Tetragonal magnetostriction (λγ,2) and elastic constants (c′, c44, and c11) for Fe100−xSixwere measured as a function of temperature (T). Compositions corresponding to the disordered A2 (x = 5), ordered D03 (x = 19.8), and mixed (x = 11.6) phases, were investigated. The magnetoelastic coupling (−b1) was determined for 77 < T < 300 K and compared with those of Fe-Ga, Fe-Ge, and Fe-Al. Both λγ,2(T) and −b1(T) of Fe-Si behave similarly to those of Fe-Ge, while other notable differences exist between the measured properties of Fe-Si and those of the other three alloys. Due to the early establishment of short range order, Fe-Si exhibits a positive, although small, slope in λγ,2(T) at 5 at. % Si, and a remarkable drop in −b1 before the solubility limit. The weaker softening of the tetragonal shear modulus with the addition of Si and the lack of strong anharmonic effects in the Fe-Si lattice inferred from the weak T-dependence of all the moduli suggest that FeSi exhibits more structural stability than the other three alloys. The distinctive behavior is likely due to the smaller size of Si compared to the sizes of Ga, Ge and Al, and therefore to the effect of the larger size difference between Fe and Si in the Fe-Si lattice.

Significant technological advances in scanning electron microscopy (SEM) have been achieved over the past years. Different SEMs can have significant differences in functionality and performance. This work presents the perspectives on selecting an SEM for research on bulk inorganic materials. Understanding materials demands quantitative composition and orientation information, and informative and interpretable images that reveal subtle differences in chemistry, orientation/structure, topography, and electronic structure. The capability to yield informative and interpretable images with high signal-to-noise ratios and spatial resolutions is an overall result of the SEM system as a whole, from the electron optical column to the detection system. The electron optical column determines probe performance. The roles of the detection system are to capture, filter or discriminate, and convert signal electrons to imaging information. The capability to control practical operating parameters including electron probe size and current, acceleration voltage or landing voltage, working distance, detector selection, and signal filtration is inherently determined by the SEM itself. As a platform for various accessories, e.g. an energy-dispersive spectrometer and an electron backscatter diffraction detector, the properties of the electron optical column, specimen chamber, and stage greatly affect the performance of accessories. Ease-of-use and ease-of-maintenance are of practical importance. It is practically important to select appropriate test specimens, design suitable imaging conditions, and analyze the specimen chamber geometry and dimensions to assess the overall functionality and performance of an SEM. For an SEM that is controlled/operated with a computer, the stable software and user-friendly interface significantly improve the usability of the SEM. SCANNING 38:864-879, 2016. © 2016 Wiley Periodicals, Inc.

In this article, we study the relationship between precipitate morphology and superconductivity in KxFe1.6+ySe2 single crystals grown by self-flux method. Scanning electron microscopy (SEM) measurements revealed that the superconducting phase forms a network in the samples quenched above iron vacancy order-disorder transition temperature Ts, whereas it aggregates into micrometer-sized rectangular bars and aligns as disconnected chains in the furnace-cooled samples.

A rare-earth supply crisis has stimulated an intensive search for alternative permanent magnets. Alnico materials, alloys containing Al, Ni, Co and Fe, are functional nanostructured alloys, which show great potential for replacing the best commercial Nd-based rare-earth alloys for applications above 200°C. However, their coercivity is ∼2–3× below theoretical limits. The coercivity of alnico depends on the nanostructure developed during spinodal decomposition. In this work, atom probe tomography, combined with advanced electron microcopy, indicate that the microstructure of alnico is sensitive to the introduction of alloying elements such as Ti and Cu, as well as the crystallographic orientation of the parent phase with respect to the direction of the imposed magnetic field during spinodal decomposition. The alnico coercivity mechanism involves interplay of size, chemistry and possibly stress at interfaces. Control of these parameters should allow reduction of the spatial dimension of the FeCo-rich precipitates and the interaction between them, which should in term increase the coercivity of alnico alloys.

Insight into microstructural features in commercial alnico alloys is revealed through electron microscopy. Significant differences were found in the chemical make-up of the primary phases and their distributions for the different classes of the alloys. Changes in the Co + Fe : Al + Ni and minor alloying elements shift the matrix phase from an ordered B2 phase found in alnico 5-7 to a L21 phase found in alnico 8 and 9. The question whether the FeCo-rich phase has a B2 or body-centered cubic structure remains open. The grain-aligned 5-7 alloy shows a well ordered array of prismatic cells of the FeCo-rich phase that is 20-40 nm in width, 100's nm in length and epitaxial with the B2 matrix. Cu is uniformly distributed in the matrix phase for alnico 5-7 whereas for alnico 8 and 9, pure Cu precipitates locate along the phase boundaries. Ti-rich precipitates and non-spinodal FeCo-rich precipitates are observed.

