TEM Study of High-Temperature Precipitation of Delta Phase in Inconel 718 Alloy

Inconel 718 is widely used because of its ability to retain strength at up to 650◦C for long periods of time through coherent metastable γ” Ni3Nb precipitation associated with a smaller volume fraction of γ’ Ni3Al precipitates. At very long ageing times at service temperature, γ” decomposes to the stable Ni3Nb δ phase. This latter phase is also present above the γ” solvus and is used for grain control during forging of alloy 718.While most works available on δ precipitation have been performed at temperatures below the γ” solvus, it appeared of interest to also investigate the case where δ phase precipitates directly fromthe fccmatrix free of γ’’precipitates. This was studied by X-ray diffraction and transmission electron microscopy (TEM). TEM observations confirmed the presence of rotation-ordered domains in δ plates, and some unexpected contrast could be explained by double diffraction due to overlapping phases

Identification of intermetallic precipitates formed during re-solidification of brazed aluminium alloys

This study shows that Fe and Mn bearing phases in Al–Si alloys containing a low level of Fe and Mn are essentially cubic greek small letter alpha-Al(Fe, Mn)Si with Im3 space group and δ-AlFeSi which is observed with both tetragonal and orthorhombic structures. As this latter phase is not expected to form according to the ternary Al–Fe–Si phase diagram, the present results suggest that it is stabilized in the quaternary Al–Fe–Mn–Si system with respect to other phases such as β-Al4FeSi

Full elastic strain tensor determination at the phase scale in a powder metallurgy nickel-based superalloy using X-ray Laue microdiffraction

Laue microdiffraction is used to determine the full elastic strain tensor of the γ and γ′ phases in grains of a nickel-based superalloy with a coarse-grained microstructure. A `rainbow' filter and an energy dispersive point detector are employed to measure the energy of Bragg reflections. For the two techniques, an uncertainty of ±2.5 × 10−3 Å is obtained for the undetermined crystal lattice parameter. Our measurements show that the filter method provides better confidence, energy resolution, accuracy and acquisition time. The sensitivity of each method with respect to the γ–γ′ lattice mismatch is demonstrated with measurements in samples with average precipitate sizes of 200 and 2000 nm. For the 200 nm precipitate size, the lattice mismatch is less than 2 × 10−3 Å and the dilatational strains are close to ±1.5 × 10−3 depending on the considered phase. For the 2000 nm precipitate size, the lattice mismatch is close to 8 × 10−3 Å and almost no elastic strain occurs in the microstructure

Study of the precipitation of dispersoids during the homogenization treatment of a 3003 aluminium alloy

The phase transformation sequences have been studied for a commercial AA3003 alloy from the as-cast state during the heating to the homogenization temperature thanks to several in situ methods i.e. electrical resistivity, transmission electron microscopy and X-ray diffraction. The as-cast structure of this alloy is characterized by an inhomogeneous solid solution with primary Al6(Mn, Fe) and a few a-Al(Mn, Fe)Si particles. The in situ analysis are in good agreement which allow to clarify the phase transformation temperatures, the morphology of the precipitated phases and their crystallography.Les séquences de transformations de phase d'un alliage commercial AA 3003 ont été étudiées au chauffage depuis l'état brut de coulée jusqu'à la température d'homogénéisation à l'aide de plusieurs techniques in situ : résistivimétrie électrique, microscopie électronique à transmission et diffraction des rayons X. A l'état initial, l'alliage est constitué d'une solution solide sursaturée, de précipités Al6(Mn,Fe) et d'une faible quantité de précipités α-Al(Mn,Fe)Si. Ces différentes analyses ont donné des résultats en très bon accord et ont permis de préciser les températures de transformations de phase, la morphologie des phases précipitées ainsi que leur cristallographie

Solid-state phase transformation in a lithium disilicate-based glass-ceramic

The solid-state phase transformation in a lithium disilicate-based glass-ceramic (IPS e.max® CAD) was revisited on the basis of quantitative data. IPS e.max® CAD is widely used as material for the dental restoration in the dental industry. In-situ X-ray diffraction and differential scanning calorimetry accompanied by scanning electron microscopy observations were applied to understand phase transformation during heat treatment in a dental ceramic. In-situ X-ray diffraction evidences the concomitant formation of cristobalite and lithium orthophosphate at 770 °C. Then, the formation of lithium disilicate occurred at the expense of a complete dissolution of cristobalite and lithium metasilicate. No phase transformation occurred during cooling. The quantitative results of microstructural features (amount of each phase, morphology, and number density of lithium disilicate and lithium metasilicate) indicate that lithium disilicate is probably formed by diffusional process at the lithium metasilicate/cristobalite interface, which acts as favorable nucleation sites. The energy barrier is probably too high for lithium disilicate nucleation in the amorphous matrix. The quantitative results will provide the background for further modeling of phase transformation kinetics, which may have potential industrial benefits

