Formation Of Primary Phase Research Articles

Processing of thick-film electrodeposited thallium oxide superconductors

Thick-film Tl-Ba-Ca-Cu-O superconductors (TBCCO) were synthesized on Ag foil and ceramic substrates using an electrodeposition process. The influence of Ag, initial stoichiometry, and reaction conditions on the phase development of TBCCO films were studied. Films deposited and reacted with no additional thallium source showed 1223 as the major phase, whereas films reacted using a two-zone thallation process showed 2223 as the primary phase formation. The TBCCO samples electrodeposited on Ag foil and annealed using the two-zone thallation process showed a critical current density of 3.2 × 10 4 A/cm 2 at 76 K in zero field. The highest critical current density for films reacted without any additional thallium source was near 10 4 A/cm 2 at 76 K in zero field.

Preferential nucleation in undercooled liquid aluminium alloys

A procedure based on nucleation theory has been developed to predict the formation of primary phases in liquid quenched Al–Cr and Al–Zr alloys as a function of powder size, heat extraction, and cooling rate in an idealised gas atomiser. The theory predicts that (i) with decreasing powder size/increasing cooling rate there will be a preferential suppression of the intermetallic compounds in favour of α-Al and (ii) there will be a hierarchy for the suppression of the various intermetallic phases. Both effects arise from differences in entropy of fusion of the competing phases and are in agreement with experiment. Maps of predicted rate limiting phase versus composition and powder diameter are presented.MST/718

Effect of hafnium on the structure and properties of wrought nickel-base alloys

1. In nickel-base alloys hafnium enters into the composition of nitrides of the type Ti(C, N) and Nb(C, N), partially replacing atoms of titanium, niobium, and aluminum in nitrides TiN, NbN, and AlN, and it also enters into the composition of the solid solution and strengthening γ-phase. 2. Hafnium promotes an increase in the amount of MC type carbides with a high carbon content, a reduction in M6C carbides, and removal of carbide liquation in nickel-base alloys. 3. An excess amount of hafnium in alloys leads to formation of primary phases of the type Ni, (Hf, Ti)2, and secondary phases: lamellar phase Ni3 (Ti, Hf) and Ni3 (Nb, Hf). 4. In worked alloys hafnium refines grains and it retards grain growth in the temperature range 1100–1200°C. 5. Hafnium promotes an increase in the strength properties of alloys at 20°C, and it does not alter ductility. Optimum additions of hafnium lead to a marked increase in time to failure for alloys in the temperature range 700–800°C. 6. On adding small amounts of hafnium to alloys there is an increase in the resistance of the metal to cracking near the seam during welding and during heating of welded joints.