Stress induced hexagonal martensite in CuZnAl

It has been shown that it is possible to induce a hexagonal long range ordered martensite phase in a CuZnAl alloy with M_S= +50°C and an electron concentration e/a=1.48. From measurements of the critical stress and of the retransformation temperature to β after reheating, it is concluded that the hexagonal and the original 3R martensite have the same formation free energy. This implies that the two structures cannot be described by their difference in stacking fault density (compared to a face centered lattice), and an energy difference which is proportional to it. It is probable that either the interaction between neighbouring stacking faults or an electronic contribution from the Fermi surface interaction with the Jones zone plays an important role.     

Phase evolution and gas-phase particle size distributions during spray pyrolysis of (Bi,Pb)SrCaCuO and Ag(Bi,Pb)SrCaCuO powders

Phase evolution, gas-phase particle size distributions and lead loss were studied during formation of (Bi,Pb)SrCaCuO powders and their composites with silver by spray pyrolysis starting from nitrate solutions. The 10 wt% Ag/90 wt% Bi_1.8Pb_0.44Sr₂Ca_2.2Cu₃O_x composite powders made at 700°C consisted of 20–60 nm grains of silver and mixed-oxide phases with a fine dispersion of Ag grains within the particles. At 700°C, the primary phases present in (Bi,Pb)SrCaCuO powders were (Bi,Pb)₂Sr₂CuO_x (2201), Ca₂PbO₄ (plumbate), (Bi,Pb)₂Sr₂CaCu₂O_x(2212), and (Bi,Pb)₃Sr₂Ca₂Cu₁O_x(3221). For T≥800°C, the powders were considerably depleted in lead, and the plumbate and 3221 phases were absent. For T = 900°C, a large number of ultrafine particles (<30 nm) were formed, probably from the PbO vapor released from the reactor walls. Using spray pyrolysis, it is easy to control stoichiometry and limit the phase segregation at the nanometer-scale so that homogeneous and phase-pure materials can be obtained rapidly during subsequent processing.