Paul J. Warren

A procedure for quantification of precipitate microstructures from three-dimensional atom probe data.

New analysis software for selecting and quantifying particles in three-dimensional atom maps has been designed. The selection of solute-rich regions is performed by connecting solute atoms which lie within a fixed distance (d), and taking clusters above a certain minimum number of solute atoms (N(min)). Other atoms within some distance L greater than d are taken to belong to the cluster. However, this results in the inclusion of a shell of matrix atoms, which must be removed through an erosion step, to define the final cluster. Data filtered in this way can be used for subsequent quantification of parameters such as size, shape, composition, number density and volume fraction with better accuracy than by manual selection. The choice of d, N(min) and L values is discussed and some methods of evaluation of these parameters are proposed. Examples are presented on the application of this new software to the analysis of early stage clustering in an Al-Mg-Si-Cu alloy and a copper-containing steel.

Investigation of precipitation in an Al–Zn–Mg alloy after two-step ageing treatment at 100° and 150°C

Abstract Fine-scale precipitation of the metastable Zn- and Mg-rich η′ phase and its precursors is essential for the mechanical properties of Al–Zn–Mg alloys. However, at present neither the precipitation sequence nor the structure and composition of the intermediate precipitate phases are completely clear. This paper deals with an investigation of precipitation in an industrial Al–Zn–Mg alloy at various stages of a conventional two-step ageing treatment at 100° and 150°C. Studies were performed using both transmission electron microscopy and atom-probe field ion microscopy. Transmission electron microscopy (TEM) analysis revealed two parallel precipitation paths; one involving formation and dissolution of the ordered GP (I) zones, the other involving formation of clusters (type II), having a different atomic arrangement compared to the Al-matrix, which transform to the η′ phase. Atom-probe study of the material after short time ageing at 100°C did not show any observable distinction between GP (I) and type II precipitates. In the peak-aged material the best classification of precipitates was obtained using their morphology (the cigar-like and the plate-like) because there was significant overlap in the range of total solute contents of each type of precipitate. Generally the Zn:Mg ratio in all observed types of precipitates was close to 1:1 and the total solute atom content increased with ageing time. Distribution of alloying elements in the precipitates and in the surrounding matrix is discussed.

PERFORMANCE OF AN ENERGY-COMPENSATED THREE-DIMENSIONAL ATOM PROBE

A wide acceptance angle first-order reflectron lens has been incorporated into a three-dimensional atom probe (3DAP) to provide improved mass resolution. This new 3DAP instrument is capable of resolving isotopes in the mass spectrum, with resolutions better than m/Δm=500 full width at half maximum and 250 full width at 10% maximum. However, use of a reflectron for energy compensation within an imaging system means that improvements in mass resolution result in degradation of the spatial resolution. This article addresses the detailed design of the energy compensated 3DAP, and the minimization and compensation of chromatic aberrations in the imaging performance of the instrument. Some applications of the new instrument are included to illustrate its capabilities in the atomic-scale analysis of engineering alloys.

Mechanical properties of partially crystallized aluminum based amorphous alloys

Abstract Melt spun and heat treated Al-Ni-Y alloy ribbons have fine structures consisting of nanocrystalline Al particles or intermetallic phases embedded in an amorphous matrix. The strength (and hardness) changes according to the composition and microstructure from approximately 1000 MPa for the as melt spun amorphous ribbon to 1500 MPa after heat treatment. In order to describe this high strength and strength change after heat treatment, a mixture model based on the volume fraction of amorphous matrix and Al particles is proposed. The nano-sized particles are treated as perfect materials and the matrix is treated as amorphous material, in which the solute concentration increases as the volume fraction of Al particles increases. The calculated results for various compositions and heat treated specimens are compared with experimental data in literature.

Partitioning of Co during crystallisation of Fe-Co-Nb-B(-Cu) amorphous alloys

Partitioning behaviour of alloying elements during the primary crystallisation of both Fe 18 Co 60 Nb 6 B 16 and Fe 19 Co 39 Nb 6 B 15 Cu 1 alloys has been studied using three-dimensional atom probe (3DAP). Cu rich clusters were found in the Cu-containing alloy, and they provided nucleation sites for the primary α-Fe(Co) particles. In the Cu clusters, concentration of both Fe and Co was greatly decreased. In both Cu-containing and Cu-free alloys Nb and B were depleted in the α-Fe(Co) particles and enriched in the remaining amorphous phase. However, Co concentration in the α-Fe(Co) was the same as the remaining amorphous phase. Based on the 3DAP analysis, the phase volume fraction has been estimated.

Crystallization behaviour of amorphous Al85Y11Ni4 alloy

Abstract The crystallization behaviour and microstructural development during the annealing of amorphous Al85Y11Ni4 ribbons has been investigated by a combination of differential scanning calorimetry, transmission electron microscopy and X-ray diffractometry. Crystallization of amorphous Al85Y11Ni4 during continuous heating takes place in three stages, with the crystallization temperature increasing with increasing heating rate. Activation energies for the three crystallization peaks determined by the Kissinger and Ozawa methods give good agreement. Isothermal annealing at temperatures in the range 511–523 K shows Johnson-Mehl-Avrami kinetics with an Avrami exponent of 2.0–2.35, in agreement with suggested linear nucleation and three-dimensional diffusion-controlled parabolic growth of dendritic particles.

