Krystyna Stiller

The precipitation sequence in Al–Mg–Si alloys

Fine-scale precipitation that occurs during age hardening of Al alloy 6061 has been studied using differential scanning calorimetry (DSC), atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). It was found that the precipitation sequence is: independent clusters of Mg and Si atoms {yields} co-clusters that contain Mg and Si atoms {yields} small precipitates of unknown structure {yields} {beta}{double_prime} needle-shaped precipitates {yields} B{prime} lath-shaped precipitates and {beta}{prime} rod-shaped precipitates. A new structure is proposed for the {beta}{double_prime} precipitate. It was found that the Mg:Si ratio in the intermediate precipitates and co-clusters was close to 1:1.

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.

High resolution microanalytical study of precipitation in a powder metallurgical high speed steel

Abstract The microstructure of a powder metallurgical high speed steel (ASP 60) has been investigated by combined analytical electron microscopy and atom-probe field-ion microscopy. The compositions of MC and M 6 C primary precipitates were determined. The tempering reaction at 560°C was studied in detail and it was shown that extremely fine M 2 C precipitates dominated at optimum hardness (3 h). Some fine MC precipitates were also present. The compositions of the secondary carbide precipitates were determined by atom-probe analysis. Overageing at 700°C resulted in the transformation of the fine secondary carbide dispersion to coarse M 6 C and M 23 C 6 and a change in the matrix morphology from martensitic plates to equiaxed recovered ferrite subgrains.

Comparison of inward and outward grown Pt modified aluminide diffusion coatings on a Ni based single crystal superalloy

Two commercial Pt modified aluminide coatings (RT22 and MDC150L) on the same single crystal Ni-based superalloy (CMSX-4) were studied by: scanning electron microscopy; transmission electron microscopy; energy dispersive X-ray spectrometry; and gravimetry. The RT22 coating is an inward grown coating (high activity), while MDC150L is produced by outward growth (low activity). Samples were oxidised in still laboratory air at 1050 °C for various times up to 2000 h. It was found that the outward grown coating produced a slower growing oxide that was more spallation resistant. Several possible reasons for this were identified including: coating purity; coating surface topography; and ductile to brittle transition temperature. The microstructural differences between the two coatings in the as-coated condition were investigated and the development of their microstructure during heat treatment was described. A model for coating growth during heat treatment was proposed.

Quantitative atom probe analysis of carbides

Compared to atom probe analysis of metallic materials, the analysis of carbide phases results in an enhanced formation of molecular ions and multiple events. In addition, many multiple events appear to consist of two or more ions originating from adjacent sites in the material. Due to limitations of the ion detectors measurements generally underestimate the carbon concentration. Analyses using laser-pulsed atom probe tomography have been performed on SiC, WC, Ti(C,N) and Ti(2)AlC grains in different materials as well as on large M(23)C(6) precipitates in steel. Using standard evaluation methods, the obtained carbon concentration was 6-24% lower than expected from the known stoichiometry. The results improved remarkably by using only the (13)C isotope, and calculating the concentration of (12)C from the natural isotope abundance. This confirms that the main reason for obtaining a too low carbon concentration is the dead time of the detector, mainly affecting carbon since it is more frequently evaporated as multiple ions. In the case of Ti(C,N) and Ti(2)AlC an additional difficulty arises from the overlap between C(2)(+), C(4)(2+) and Ti(2+) at the mass-to-charge 24 Da.

A comparative study of two inward grown Pt modified Al diffusion coatings on a single crystal Ni base superalloy

Abstract Two inward grown Pt modified aluminide diffusion coatings (RT22 and SS82A) on the same Ni-based superalloy substrate (CMSX-4) have been studied. The specimens were oxidised in still laboratory air at 1050°C for times up to 2000 h and investigated by gravimetry. Specimens oxidised up to 500 h were studied by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDS). The gravimetric investigations revealed that the oxide on RT22 grew faster than the oxide on SS82A. Moreover, it was found that in the as-coated condition, both coatings contained precipitates of α-W, α-Cr, μ-phase and σ-phase. After short time heat treatment at 1050°C, only μ-phase precipitates were present. The volume fraction of the precipitates was measured and compared for different heat treatments. Compositional profiles across the coatings were acquired by TEM-EDS before and after heat treatment. They showed that considerable redistribution of elements takes place during oxidation and the initially steep Pt and Al profiles became smeared out. The RT22 coating formed γ′-Ni3Al grains at the coating/oxide interface.

Structure, chemistry, and stress corrosion cracking of grain boundaries in alloys 600 and 690

The microstructure in six commercial batches of alloys 600 and 690 has been investigated using scanning electron microscopy (SEM), analytical transmission electron microscopy (ATEM), atom probe field ion microscopy (APFIM), and secondary ion mass spectroscopy (SIMS). The materials were also tested with respect to their resistance to intergranular stress corrosion cracking (IGSCC) in high-purity water at 365 °. Applied microanalytical techniques allowed direct measurement of carbon concentration in the matrix together with determination of grain boundary micro structure and microchemistry in all material conditions. The distribution of oxygen near a crack in material tested with respect to IGSCC was also investigated. The role of carbon and chromium and intergranular precipitates on IGSCC is discussed.

APFIM investigation of fine-scale precipitation in aluminium alloy 6061

The results of an APFIM investigation of precipitation during ageing of aluminium alloy 6061 at 70°C and 175°C are presented. The first stage of precipitation was found to involve the separate clustering of Si atoms and Mg atoms. This was closely followed by movement of Mg atoms to the Si clusters. The Mg:Si ratio in these small precipitates increased slowly during ageing at 70°C and more rapidly during ageing at 175°C, with most precipitates attaining a Mg:Si ratio of close to one at both temperatures. A needle-shaped precipitate formed during ageing for 24 h at 175°C was also found to have a Mg:Si ratio of close to one.

Oxidation of Simple and Pt-Modified Aluminide Diffusion Coatings on Ni-Base Superalloys—II. Oxide Scale Failure

The spallation behavior of oxides formed during isothermal oxidation at 1050°C of one simple (PWA73) and three Pt-modified (RT22, SS82A and MDC150L) aluminide diffusion coatings on the same Ni-base, single-crystalline superalloy (CMSX-4) was investigated by scanning electron microscopy (SEM) and transmission-electron microscopy (TEM). It was found that the main spallation mechanism was the formation of large Kirkendall voids at the oxide–coating interface. It is believed that the void formation was caused by counter-current flow of vacancies to the diffusion of Ni away from the interface as Al is consumed by the oxide. The magnitude of the vacancy current was determined by the oxidation rate. The properties of the void-formation mechanism are discussed in view of previous data on the microstructure of the oxide scales.

Microstructure of a boron containing high purity nickel-based alloy 690

Abstract A high purity model alloy with major composition Ni–30Cr–10Fe–0.024C–0.006B (wt.%), corresponding to the commercial Alloy 690, was investigated using secondary ion mass spectroscopy, atom probe analysis and transmission electron microscopy in combination with energy dispersive X-ray analysis and electron energy loss spectroscopy. Solution annealing at 1200°C for 20 min put most carbon and boron into solution, but some intergranular borides and carbides, probably formed during quenching, were observed. Heat treatment at 700°C for 1 and 100 h resulted in substantial intergranular precipitation of Cr 23 C 6 and Cr 2 B. The influence of boron on the precipitation process in Alloy 690 is discussed and compared with precipitation in Alloy 600.