The present study reinvestigates the Al-Ce and Al-Nd phase diagrams and reoptimizes their thermodynamics using the CALPHAD method. First-principles energy calculations play an important role in terms of sublattice formalism and phase-stability prediction, demonstrating that they should be effectively integrated into experimental investigations and thermodynamic assessments. Specifically, current experimental results and theoretical calculations show that Al 2 Nd (or Al 2 Ce) should be treated as a stoichiometric compound phase rather than as the solution phase that was proposed in previous studies. Further, a new compound, AlCe 2 , is found stable at high temperatures (648 °C to 775 °C) in the Al-Ce system. It forms through a peritectic reaction of liquid and AlCe phases at 775 °C, and decomposes into AlCe and ␤AlCe 3 at 648 °C and below. Since the AlCe 2 phase is not retained at room temperature by quenching experiments, it is suggested that AlCe 2 may be isostructural with the previously known compound AlNd 2 (oP12). Based on current differential thermal analysis (DTA) measurements and theoretical calculations, it is also proposed that there is an ␣/␤Al 3 Ce polymorphous transition occurring at 973 °C in the Al-Ce system and an ␣/␤Al 3 Nd polymorphous transition occurring at 888 °C in the Al-Nd system. The ␤Al 3 RE phase may be isostructural with ␤Al 3 Y (hP12). Finally, the previously described ␤Al 11 RE 3 phase (rare earth elements (RE) ϭ La, Ce, Nd, or Pr) is proposed to have a stoichiometry of Al 4 RE (tI10), based on direct evidence from differential scanning calorimetry (DSC) measurements.

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Selected corrosion properties of the recently developed amorphous Al-Co-Ce alloy system are reported over a range of cobalt and cerium compositions. Improved resistance to halide-induced pitting was observed in 0.6 M NaCl solution when fully amorphous and aged alloys ͑100°C for up to 1000 h͒ were compared to polycrystalline aluminum. Corrosion behavior was strongly influenced by the cobalt concentration in solid solution in the amorphous state. Tunable corrosion properties based on pre-selection of Co content is shown to be feasible. These corrosion properties remained after heat treatment at 100°C. However, complete recrystallization at 550°C removed the benefits of the amorphous state.Over the past several years, a wide range of novel amorphous alloys have been produced with early transition metal-late transition metal-simple metal ͑ETM-LTM-SM͒ compositions. These amorphous alloys offer significant enhancements of both mechanical strength and corrosion resistance over their fully crystalline counterparts. 1-5 One subset of the ETM-LTM-SM compositions showing particular promise as high-strength low-density material is the class of Al-TM-RE alloy compositions. These alloys have demonstrated high glass-forming abilities 4,6 as well as improved corrosion resistance over conventional aluminum alloys. 7-9 In addition, this class of alloys possesses the capacity to develop nanocrystals in an amorphous matrix upon appropriate heat treatment. 10 Such nanocrystals have been shown to further enhance the mechanical properties in other Al-TM-RE glasses without adversely affecting the improved corrosion resistance normally associated with fully amorphous alloys. 11,12One alloy system within the Al-TM-RE class of amorphous alloys that has recently been characterized is Al-Co-Ce. 13,14 This system has shown to have high thermal stability, as well as a large compositional range over which an amorphous microstructure has been obtained. 14 By making use of these unique physical characteristics, as well as the beneficial influence which transition metals have on the barrier corrosion properties 7,8 and sacrificial coating capabilities 3,5 when added to solid solution of amorphous aluminum, the opportunity exists to create a multifunctional metallic coating. An additional benefit of this system as a coating comes from the possibility of an active corrosion inhibition function provided by the release and delivery of the oxidation products of Co and Ce. 15 Although no known previous studies have focused on the release of these elements from solid solution of aluminum, Co͑II͒ and Ce ͑III͒ have shown great promise as inhibitors when deposited on the surface of aluminum from aqueous solution. [16][17][18][19][20][21][22] Recent preliminary studies have demonstrated active corrosion inhibition where an AlCo-Ce coating is placed in close proximity to 2024-T3 through release and transport of Co͑II͒ and Ce͑III͒ in the aqueous solution. 15 To optimize each of these coating functions, a full understanding is needed regarding the influence of ...

AKIN ODABAŞI, NEC _ IP Ü NLÜ , GÜ LTEK _ IN GÖ LLER, and MEHMET N _ IYAZ _ I ERUSLUThe effect of heat input from laser beam welding (LBW) on the microstructural evolution of superalloy Inconel 718 was investigated. LBW was carried out on 1.6-mm-thick sheets with an average grain size of 13 lm (ASTM # 9.5), and four different heat inputs in the range of 74.5 mm À1 to 126.6 J mm À1 were used. Full penetration was achieved in all weld experiments.Microstructures of the welds were evaluated using an optical microscope and a field emission scanning electron microscope. Increasing the heat input changed the resulting weld shape from a wine glass shape to a stemless glass shape with wider surface bead widths, and the measured average dendrite arm spacing was increased from 1.06 lm to 2.30 lm, indicating the corresponding solidification rate in the range of 1.75 9 10 5 K s À1 (°C s À1 ) to 3.5 9 10 6 K s À1 (°C s À1 ). The welds also were free from microfissuring even at the lowest heat input trials. The Nb concentration of Laves phase for the current LBW samples was %20.0 wt pct. The coefficients of partition and distribution for Nb were determined to be approximately 3.40 and 0.50, respectively.

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