants for the particles in a given 8 matrix grain is equal to 6 (n=6). Fig. 1: Morphology of the Tphase which is embedded inside 8 matrix. M
The determination of the crystal structure of the T phase was carried out by d
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