Abstract
In this work the Laves phase was found to appear in two different morphologies, namely granular shapes and in an elongated shape. No difference in crystallography could be detected between these morphologies. The Laves phase was only observed in its elongated form in Cu-containing steels, where it was the primary morphology present after short term ageing. After long term ageing, the elongated Laves phase was replaced by the granular morphology. It is speculated that Cu precipitates act as nucleation sites for the elongated Laves phase, resulting in an unstable orientation relationship with the matrix, an in the meta-stable elongated morphology of Laves phase precipitates.
Highlights
IntroductionThe precipitation process of Laves phase in these steels is normally very long, and can continue thousands of hours during service at 600°C [2,3]
The 12CrTa and 12CrTaCu specimens in figure 1 have been aged at 650°C for 1000h each, and show a very different morphology of the Laves phase, with a granular Laves phase in the steel without copper, and an elongated Laves phase in the steel containing copper
There was no difference in crystallography or composition between these morphologies, and no Cu could be detected at the interface with the matrix in either case
Summary
The precipitation process of Laves phase in these steels is normally very long, and can continue thousands of hours during service at 600°C [2,3]. This intermetallic phase has previously been observed to appear with two different morphologies, most frequently with almost equiaxed granular shapes but sometimes with elongated needle shape [4]. 2. Experimental The alloys investigated in this work were based on a very low carbon 12%Cr composition with Nb or Ta instead of vanadium, Z-phase strengthened steels [5,6], which are slightly different from most other 9-12%Cr steels. Some of the very small precipitates may not have been recovered by this sedimentation process
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