Understanding Laves phase precipitation induced embrittlement of modified 9Cr–1Mo steel

Abstract

Precipitation of Laves phase in steels is often identified as the primary cause for the degradation of mechanical properties. Analytical electron microscopy and thermodynamic modelling have been used in this study to understand the Lave phase precipitation induced damage initiation process in modified 9Cr-1Mo steel, exposed to high temperature for 10,000 h. The results have suggested that in addition to void nucleation at the interface and de-cohesion, the limited plasticity of the Laves phase is also a predominant mechanism for crack initiation in this material. The thermodynamic propensity of Laves phase precipitation around the carbides and the underlying role of local chemistry have also been studied. Binary Laves phases have been found to be stabilized by ternary additions.

Laves phases: a review of their functional and structural applications and an improved fundamental understanding of stability and properties

“…Most prominent representative of this group of ferritic-martensitic steels is socalled P91 steel (9Cr-1Mo ? various minor additions), see, for example, [639,652,656,[663][664][665][666][667][668]. While in 9Cr-2Mo steels, a significant loss of toughness after only 1000 h at 600°C was observed that was attributed to precipitation of MoFe 2 Laves phase [669][670][671], P91 (1 wt% Mo) steels were successfully creep-tested for more than 100000 h at 550°C [672,673] and 600°C [652] without fracturing but finally also showing creep damage that at least partially was attributed to strongly coarsened MoFe 2 Laves phase precipitates.…”

Laves phase formation in Fe-based alloys from strengthening particle to self-healing agent: a review

“…On the other hand, Laves-phase strengthened steels seems to be a complex challenge at least in martensitic/ferritic steels, mainly due to the crystallographic features of this phase in comparison with the host matrix [41,81]. Laves phase has a partially semi-coherent heterogeneous nucleation in grain boundaries [10,11]. Nevertheless, researchers have found that the addition of rare earths promotes a high fraction of fine Laves phase particles, therefore increasing its effectiveness as a strengthening particle [59,132].…”

“…Laves phase is frequently identified with degradation of creep properties in high chromium steels. Even though these precipitates reinforce the matrix initially, their fast growth can promote long-term creep deformation [10,11]. If the precipitate size is stabilized, Laves-phase is considered one of the most effective reinforcements by precipitation, which has led to the development of ferritic steels with chromium content higher than 13%wt.…”

“…If the precipitate size is stabilized, Laves-phase is considered one of the most effective reinforcements by precipitation, which has led to the development of ferritic steels with chromium content higher than 13%wt. In these cases, Laves-phase acts by anchoring dislocations and grain boundaries [10,12]. Recent studies of the heterogeneous nucleation of Laves phase in creep cavities have been demonstrated that its growth can increase the lifetime of metals under creep conditions by preventing coarsening, agglomeration, and crack propagation.…”

Evaluation of Cleavage Fracture Behavior of C14 Fe₂W Laves Phase by First-principles Calculations and Crystal Orientation Analysis

“…5) They mainly precipitate adjacent to M 23 C 6 on prior austenite grain, packet/block, and subgrain boundaries. [5][6][7][8][9] Several studies [10][11][12][13][14][15][16][17][18] on the relationship between the Fe 2 M Laves phase (M is mainly Mo, W, and Nb) precipitated in ferritic heat-resistant steels and the decrease in creep ductility and toughness of steels have been reported. However, Zhong et al 19) confirmed that the ductility of the specimens embrittled by aging at 600°C was recovered by reheating at 700°C.…”