Ti CxNi TiC xNi

and a temperature-time profile for this reaction is shown in Fig. 9 (Ref 29). The reaction temperature exceeds the melting point of nickel (1453 °C) for about 20 s. Liquid nickel is present during this time period, and in fact the slight shoulder observed on the profile upon cooling at 1450 °C corresponds to the solidification of the nickel phase. Similar behavior is also noted for the simultaneous reaction of TiC and NiAl and TiC and Ni3Al from titanium, carbon, nickel, and aluminum powder mixtures (Ref 30).

Fig. 9 Time-temperature profiles for the reactive synthesis of Ti + C + 25 wt% Ni >TiC-Ni. Source: Ref 29

Alloying additions can also influence the diffusion rates between the reacting elements and minimize Kirkendall porosity. An example of this is the reactive sintering of nickel-modified iron aluminides (Ref 31). Without nickel additions, iron aluminide swells upon reactive sintering. The addition of nickel (substituting for iron) results in a high-density reactively sintered product (Fig. 10). The nickel inhibits the rate of aluminum diffusion in the iron, thus eliminating the pores that develop at the prior aluminum particle sites (Ref 31). However, this only occurs in Fe-Al-Ni powder billets rolled to an 80% reduction prior to reactive sintering at 650 °C for 15 min. This effect does not occur in similar material that was die pressed. This is due to differences in the green structure that develop during rolling compared to die pressing, as discussed below.

Fig. 10 Effect of nickel content on the porosity of reactive sintered (650 °C, 15 min) iron-aluminides (elemental powder mixtures corresponding (in wt%) to 86.4(Fe + Ni) + 13.6Al. Source: Ref 31. Compacts of elemental powder mixtures were rolled to 80% reduction prior to sintering.

Influence of Green Compaction. As mentioned previously, Fe-Al-Ni powder billets rolled to an 80% reduction prior to reactive sintering at 650 °C for 15 min were dense; however, similar composition of elemental powders, die pressed at more than 550 MPa, swelled upon sintering (Ref 31). Similar behavior is observed during reactive sintering of TiAl. Die-pressed titanium and aluminum powders swell upon reactive sintering. However, Dahms and coworkers (Ref 5, 6, 32, and 33) were able to produce 97% dense TiAl by reactive sintering extruded billets of titanium and aluminum powders, as illustrated in Fig. 11. The extrusion ratio played an important role in the densification of TiAl, with higher extrusion ratios resulting in denser (less porous) pressureless sintered billets.

o Extrusion ratio - 17

■ cjuru a Exlru

biun i dLJi sion ratic

} - o 5 = 350

t

—*

400 600 600 1000 Temperature, °C

1200 1400

400 600 600 1000 Temperature, °C

1200 1400

Fig. 11 Porosity of reactive sintered TiAl specimens as a function of sintering temperature and extrusion ratio. Source: Ref 33. Compacts of elemental titanium and aluminum were extruded prior to sintering.

It is evident that the high pressures that can be generated during rolling and extrusion influence the green structure in a beneficial manner. The deformation of the elemental powders that occurs during extrusion and rolling results in increased surface contact area, decreased initial pore size, and improved aluminum distributions in the green microstructure. This results in minimal pore formation during reactive sintering.

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