Effects of Si on Phase Stability and Precipitation Behavior of C14 Laves Phase (Fe,Cr)2(Nb,Mo) in High Cr αFe-base Alloys

ABSTRACTThe growth mechanism of C14 Laves phase in the bcc αFe matrix was determined as the ledge mechanism on the (110)α//(0001)C14 habit plane in Fe-20Cr-0.5Nb-1Mo (at%) alloys annealed at 1073 K for 24 hours, using conventional and scanning transmission electron microscopy. Terrace planes are the basal plane of hcp-based C14 structure. Precipitation particles tend to grow in plate shape depending on the anisotropic difference of lattice mismatch. The addition of Si with Mo remarkably enhances C14 Laves phase precipitation. The area fraction of Laves phase increases from 5.9% to 12.1% by the 2Si addition on Fe-20Cr-0.5Nb-2Mo alloys. Contrary to this, the addition of Si is not effective to increase Laves phase precipitation. It is indicated that Si improves the phase stability of C14 Laves phase while the partitioning of Mo into C14 Laves phase would be promoted due to the attractive interaction between Mo and Si.

The Development of Interphase Precipitated Nanometre-Sized Carbides in the Advanced Low-Alloy Steels

In this work, the investigation of transmission electron microscopy has elucidated the morphologies of the interphase precipitated carbides in an experimental Ti-Mo-bearing steel into three types: (1) planar interphase precipitation with regular sheet spacing (designated as PIP), (2) curved interphase precipitation with regular sheet spacing (designated as Regular CIP), and (3) curved interphase precipitation with irregular sheet spacing (designated as Irregular CIP). The planar sheets of carbides have also been analyzed and found to be oriented close to ferrite planes {211}, {210} and {111}; the results of transmission electron microscopy provide strong evidence to suggest that the development of interphase-precipitated carbides can be associated with the growth of incoherent ferrite/austenite interface by the ledge mechanism. The sheet spacing and inter-carbide spacing in the sheet have been measured and estimated in this work. The sheet spacing is found to be finer than the inter-carbide spacing in the sheet for all samples investigated. The result reflects that the distribution of interphase-precipitated carbides is anisotropic and cannot be considered random distribution. The relevance of the Orowan mechanism to the non-random distribution of interphase-precipitated carbides has been considered. The contribution of the dispersion of interphase-precipitated carbides to the yield strength of the steel studied has been estimated. It is revealed that an optimum component about 400 MPa contributed by interphase-precipitated carbides can be achieved, and the finding is consistent with the hardness data. Other examples of the different alloy steels are also addressed.

Precipitation Process in a Cu-Ni-Be Alloy

The precipitation process in an aged Cu-1.9wt%Ni-0.3wt%Be alloy has been examined by high-resolution transmission electron microscopy. The precipitation sequence found is: Guinier- Preston (G.P.) zones → γ'' → γ' → stable γ. The disk-shaped G.P. zones and the disk-shaped γ'', γ' and γ precipitated phases are composed of monolayers of Be atoms on {100}αof the Cu matrix and alternative Be and Ni matrix layers parallel to {100}α. The γ'' phases consisting of two to eight Be-layers has a body-centered tetragonal (bct) lattice witha=b=0.24 nm andc=0.28 nm. The γ' or γ phase is bct witha=b=0.24 nm andc=0.26 nm ora=b=0.26 nm andc=0.27 nm. The γ'', γ' or γ phase aligns with the matrix according to the Bain orientation relationship. The growth kinetics of disk-shaped γ precipitates on aging at 500°C has been also investigated. The {001}αhabit planes of the γ precipitates migrate by a ledge mechanism. The average thickness of the γ disks increases with aging timetast1/2. An analysis of experimental data using a kinetic model yields the diffusivity of solute in the Cu matrix, which is in agreement with the reported diffusivity of Ni in Cu.

Deformation-induced α2 ↔ γ phase transformation in a Ti–48Al–2Cr alloy

The structure of γ–α2 interfaces in deformed Ti–48Al–2Cr alloy was analyzed by high-resolution transmission electron microscopy (HREM) and image simulations. Growth of γ–TiAl plate in α2–Ti3Al phase was found to be a result of a ledge mechanism consisting of Shockley partial dislocations on alternate (0001)α2 planes. The height of the ledges was always a multiple of two (0001)α2 planes. The γ → α2 phase transformation was also an interface-related process. Large ledges of six close packed planes (111)γ high were often observed at the γ–α2 interface. Every large ledge consisted of six Shockley partial dislocations that originated from the γ–a2 interfacial lattice misfit. The movement of these partial dislocations accomplished the transformation of γ → α2 phase. Comparing the experimental and simulated HREM image, it was found that atomic reordering appears during the deformation-induced γ↔α2 transformation.