It is well established that the toughness properties of duplex alloys are reduced in the presence of intermetallic phases and that the extent of the reduction increases with higher intermetallic contents (refs 1, 2, 3). At relatively high contents, the mechanism may be easily explained by the formation of an interconnected network of the embrittling phases. However, there is evidence that lower volume fractions also reduce toughness significantly (ref. 4), even though individual particles are present in a ‘ductile matrix’. Another issue is that intermetallic precipitation may influence fracture toughness and impact tests in a different manner, such that it is not possible to extrapolate Charpy impact test data to fracture toughness requirements. The current paper addresses these issues and draws conclusions on these two topics.

The work concluded that brittle intermetallic precipitates (e.g. sigma phase – s ) can either fracture or disbond from the surrounding ferrite (d ) and austenite (g ), act as local stress raisers, and encourage cleavage through the ferrite phase. This implies that intermetallic precipitates assist crack initiation and produce proportionally lower CTOD results than comparable Charpy V-notch results might suggest.

Notwithstanding the conclusion above, a correlation between Charpy and CTOD toughness was found to hold in the presence of up to 8% intermetallic phases. This allows for the presence of moderate intermetallic contents to be ignored when determining Charpy impact test requirements in procedure codes. However, this correlation should be used with caution, especially if the temperature of interest is below the ferrite ductile-to-brittle transition temperature (DBTT). In such instances, intermetallic precipitates may encourage crack initiation and cleavage of the ferrite, which would not necessarily be highlighted by a fast strain rate test such as Charpy V-notch.