Specimens can be quenched rapidly, without cracking ampules.
Improved crucibles consisting mainly of metal-reinforced ceramic ampules have been developed for use in experiments in which material specimens are heated in the crucibles to various high temperatures, then quenched by, for example, plunging the crucibles into water at room temperature. A quench crucible of the traditional type intended to be supplanted by the improved crucibles consists mainly of a ceramic or graphite ampule inside a metal cartridge, with a gap between the metal and the cartridge, as shown on the left side of the figure.
The need for the improved quench crucibles arises as follows: In a traditional quench crucible, the gap between the ampule and the metal cartridge impedes the transfer of heat to such a degree that the quench rate (the rate of cooling of the specimen) can be too low to produce the desired effect in the specimen. One can increase the quench rate by eliminating the metal cartridge to enable direct quenching of the ampule, but then the thermal shock of direct quenching causes cracking of the ampule.
In a quench crucible of the present improved type, there is no gap and no metal cartridge in the traditional sense. Instead, there is an overlay of metal in direct contact with the ampule, as shown on the right side of the figure. Because there is no gap between the metal overlay and the ampule, the heat-transfer rate can be much greater than it is in a traditional quench crucible. The metal overlay also reinforces the ampule against cracking.
The choice of ampule material and metal depends on the specific application. In general, the ampule material should be chemically compatible with the specimen material. The overlay metal should be chosen to have a coefficient of thermal expansion (CTE) as close as possible to that of the ampule material. Examples of suitable ampule/metal-overlay material pairs include the following:
- graphite (CTE = 8.0 × 10–6 K–1) and stainless steel (CTE = 9.9 × 10–6 K–1)
- aluminum nitride (CTE = 5.2 × 10–6 K–1) and tungsten heavy alloy (CTE = 5.0 × 10–6 K–1) and
- silicon carbide (CTE = 4.5 × 10–6 K–1) and tungsten heavy alloy (CTE = 5.0 × 10–6 K–1).
Several thermal-spray processes for applying metal overlays to ampules were investigated. Of these processes, vacuum plasma spraying was found to yield the best results.