Creep Laves phases

Similar effects have been observed in other intermetallic NiAl-base alloys with less regular distributions of hexagonal C14 Laves phases (Machon, 1992 Sauthoff, 1993 a), and have been discussed by Sauthoff (1991b). In those alloys with coarse phase distributions the observed secondary creep rates follow a rule of mixtures at a first approximation, and additional strengthening effects are only observed for alloys with fine phase distributions. From this it is concluded that particulate and nonparticulate intermetallic alloys creep in similar ways and can be described by the same constitutive equations as conventional multiphase alloys. 

Another example is given in Fig. 27, which illustrates the strengthening of NiAI by ternary Cl4 Laves phases. The data points for TaNiAl-NiAl seem to follow a rule of mixtures, but the data for NbNiAl-NiAl clearly indicate a significant deviation from a lineal superposition for the creep resistances. Furthermore, the extrap-... 

Figure 27. Creep resistance (compressive stress for 10" s secondary creep rate) at 1200 °C as a function of the volume fraction of the second phase for the inter-metallic NiAl-base alloys NiAl-NbNiAl and NiAl-TaNiAl with the C14 Laves phases NbNiAl and TaNiAl, respectively (SautholT, 1990a Machon, 1992).

Since the transition metal elements in these ternary Laves phases can substitute for each other freely, and since the distribution of Laves phase can be controlled by thermomechanical treatments there are possibilities for optimizing such NiAl alloys with Laves phases with respect to creep resistance and the brittle-to-ductile transition temperature (BDTT) by controlling the composition and phase distribution, and this is being studied presently... 

A heat resistant alloy is based on X15CrMoV12-l and contains 0,2 % nitrogen (trade name HNS 15). It precipitates fine V(C,N) and provides a good creep resistance and heat resistance up to 650 °C. Above this temperature the appearance of Laves-phase restricts its usage. 

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