Konstantina Lambrinou

Synthesis of MAX Phases in the Zr-Ti-Al-C System

This study reports on the synthesis and characterization of MAX phases in the (Zr,Ti)n+1AlCn system. The MAX phases were synthesized by reactive hot pressing and pressureless sintering in the 1350-1700 °C temperature range. The produced ceramics contained large fractions of 211 and 312 (n = 1, 2) MAX phases, while strong evidence of a 413 (n = 3) stacking was found. Moreover, (Zr,Ti)C, ZrAl2, ZrAl3, and Zr2Al3 were present as secondary phases. In general, the lattice parameters of the hexagonal 211 and 312 phases followed Vegards law over the complete Zr-Ti solid solution range, but the 312 phase showed a non-negligible deviation from Vegards law around the (Zr0.33,Ti0.67)3Al1.2C1.6 stoichiometry. High-resolution scanning transmission electron microscopy combined with X-ray diffraction demonstrated ordering of the Zr and Ti atoms in the 312 phase, whereby Zr atoms occupied preferentially the central position in the close-packed M6X octahedral layers. The same ordering was also observed in 413 stackings present within the 312 phase. The decomposition of the secondary (Zr,Ti)C phase was attributed to the miscibility gap in the ZrC-TiC system.

Low-Temperature Embrittlement of Lead-Free Solders in Joint Level Impact Testing

The low-temperature embrittlement of tin-based (Sn- based) lead-free (Pb-free) solder alloys has been previously reported for bulk specimens tested in impact by means of a conventional Charpy V-notch test setup. The brittle failure of Sn-based Pb-free solders at low temperatures (i.e. below -30degC) is of great concern for electronic assemblies that operate at harsh conditions, because it can lead to occurrences of early failures. A miniaturized impact test setup has been devised to perform a scaled-down version of the conventional Charpy impact test on specimens, which lie in the size range of 300 mum to 3 mm. In this work, the impact tests have been carried out in the temperature range between 23 degC and -110degC. The specimens consisted of SuperBGAreg (SBGAreg) substrates bumped with 4 different solder alloys: Sn3%Ag0.5%Cu (SAC 305), Sn4%Ag0.5%Cu (SAC 405), Sn3.5%Ag, and Sn37%Pb. The energy absorbed during the fracture of the solder joints was seen to decrease as a function of temperature for the SAC 305 and SAC 405 alloys, while for the Sn37%Pb and Sn3.5%Ag this trend was less pronounced. The ductile-to-brittle transition in the fracture behavior of Sn- based Pb-free solders was confirmed by scanning electron microscopy (SEM) investigations. These investigations showed the progressive bulk embrittlement of the Sn matrix of SAC 305 and SAC 405 solder alloys. The onset of the Sn embrittlement was observed between -40degC and -50degC leading to the complete brittle fracture of the solder joint at around -70degC.

The double solid solution (Zr, Nb)2(Al, Sn)C MAX phase: a steric stability approach

The addition of Nb and Sn to Zr2AlC is investigated, targeting the synthesis of a Zr-rich bulk MAX phase free of ZrC. The 211 phase formation in the two quaternary Zr-Nb-Al-C and Zr-Al-Sn-C systems is evaluated. Solubility over the entire compositional range in (Zr, Nb)2AlC and Zr2(Al, Sn)C is observed. In terms of effectiveness, the addition of Sn is preferred over the addition of Nb, as the former is selectively incorporated into the 211 structure. A combinatorial approach results in the formation of phase-pure (Zr0.8, Nb0.2)2(Al0.5, Sn0.5)C. The effect of the added solutes on the microstructure and crystallographic parameters is investigated. The addition of Nb and Sn reduces the distortion parameter of the trigonal prism compared to pure Zr2AlC. Therefore, an attempt is made to establish a more general stability criterion for the M2AC structure based on the steric relationship between the atoms in the M6A trigonal prism. Inspired by the Hume-Rothery rules, it is suggested that comparable atomic radii of the M- and A-atoms provide a good starting point to obtain a stable 211 MAX phase.