Evolution of defects in titanium grade 2 under Ti2+ion irradiation

The complexity and diversity of microstructure involved in titanium alloysmakeit rather difficult to quantitatively describe defect evolution due to irradiation. This paper focusesondefect evolutions of commercially pure titanium grade 2 under Ti2+ ion irradiation considering the effect of dose (0.6 and 3 dpa), temperature (300°C and 430°C) and flux (15:1 ratio). An irradiation damage profile was predicted using SRIM software to obtain a homogeneous damage on at least 500 nm depth for TEM observations and simulated using JANNUS-Saclay facility. The details regarding the quantification methodologies of the defects from dark field images are provided, as are the origins of the associated uncertainties. In addition to a tangled dislocation network, presence of the -type and -component loops is observed. The latter was scarcely reported in the literature in the case of titanium alloys. At low temperature, the size distribution of the -type dislocation loops remained similar regardless of the dose and flux whereas these parameters have highly influence at 430°C. A widening of the size distribution and an increase of the threshold incubation dose (TID) was noted with the temperature. In the case of the -component loops, it was shown that the nucleation occurred in spite of the 0.6 dpa low dose

Effect of Processing Route on Microstructure and Mechanical Properties of a Ti-3Al-2.5V/TiB Composite

A Ti-3Al-2.5V matrix composite reinforced with 8.5 vol.% TiB was produced using a powder metallurgy route. Processing included the mechanical alloying of Ti-3Al-2.5V and TiB2 powders and Hot Isostatic Pressing (HIP) of the resultant composite powders, to produce a dense billet. These billets were subsequently extruded and/or subjected to various Conversion Heat Treatments (CHT), to complete the transformation of the TiB2 particles into TiB needles. The CHT was performed either before or after extrusion. Microstructures and tensile properties of the materials at each stage of the processing routes were investigated and compared to those of a non-reinforced Ti-3Al-2.5V material, manufactured by the same powder metallurgy route. It has been demonstrated that the processing routes have a great impact on the mechanical properties, through modifications of the matrix and reinforcement characteristics. Well-chosen processing routes lead to more ductile composites, though this gain in ductility leads to slightly lower stiffness and strength values. This study clearly demonstrates the possibility to produce, at an industrial scale, a ductile version of a highly reinforced titanium matrix composite, showing important application potential

Phase transformation of the Ti-5553 titanium alloy subjected to rapid heating

The a -> b phase transformation upon heating in the Ti-5553 alloy with lamellar-nodular bimodal microstructure was tracked in situ with high energy X-ray diffraction. Rapid heating at 10, 50 and 100 °C s-1 from room temperature to 1050 °C was tested. Phase transformation on heating was studied by a combined analysis of the microstructural features that provides estimates of mass fractions, mean lattice parameters and full width at half maximum for the two phases. In comparison with equilibrium conditions, the experimental mass fractions reveal a shift of the transformation domain toward high temperatures when the heating rate increases. Also, the dissolution of the a phase is largely impacted by its morphology, the transformation being faster for a lamellae. The combined analysis of mean lattice parameters and full width at half maximum suggests that the a -> b phase transformation on heating is diffusion controlled. The b phase therefore inherits the solute content of the adjacent parent a phase, leading to chemical heterogeneities in the b phase regardless of the heating rate

Ferrite recrystallization and austenite formation during annealing of cold-rolled advanced high-strength steels: In situ synchrotron X-ray diffraction and modeling

Ferrite recrystallization and austenite formation occurring during annealing of cold-rolled advanced high-strength steels are key mechanisms as they largely determine the final microstructure and mechanical properties. However, the influence of processing parameters on these mechanisms and their interactions is still not fully understood. This is particularly the case for Dual-Phase steels having an initial cold-rolled microstructure con-sisting of ferrite and martensite before annealing, which were scarcely investigated compared to ferrite-pearlite initial microstructures. In situ synchrotron X-ray diffraction experiments together with post-mortem metallo-graphic analysis allowed clarifying both ferrite recrystallization and austenite formation during annealing of a ferrite-martensite initial microstructure depending on the process parameters of the annealing cycle. Results showed a major influence of recrystallization state on austenite formation, leading to an unexpected effect of heating rate on austenite formation kinetics. A modeling approach was undertaken to rationalize the influence of heating rate on austenite formation by taking into account the bi-phased ferrite-martensite initial microstructure and the effect of ferrite recrystallization state

Influence of the ageing conditions and the initial microstructure on the precipitation of α phase in Ti-17 alloy

The precipitation of α phase during ageing was investigated in the near-β titanium alloy Ti-17 considering either a fully βmetastable initial microstructure or a 35% αprimary + 65% βmetastable initial microstructure. In-situ electrical resistivity and high energy X-ray diffraction measurements revealed the influence of the initial microstructure, with different α morphologies (size and distribution of αprimary), as well as the heating rate on the precipitation sequences and kinetics following the decomposition of the β-metastable phase. Various amounts of metastable phases (ωisothermal and α″isothermal) precipitate in temperature ranges that increase with the heating rate. From temperatures about 500 °C, the orthorhombic α″isothermal structure evolved towards the hexagonal close-packed α as temperature increased. SEM microstructure characterisations showed that slow heating rates promoted a fine and dense α precipitate distribution through the formation of ωisothermal and/or α″isothermal, leading to higher hardness values. A higher heating rate restricted the precipitation of α″isothermal and shifted to the one of α at a higher temperature, leading to coarser precipitates. Furthermore, precipitation kinetics of α″isothermal/α were quicker considering an initial intragranular α precipitation as compared to α colonies