Chromium – Molybdenum – Nickel

Abstract

The first experimental work on the ternary phase diagram is from [1925Sie]. These authors constructed a liquidus projection of the partial system with less than 50 at.% Mo. [1950Put] detected the first ternary phase in this system and found it isotypic to the already known F phase. They determined roughly the range of stability of this phase. [1951Rid] studied this range in more detail in a 1200°C isothermal section. They found an additional similar but different ternary phase and called it P phase. By [1953Rid] these results were combined to the quaternary system Co-Cr-Mo-Ni. [1954Blo] by more than 100 alloys investigated the 1250°C isothermal section and the liquidus surface. Tentatively also the invariant equilibria above 1250°C are given. These authors assumed a solid solution, based on an allotropic modification of pure Cr, which is not accepted to be a stable phase in the newer review of the Cr-Ni binary system of [1986Nas]. [1960Vas] studied a diffusion couple Cr-Mo with a thin interlayer of Ni at 1250°C. They deduced Ni solubilities in the Mo-rich and Cr-rich solid solutions at 1250°C. The same method was used by [1988Kod] to establish tie lines between (Ni) or (Cr,Mo) and the ternary intermediate phases P or F at 1152°C. The boundary of the (Ni) solid solution at 800, 1000 and 1200°C was determined by [1962Cla]. These authors studied also the influence of Si or Mg (added as Ni2Mg) deoxydants on the velocity of F or P phase precipitate formation in weld areas and found much slower precipitation with Mg than with Si. The equilibria between (Ni) and the F, P and * phases were repeatedly investigated by [1969Bra, 1973Mor, 1984Rag, 1988Kod, 1991Goz], also in presence of further elements. The results agree within a few at.%. [1984Rag] and [1991Goz] found an additional phase, :, between the * and P phases at 850°C or at 700, 850 and 1000°C, respectively. This phase was not recognized by [1962Cla], eventually because annealing 30 h at 800°C is too short. All other investigations did not include the stability range of this phase (below 1000°C and below 20 at.% Cr). The crystal structure of the P phase was established by single crystal X-ray diffraction [1955Bri]. The parameters of the atomic positions were refined by the same research group [1957Sho]. This phase belongs to the topologically close packed (TCP) phases like the other ternary phases of this system (F, P, :) and *MoNi. Besides the TCP phases there exist three different ordered superstructures of the (Ni) solid solution as intermediate phases. MoNi4 and MoNi3, are binary phases of the Mo-Ni system and dissolve only traces of Cr. Both form two-phase fields with the disoredered (Ni) phase. Addition of 3.8 mass% Cr to a MoNi4 alloy suppresses the long range ordering of the MoNi4 superstructure [1985Vas]. The CrNi2 ordered phase extends metastably until MoNi2. The kinetics of this ordering were studied by [1991Kum] in an alloy Mo25Cr8Ni67. Karmazin studied also these kinetics as well as the upper temperature limit as function of the Ni concentration in binary Cr-Ni alloys [1982Kar] (maximum at about 570°C, 67 at.% Ni) and in ternary alloys of the section (Cr0.25Mo0.75)1–xNi2+x [1994Kar] (maximum at about 670°C, 65 at.% Ni). The ordering proceeds continuously like in a second order transformation, but it cannot be excluded that equilibrium two-phase regions ordered + disordered exist. The velocity of ordering is only slightly faster than precipitation of TCP phases from the supersaturated (Ni) solid solution [1994Kar]. [2002Ary] compared by TEM the formation of the three different superstructures in an alloy Mo24Cr6Ni70. The as quenched samples show only short range order (SRO). Annealing at 600°C produces a mixture of MoPt2and MoNi4-type domains, fully coherent mutually and with the matrix. Annealing at 800°C results in a cellular decomposition into MoNi3 and matrix plates. At 700°C a mixture of both structures is formed. An attempt to calculate theoretically the relative phase stabilities agrees well with the experimental findings. Solidification paths of compositions near Hastelloy C22 and C276 were calculated after the Scheil-Gulliver model [2003Per, 2006Per]. These calculations agree well with microscopic observations of the resulting microstructures in slowly cooled (DTA) as well as in rapidly cooled (weld) samples. No experimental determinations of thermodynamic properties are reported for ternary Cr-Mo-Ni alloys. Thermodynamic datasets were assessed by [1974Kau, 1990Fri]. The description of Cr-Mo-Ni in the SGTE