Processing of iron-titanium powder mixtures by transient liquid phase sintering
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I.
Transient Liquid Phase Sintering
INTRODUCTION
TRANSIENT liquid phase sintering has attained recent widespread attention for high performance material systems. 1-16 Mixed elemental powders are a useful basis for forming alloys and avoid the expense and compaction difficulties of prealloyed powders. The transient liquid offers the possibility for high sintered densities with uniform dimensional change and minimal microstructural coarsening. In spite of the progress in applying transient liquid phase sintering to several systems, the processing effects are not well understood. Figure 1 gives two hypothetical binary diagrams for systems which could be processed by transient liquid phase sintering using mixed elemental powders. Liquid phase sintering depends on a low temperature wetting liquid with solubility for the solid. 17For a transient liquid, the base material must also have a high solubility for the additive at the sintering temperature. Liquid formation depends on the interaction between the constituents during heating. Slow heating diffusionally homogenizes the compact, suppressing liquid formation and giving swelling.~S'~9 Thus, a strong sensitivity to processing conditions is anticipated in transient liquid phase sintering systems. In this study, transient liquid phase sintering is applied to Fe-Ti alloys while considering several processing variables. Iron was selected as the base material because of its availability, common use in powder metallurgy, and extensive use in enhanced sintering studies. 2~A survey was conducted of several candidate iron-based systems, 21-24leading to the selection of Ti as detailed in the thesis by Dunlap. 2~ The Fe-Ti binary phase diagram is shown in Figure 2 based on Moffatt. 25 The use of a volumetric scale for the phase diagram aids in subsequent analysis of swelling and shrinkage behavior. The diagram has features similar to those suggested as ideal in Figure 1; however, there is the complication of two eutectic reactions. A mixture of Fe and Ti powders will potentially form the first liquid during heating at 1085 ~ The second eutectic occurs at 1289 ~ Both melts can be expected to be transient in character under appropriate processing conditions.
start with a mixture of B and ~A powders
\-'~-"sin-teri~n ....... temperature
temperature
• final composition
,~.jfinal composition
I I I I
B
composition A B composition B=bose A=additive
A
Fig. 1--Two binary phase diagrams indicating sintering temperature and alloy compositions which can be processed by transient liquid phase sintering of mixed powders.
Sintering studies on the Fe-Ti powder system have been reported by Kiebeck et aI.,3'~3 Kivalo et al. ,26,27 and Baek and German.14 The experiments by Kivalo et al. demonstrate swelling at sintering temperatures below 1200 ~ They did not consider sintering at temperatures high enough to form the 1289 ~ eutectic. Kiebeck eta/. 3't3 and Baek and
1600o 1400-
Fe2Ti ~, F e T i j ~ L
~
1200-
E 1000-
1085
800 -
V!
600-
1 590 i
R.M. GERMAN is Professor of Materi
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