COST 507 - Repositório Aberto da Universidade do Porto

3.4 Electronmicroscopy (SEM) and X-ray diffraction
The microstructure of nearly all investigated quaternary alloys consists of only three
phases. In all investigated alloys the binary phase Ti 3 Al was found. Furthermore
complex binary and ternary phases like Ti x Al y Cu z and TiNi 3 were determined. Phases
containing all four system elements (Ti, Al, Cu, Ni) were also detected. The results of
the X­ray diffraction are lines from binary and ternary compounds and some
unidentified lines. The lack of data from possible quaternary phases prevents the
assignment. The comparison of these two methods explain that there is no agreement
between the results. The figures 3­6 in the appendix indicate the results for some
selected alloys. The element distribution is originated from a SEM­analysis and the
phase determination is based on the X­ray diffraction.
3.5 Wetting and Joining tests
The wetting and joining tests were carried out at 1100°C for the systems with a lower
melting temperature e.g. Til5Al­CuNi. Systems with a higher melting temperature e.g.
Ti20Al­CuNi, are investigated at 1250°C. In general, there are three different cases of
wetting behaviour obvious:
1. a complete melting and spreading with a small wetting angle (less than 30°)
2. no complete melting but some reactions between filler metal and substrate
3. no melting
The behaviour in cases two and three originate from a possible difference between
melting­ and brazing temperature of these systems mainly Ti20Al­based alloys. Due to
high activity between Titanium and the ceramic crucible a thermodynamic reaction
occurs, with the result that the melting temperature of some alloys could not be
determined precisely. In comparison to the TÌ25A1­ and Ti20Al­systems the Ti 15Albased
alloys show bigger wetting angles (Fig. 7) and a well­defined diffusion zone
between the Ti 3 Al­substrate and the filler matrix (Fig. 8) with a thickness of ca. 30μπι.
The SEM­analysis reveals that this zone consists of mainly TiAlCu­ and TiNicompounds.
The Ti25Al­based systems show some erosion of the substrate (Fig.9) and
a few pores within the filler matrix (Fig. 10). It appears from the SEM analysis that
there is a decrease of Cu­ and Ni­diffusion within the interface. Furthermore there is a
tendency for TiAl phase formation during the braze process.
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