A. Menand

Interstitial solubility in γ and α2 phases of TiAl-based alloys

Measurements of interstitial concentrations were performed on (α2 + γ) two-phase TiAl alloys and on γ single-phase TiAl alloys. This study allowed the determination of the maximum solubility of oxygen and carbon in the γ phase. The influence on this solubility of various parameters such as temperature, α2 and γ phase stoichiometry and the alloying addition of a third element will be discussed. This paper also presents an explanation for the higher solubility of oxygen in α2 phase compared to γ phase. From our results it is shown unambiguously that the better ductility of (α2 + γ) two-phase alloys can no longer fit with the widespread idea of a scavenging effect of the α2 phase which would lower in that way the interstitial level in γ phase.

Phosphorus segregation in nanocrystalline Ni–3.6 at.% P alloy investigated with the tomographic atom probe (TAP)

Abstract The microstructures of electroless plated and thermally aged nanocrystalline nickel–3.6 at.% phosphorus layers were investigated on an atomic scale with a tomographic atom probe (TAP). After heat treatments at 250 and 400°C, a continuous P-segregation in the grain boundaries of the nanocrystalline structure was directly proved for the first time. This segregation effect explains the comparatively high thermal stability of the material. Assuming the existence of a metastable equilibrium, a simple mass balance calculation, which uses experimentally determined data exclusively, makes it possible to predict grain sizes of other NiP alloys within the thermal stability region.

A model accounting for spatial overlaps in 3D atom-probe microscopy

The spatial resolution of three-dimensional atom probe is known to be mainly controlled by the aberrations of ion trajectories near the specimen surface. An analytical model accounting for the spatial overlaps that occur near phase interfaces is described. This model makes it possible to correct the apparent composition of small spherical precipitates in order to determine the true composition. The prediction of the overlap rate as a function of the particle size was found in remarkably good agreement with the simulations of ion trajectories that were made. The thickness of the mixed zone around beta precipitates was found to be of 0.3 nm for a normalised evaporation field of beta phase of 0.8. Using simulations, the overlap rate could be parameterised as a function of the apparent atomic density observed in particles. This model has been applied to copper precipitation in FeCu.

Homogeneous Cu–Fe supersaturated solid solutions prepared by severe plastic deformation

A Cu–Fe nanocomposite containing 50 nm thick iron filaments dispersed in a copper matrix was processed by torsion under high pressure at various strain rates and temperatures. The resulting nanostructures were characterized by transmission electron microscopy, atom probe tomography (APT) and Mössbauer spectrometry. It is shown that α-Fe filaments are dissolved during severe plastic deformation leading to the formation of a homogeneous supersaturated solid solution of about 12 at% Fe in fcc Cu. The dissolution rate is proportional to the total plastic strain but is not very sensitive to strain rate. Similar results were found for samples processed at liquid nitrogen temperature. APT data revealed asymmetric composition gradients resulting from deformation-induced intermixing. On the basis of these experimental data, the formation of the supersaturated solid solutions is discussed.

Atom-probe investigations of TiAl alloys

Abstract Atom probe field ion microscopy (APFIM) and tomographic atom probe (TAP) have been used to study TiAl-based alloys. The element concentrations, the influence of additional elements such as Cr or Nb as well as the solubility of oxygen in α2 (Ti3Al) and γ (TiAl) phases in compounds with nominal concentration Ti54Al46 and Ti58Al42 have been determined. By using the detection of oxygen atoms as a very local probe, the present investigation revealed the existence of some intermediate phases during the phase transformation α→γ. The presence of the oxygen atoms during this transformation gives some peculiarities on the transformation path. The appearance of some metastable phases may be explained by the existence of the homologous series Ti2n−1Aln where n is an integer varying from 1 (stoichiometry TiAl) to ∞ (phase α3 Ti2Al).

Pinning points anchoring ordinary and shockley dislocations in TiAl alloys

Abstract Pinning points anchoring Shockley dislocations and ordinary dislocations in the γ phase of two TiAl alloys (a single-phase alloy and a lamellar two-phase alloy) are studied by in situ straining experiments performed inside a transmission electron microscope. The overcoming processes of these pinning points are observed. Their densities are measured and compared. The results indicate that the pinning points are extrinsic to the dislocations. The measured densities do not correspond to a homogeneous distribution of the individual oxygen atoms present in the crystal. It is proposed that they are due to the segregation of part of the interstitial oxygen atoms.

Three-dimensional atomic scale microscopy with the atom probe

A new type of high resolution nanoanalytical microscope, the three-dimensional atom-probe, has been recently developed. The tomographic atom-probe (TAP) developed in our laboratory provides three-dimensional maps of chemical heterogeneities in a metallic material on a near-atomic scale. The basic principle of this new generation apparatus relies on the field evaporation and ionisation of atoms from the material. Chemical species are identified by time-of-flight mass spectrometry. The position of atoms at the specimen surface is determined with the aid of a specially designed position-sensitive multidetector. In this paper the high spatial resolution of the TAP is illustrated through some metallurgical examples by studying the very early stages of unmixing and ordering reactions and the mapping of interfacial segregations.

3D reconstruction and analysis of GP zones in Al1.7Cu (at%): a tomographic atom probe investigation

The tomographic atom probe (TAP) has been used to analyse Guinier-Preston (GP) zones in an Al-1.7Cu (at%) single crystal alloy aged at 373 K for 30 h, known to contain a majority of monolayer GP(I) zones. This work is a challenge for the TAP because of the extreme thinness of the metallurgical objects to be analysed and reconstructed. The GP zones whose habit plane is parallel to the analysis direction are quite successfully reconstructed in the form of thin copper rich platelets. However, the expected single layer structure is not preserved, due to a local magnification effect. Despite the high depth resolution of the TAP, the investigation of the GP zones perpendicular to the analysis direction, that was carried out at 35 K, did not provide any direct quantitative data related to the real thickness and the composition of GP zones. The observed phenomenon can be interpreted in terms of preferential evaporation of Al and preferential retention of Cu atoms.

Atom-probe determination of interstitial element concentration in two-phase and single-phase TiAl-based alloys

Abstract Interstitial elements (O, C, N) seem to have a key role with respect to mechanical properties of TiAl-based alloys. This paper presents an atom-probe study of the distribution of interstitial elements in the α2 and γ phases of two-phase TiAl alloys. These elements are found to preferentially segregate in the α2 phase. This effect is particularly important for oxygen. Measurements of oxygen concentration in the γ phase of two-phase and of single-phase alloys were performed. This allows the maximum solubility of oxygen in the γ phase to be determined. The result shows that the poor low-temperature ductility of single-phase TiAl alloys cannot be simply due to the presence of oxygen in the γ phase.

Field evaporation behaviour in the γ phase in Ti–Al during analysis in the tomographic atom probe

A Ti-48 at% Al alloy has been successfully investigated, using atom probe field ion microscopy and transmission electron microscopy. After a specific heat treatment, this alloy has a (alpha2 + gamma) lamellar microstructure. Using the tomographic atom probe (TAP), it has been possible to image the stacking of superlattice planes of gamma and to identify titanium as the highest evaporation field element. In addition, the influence of analysis site on atom probe measurements has been estimated for this phase. A TAP analysis has also made it possible to observe an extremely thin step along a gamma/gamma interface at a near atomic scale.