Densification during Reactive Sintering

Figure 2 shows a microstructure of Ni3Al produced by reactive sintering nickel and aluminum powders at 750 °C for 15 min in vacuum. The elemental powder mixture was die pressed at room temperature to a green density of about 70% and heated at a rate of 20 K/min to the sintering temperature. As is evident, near-fully dense (>97% theoretical) Ni3Al can be produced utilizing processing cycles consisting of short sintering times and low homologous sintering temperatures (TH = 0.60 and is defined as the sintering temperature, 1023 K, divided by the melting temperature of compound, 1660 K). Densification coincides with the initiation of the reaction, as shown by the dilatometer and differential thermal analysis scans illustrated in Fig. 3. The high final density is a result of:

• The relatively small volume change (-5.2%, see Table 1) that occurs when the nickel and aluminum powders transform to Ni3Al

• The presence of a transient liquid phase during the sintering cycle

The processing parameters that influence the reaction between the constituent powders and alter the amount, distribution, and duration of the liquid phase impact the density of the final product. These processing parameters include: powder particle sizes, composition, green density, heating rate, and sintering atmosphere. Proper manipulation of these parameters can enhance the density of the reactively sintered product.

Fig. 2 Microstructure of reactive sintered Ni3Al (750 °C, 15 min). (a) As-polished. (b) Etched with Kallings solution



1 1

1 1 1

Compacted Ni

and AI powders

70% dense

10 K/min

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I ! I

250 500 750 1000 1250 Temperature, °C

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