Johan Angenete

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.

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.

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.

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.

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

The isothermal oxidation at 1050°C of one simple (PWA73) and three Pt-modified (RT22, SS82A and MDC150L) aluminide diffusion coatings, deposited on the same single crystalline Ni-base superalloy, CMSX-4, was investigated. The oxidation was studied by gravimetry, scanning-electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDS) and grazing angle X-ray diffraction (XRD). TEM samples were prepared by focused ion beam (FIB) milling. It was found that the oxide on the simple-aluminide coating grew much faster and started to spall much earlier than those on the Pt-modified coatings. This was related to the higher amount of other phases than α-Al2O3 in the oxide scale on the simple-aluminide coating. It was shown that the presence of Pt in the coating suppressed the formation of deleterious phases such as spinels in the oxide scale, but also that the surface morphology of the coating prior to oxidation plays an important role.

Combining Scanning Probe Microscopy and Transmission Electron Microscopy

This chapter is a review of an in situ method where a scanning probe microscope (SPM) has been combined with a transmission electron microscope (TEM). By inserting a miniaturized SPM inside a TEM, a large set of open problems can be addressed and, perhaps more importantly, one may start to think about experiments in a new kind of laboratory, an in situ TEM probing laboratory, where the TEM is transformed from a microscope for still images to a real-time local probing tool. In this method, called TEMSPM, the TEM is used for imaging and analysis of a sample and SPM tip, while the SPM is used for probing of electrical and mechanical properties or for local manipulation of the sample. This chapter covers both instrumental and applicational aspects of TEMSPM.

Microstructural studies of NiAl-based model alloys and commercial coatings after isothermal oxidation

Abstract Four commercial aluminide diffusion coatings (one Pt-free) and four polycrystalline NiAl model materials, isothermally oxidised in laboratory air at 1050°C for 1 hour, have been investigated. The coatings were deposited on a single crystal Ni-based superalloy, CMSX-4. Two of the model materials are binary alloys containing different Al amounts. The two other have the same Ni/Al ratio as the binary alloys but were alloyed with 4 at% Pt. In order to control the surface roughness, all materials were polished prior to oxidation. Microstructural studies have been conducted using X-ray diffraction and scanning electron microscopy. To get a deeper insight on the platinum influence on the oxide structure, two commercial coatings (one Pt free and the other modified with Pt) were also investigated using transmission electron microscopy. The results obtained are discussed according to the influence of Al and Pt contents in the materials.

Investigation of Oxide Formation during Short-Term Oxidation of Model NiAl Alloys

Two β-NiAl model materials, one stoichiometric (50 at% Al) and one with low Al (39 at%) content were oxidized at 1050 °C for 1 h. FIB cross sections were analyzed by SEM and TEM. Phase identification was also performed by XRD. Strong influence of metal grain orientation on the diffusion and oxidation was observed. The morphology of the scale surface was similar in both materials and consisted of regions with needle-like oxide and small smoother pits. TEM and SEM analyses showed that the pits correspond to the contact areas between the oxide and the underlying alloy. Moreover TEM investigations revealed that these contact areas consist of α-Al2O3, while the detached oxide scale contains mainly transient-Al2O3 phases. The measured oxide thickness was similar in both contact and detached areas. The observed voids at the metal side were much more faceted in the Al-poor material. Possible mechanisms responsible for these observations are discussed.

MEMS sensor for in situ TEM-nanoindentation with simultaneous force and current measurements

A capacitive force sensor for in situ transmission electron microscope (TEM)-nanoindentation with simultaneous force and current measurement has been developed. The sensor was fabricated using bulk micro machining methods such as deep reactive ion etch, thermal oxidation, metal deposition and anodic bonding. Two different geometries of the sensor were designed to allow in situ TEM electromechanical experiments in the most common TEM instruments. Electrical probing is enabled by an on-chip insulator, electrically separating the indenter tip and the capacitor used for force measurements. The sensor was designed for the force range of 0 to 4.5 mN. Finally, we demonstrate for the first time in situ TEM-nanoindentation with simultaneous force and current measurements.

SEM and TEM studies of PtAl diffusion coatings under isothermal oxidation

Abstract Three commercial Pt modified aluminide coatings (RT22, SS82A and MDC150L) and one conventional aluminide diffusion coating (PWA73), applied to the same substrate (CMSX4), were oxidised in still laboratory air at 1,050°C for times up to 2,000 h. The samples were investigated by gravimetry, scanning electron microscopy, (SEM), transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy (EDS). It was found that among the three Pt modified coatings, RT22 produced the fastest growing oxide and that at about 500 h, a decrease in growth rate occurred for all three coatings. The oxide on RT22 started to spall after 1,000 h, while the oxide on SS82A and MDC150L remained adherent. The conventional aluminide diffusion coating suffered from severe spalling after 1,000 h. Furthermore, results from microstructural and compositional measurements of the Pt modified aluminide coatings are presented.