Takeshi Sano

Electron microscopy study of Ni/Ni3Al diffusion-couple interface. II: Diffusivity measurement

Abstract Interdiffusivities have been determined from compositional variations measured across the Ni/Ni 3 Al diffusion-couple interface with an analytical electron microscope (AEM) and an electron probe microanalyser (EPMA). The AEM was used for measurement at lower temperatures and the EPMA was used at higher temperatures. The Boltzmann-Matano technique has been applied to the determination of interdiffusivities in the Ni-rich disordered phase and those in the Ni 3 Al ordered phase. The interdiffusivities obtained in this study are compared with those reported in the literature including self-diffusivities and impurity diffusivities. It is shown that the present interdiffusivities are consistent with those predicted from the modified Darkens equation.

Structure of perovskite carbide and nitride precipitates in L10-ordered TiAl

Abstract The crystallographic structures of carbide and nitride formed in TiAl containing carbon and nitrogen have been investigated using transmission electron microscopy. The needle-shaped carbide and nitride are formed parallel to only the [001] direction of the L10-ordered TiAl matrix. Detailed analyses in terms of selected area electron diffraction and dark-field electron microscopy revealed that the needle-shaped carbide and nitride have a crystal structure of perovskite type and have long-range carbon-vacancy and nitrogen-vacancy ordering below a critical temperature. The domain of the long-range order rotates periodically about the axis of the needle by 90[ddot] resulting in the repeated formation of antiphase boundaries along the needle. At higher temperatures, long-range ordering disappears indicating uniform distribution of carbon and nitrogen in the needle-shaped perovskite carbide and nitride.

An extrapolation method for the determination of Cliff-Lorimer kAB factors at zero foil thickness

An extrapolation method is proposed to be useful for the determination of the Cliff‐Lorimer kAB factor at zero foil thickness. The method consists of measuring kAB factors as a function of the measured foil thickness, tM, and extrapolating the relationship toward tM=0. The intersection between the extrapolated line and the ordinate of tM=0 gives (kAB)0 which is free from the effect of absorption. The straight line extrapolation that can be achieved by a linear‐least squares method is particularly developed to eliminate arbitrariness introduced in the extrapolation process.

Applicability of the differential X-ray absorption method to the determinations of foil thickness and local composition in the analytical electron microscope

Abstract In this paper, we examine the applicability of the differential X-ray absorption (DXA) method of Morris, Ball and Statham which is capable of determining foil thickness and local composition concurrently in an analytical electron microscope. The method is first modified to take a general form where the thickness determination is achieved with a pair of characteristic lines from different elements. When this form is used, the pair is not necessarily limited from the same elements. It is shown then that there are four important factors affecting the precision and accuracy of the foil thickness. A comparison is carried out between the thicknesses determined with the DXA method and those measured with the convergent-beam electron diffraction method. The composition analysed with the DXA method is also compared with the nominal composition of the sample.

Hardening of ordered γ′-Ni3(Al, Ti) by precipitation of disordered γ

Abstract 1. (1) A uniform solid solution of γ′-L1 2 ordered Ni 3 (Al,Ti) supersaturated with disordered γ can be obtained by solution annealing at a suitable temperature. 2. (2) The initial shape of γ precipitated from supersaturated γ′ on aging is spherical or round-cubic. The γ precipitates grow into platelets as aging proceeds. The γ-precipitates are completely coherent with the matrix γ′ as long as the γ-precipitates are plates. 3. (3) The γ′-L1 2 ordered Ni 3 (Al, Ti) hardens appreciably by fine precipitation of disordered γ.

A new form of the extrapolation method for absorption correction in quantitative X-ray microanalysis with the analytical electron microscope

Abstract The extrapolation method may be used to correct X-ray absorption without knowledge of thickness in quantitative microanalysis with the analytical electron microscope. This paper presents a new form of the method which permits the analysis of a region equivalent to a focused beam size or of a specimen with a uniform thickness. The development of the method is based on tilting the specimen and introducing a geometrical factor into the earlier form. The new form is general because it includes not only the data for different thicknesses as used in the earlier form but also the data at different tilt angles. The applicability of the new form is demonstrated with reference to experimental data for a Ni-Al-Ta alloy and of Maher et al. [in: Analytical Electron Microscopy - 1981, p. 29].

Microstructure of precipitation strengthened Ni3Al and TiAl

Abstract Structural studies have been performed on precipitation hardening found in Ni 3 Al- and TiAl-based ordered alloys using transmission electron microscopy. In an L1 2 -ordered Ni 3 Al alloy with appropriate titanium content, fine precipitates of coherent disordered γ are formed during aging. The γ precipitates are initially spherical or rounded cubic in shape and grow into platelets as aging proceeds. Loss of coherency is initiated by the introduction of dislocations at the γ/γ′ interface and results in step formation at the dislocations. The γ precipitates become globular after the loss of coherency. In the alloys with 4 mol.% of chromium and 0.2–0.5 mol.% of carbon, fine octahedral precipitates of M 23 C 6 type carbide, which has the cube-cube orientation relationship with the matrix, appear during aging. In the L1 0 -ordered TiAl containing 0.1–2.0 mol.% of carbon or nitrogen, fine needle-shaped Ti 3 AlC and Ti 3 AlN of perovskite type are formed only along the [001] direction of the L1 0 matrix during aging. In the γ′ phase hardened by the precipitation of disordered γ phase, dislocations are attracted into the disordered γ phase and cut through the particles during deformation at any stage of aging. In the alloys containing fine dispersion of carbide and nitride particles, different behavior of dislocations has been observed.

Delocalization corrections using a disordered structure for atom location by channelling-enhanced microanalysis in the Ni-Al system

Abstract A correction procedure for the delocalization effect when determining the site occupancy of an impurity element by atom location by channelling-enhanced microanalysis is proposed. The procedure utilizes a disordered structure with a composition similar to the ordered structure. Correction factors are derived by illuminating a sample with a disordered structure in the same orientation as for the ordered structure. The correction procedure is applied to a determination of the Ti occupancy in a Ni3Al intermetallic compound.

Application of analytical electron microscopy to diffusivity measurement requiring X-ray absorption correction

Abstract X-ray microanalysis using an analytical electron microscope is applied to diffusivity measurements in an alloy system where the X-ray absorption correction is imperative. It is shown that the differential X-ray absorption (DXA) method is useful for the absorption correction in the determination of a concentration gradient across a diffusion couple interface.

An application of the differential X-ray absorption method to thickness measurement in precipitate-containing Ni3(Al, Ti) and TiAl compounds

Abstract The differential X-ray absorption (DXA) method is applied to the thickness measurement of the two types of intermetallic compound, Ni3(Al, Ti) and TiAl, containing finely dispersed precipitate particles. It is shown that the DXA method is applicable even in the presence of particles with coherent strain fields where it is not possible to use the convergent-beam electron diffraction method. It is proposed that the absorption-free intensity ratio required for the DX A method can be determined using the extrapolation method without knowledge of specimen compositions.