Metallic glass formation in multicomponent (Ti, Zr, Hf, Nb)–(Ni, Cu, Ag)–Al alloys

Abstract A wide range of novel multicomponent amorphous alloys have been manufactured by a new method of equiatomic substitution for the early and late transition metals in Zr-based amorphous alloys. (Ti 33 Zr 33 Hf 33 ) 90− x (Ni 50 Cu 50 ) x Al 10 , (Ti 33 Zr 33 Hf 33 ) 90− x (Ni 33 Cu 33 Ag 33 ) x Al 10 , (Ti 25 Zr 25 Hf 25 Nb 25 ) 90− x (Ni 50 Cu 50 ) x Al 10 and (Ti 25 Zr 25 Hf 25 Nb 25 ) 90− x (Ni 33 Cu 33 Ag 33 ) x Al 10 alloys with composition range x =20–70 at.% have been prepared by melt-spinning and the range of glass formation characterized by X-ray diffraction and differential scanning calorimetry. Amorphous alloys were formed over the composition range x =20–70 at.% for the (Ti 33 Zr 33 Hf 33 ) 90− x (Ni 50 Cu 50 ) x Al 10 and (Ti 25 Zr 25 Hf 25 Nb 25 ) 90− x (Ni 50 Cu 50 ) x Al 10 alloys. Addition of Nb with a higher melting point than Ti, Zr and Hf increased the thermal stability of the amorphous phase for the whole composition range x =20–70 at.%. The most stable amorphous alloy was (Ti 33 Zr 33 Hf 33 ) 40 (Ni 50 Cu 50 ) 50 Al 10 with a crystallisation temperature of T x =545 °C. Addition of Ag decreased the composition range of the amorphous phase to x =20–40 at.% for the (Ti 33 Zr 33 Hf 33 ) 90− x (Ni 33 Cu 33 Ag 33 ) x Al 10 and (Ti 25 Zr 25 Hf 25 Nb 25 ) 90− x (Ni 33 Cu 33 Ag 33 ) x Al 10 alloys. However the amorphous alloy with the largest supercooled liquid region was (Ti 33 Zr 33 Hf 33 ) 50 (Ni 33 Cu 33 Ag 33 ) 40 Al 10 with a crystallisation–glass transition temperature difference of T x − T g =103 °C.

Observation of atomic planes in 3DAP analysis

Abstract Three-dimensional atom probe (3DAP) reconstruction of a tungsten specimen containing grain boundaries has revealed a wide range of atomic planes. It has been found that planes are visible even for crystallographic poles up to 45° away from the surface normal. These results suggest that, at least for pure elements, it may be possible to reconstruct the atomic lattice originally present in the material using 3DAP techniques.

Lateral and depth scale calibration of the position sensitive atom probe

Abstract In this paper a method is outlined to calibrate both the lateral and depth scales using a combination of FIM and PoSAP microanalysis. Results are given for the following alloy systems: Fe-45at%Cr, Cu-1at%Co, Al-3at%Zn-3 at%Mg-1at%Cu and Fe-1.3wt%Cu-1.4wt%Ni. For each system a direct relationship between the applied voltage and tip radius was found from indexed FIM images and estimates of the image compression factor. The reconstructed PoSAP data was divided into a series of frames in which each represented a fraction of an atomic layer evaporated. Individual rings were seen to collapse inwards when the frames were displayed in succession. The visual effect is similar to watching ring collapse in a FIM image when DC field evaporation occurs. A depth scale calibration was determined directly from the number of ions detected per plane evaporated. The lateral resolution of the wedge and strip anode was analysed by placing a thin mask, consisting of a series of small circular apertures, arranged in a concentric ring pattern, in front of the double channel plate assembly. A series of FIM images was generated and compared with a simulation of the expected distribution of ion impacts. In order to simulate a uniform detector resolution a Gaussian scatter was added to each coordinate in the x and y directions of the simulated impact positions. A comparison between the observed impact positions and simulations showed that approximately 96% of observations would be located to within one atomic spacing during a typical PoSAP experiment.

Crystallization behaviour in a new multicomponent Ti16.6Zr16.6Hf16.6Ni20Cu20Al10 metallic glass developed by the equiatomic substitution technique

A new amorphous Ti16.6Zr16.6Hf16.6Ni20Cu20A110 alloy has been developed using the novel equiatomic substitution technique. Melt spinning Ti16.6Zr16.6Hf16.6Ni20Cu20A110 forms an amorphous phase with a large supercooled liquid region, ΔT=70°C. After isothermal annealing within the supercooled liquid region for 3 h at 470°C, the amorphous alloy crystallizes to form a fine-scale distribution of 2–5 nm nanocrystals, and the supercooled liquid region increases to ΔT=108°C. Atomic-scale compositional analysis of this partially crystalline material using a three-dimensional atom probe (3DAP) is unable to detect any compositional difference between the nanocrystals and the remaining amorphous phase. After annealing for 1 hr at 620°C, the amorphous alloy crystallizes to form 20–50nm equiaxed grains of a hexagonal-type C14 Laves phase with lattice parameters a = 5.2Å and c = 9.0 Å. 3DAP analysis shows that this Laves phase has a composition very close to that of the initial amorphous phase, suggesting that the alloy crystallizes via a polymorphic rather than a primary crystallization mechanism, despite the complexity of the alloy composition.