Tetragonal magnetostriction (λγ,2) and elastic constants (c′, c44, and c11) for Fe100−xSix were measured as a function of temperature (T). Compositions corresponding to the disordered A2 (x = 5), ordered D03 (x = 19.8), and mixed (x = 11.6) phases, were investigated. The magnetoelastic coupling (−b1) was determined for 77 < T < 300 K and compared with those of Fe-Ga, Fe-Ge, and Fe-Al. Both λγ,2(T) and −b1(T) of Fe-Si behave similarly to those of Fe-Ge, while other notable differences exist between the measured properties of Fe-Si and those of the other three alloys. Due to the early establishment of short range order, Fe-Si exhibits a positive, although small, slope in λγ,2(T) at 5 at. % Si, and a remarkable drop in −b1 before the solubility limit. The weaker softening of the tetragonal shear modulus with the addition of Si and the lack of strong anharmonic effects in the Fe-Si lattice inferred from the weak T-dependence of all the moduli suggest that Fe-Si exhibits more structural stability than the other three alloys. The distinctive behavior is likely due to the smaller size of Si compared to the sizes of Ga, Ge and Al, and therefore to the effect of the larger size difference between Fe and Si in the Fe-Si lattice.

For Fe-Ga single crystals, faster cooling rates achieved by metallic quench and ice-water quench extended the tetragonal magnetostriction constant, (3/2) lambda(100), to higher Ga contents than reported previously. The maximum magnetostriction measured was 420 ppm at a composition of 21.4 at.% Ga. Using these faster cooling rates, the formation of short-range order (SRO) was obviously suppressed in A2 single phase, while this change in SRO did not significantly affect (3/2) (lambda 100). (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We previously showed that extended 4-week anneals at 1223 K were required to achieve structural and chemical homogeneity in the off-stoichiometric Ni–Mn–Sn Heusler alloy solid solution. To insure that the transition from the high temperature B2 to the room temperature L21 phase was complete, we performed low temperature (773 K) anneals, ∼150 K below the reported B2 → L21 transition temperature, on the 1223 K homogenized samples. Drastic changes in the magnetization measurements of the annealed samples suggested that the intermediate compositions of the L21 phase were metastable. TEM and XRD analyses confirmed the decomposition of the single-phase Heusler alloys into two phases with compositions close to Ni54Mn45Sn1 and Ni50Mn30Sn20. The observed phase decomposition indicated that the off-stoichiometric Ni–Mn–Sn Heusler alloys, that feature martensitic transformations, are metastable at 773 K.

Fe–Ga alloys are rapidly oxidized when exposed in air, forming both amorphous and crystalline surface oxides. These oxides hinder the observation of the ordered phases of B2 and D03 in Fe–Ga alloys by dark-field imaging and high-resolution imaging of transmission electron microscopy (TEM) techniques. Proper imaging techniques and reduction of surface oxides are necessary to obtain representative microstructural features to the bulk alloys by TEM.

The tetragonal magnetostriction constant, (3/2)λ100, of Fe–Si single crystals was measured and was found to be structure dependent. Similar to that of Fe–Ge single crystals, (3/2)λ100 is positive in the single phase A2 regime, becomes negative in the single phase D03 regime, and changes from positive to negative between the two regimes. Short-range order in the A2 regime decreases the magnetostriction prior to the onset of long range order. In the single phase regions of both A2 and D03, thermal history does not show any obvious effect on the magnetostriction, contrary to that found for Fe–Ga alloys. However, in the regions of phase mixture involving A2, B2, and D03 phases, quenching pushes the change in magnetostriction from positive to negative to higher Si contents.

The magnetostrictive atomic strain in a pure Fe single crystal was measured by differential x-ray absorption spectroscopy. The obtained tetragonal magnetostriction constant, (3/2)λ100, was determined to be 45 ppm, consistent with the previously reported theoretical value calculated from a spin-orbit coupling theory. These results provide a foundation for understanding the origin of magnetostriction in pure Fe as well as Fe-based binary alloys.

The origin of the induced magnetic anisotropy in stress-annealed single crystalline Fe 80.5 Ga 19.5 was investigated by high energy X-ray diffuse scattering. A compressive stress was applied along [010] during annealing. Superlattice reflections associated with D0 3 chemical order were analyzed to clarify the origin of the stress-annealing-induced magnetic anisotropy. Results showed that the D0 3 precipitates in the microstructure exhibited long range ordering of Ga. These clusters were found not to have a crystallographic preference relative to the applied stress axis and no structural anisotropy could be detected. The results indicate that the induced anisotropy following stress annealing likely comes from the anisotropy of the magnetic domain alignment and is not related to the underlying microstructural features of the alloy.

This work presents detailed information of phase identification of a single-crystalline Fe 25at.%,. Ga quenched from 1000 degrees C by transmission electron microscopy. The alloy was round to contain A2, B2 and D0(3), phases. Technical difficulties of the phase identification and their solutions are discussed. The discussion can also be applied to other alloys with similar assemblages of phases. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Magnetic and structural transitions in the Ni50Mn50-xSnx (x=10-25) ferromagnetic shape memory alloys are currently of interest. As in Ni-Mn-Ga, these alloys feature high-temperature austenite and low-temperature martensite phases, where the magnetic state is strongly composition dependent. To study the role of chemical ordering in fine-tuning their magnetostructural properties, they were first annealed for 4 weeks/1223 K to achieve structural and compositional homogeneity, and were then further annealed for 1 week (similar to 150 K below the reported B2 to L2(1) transition) at 773 K to increase the degree of chemical ordering. For x=11, this anneal resulted in a dramatic change in the magnetic ordering temperature. Following the 1223 K anneal, the sample exhibited ferromagnetic ordering at 140 K. After the 773 K anneal, the ferromagnetic transition is at 350 K, a characteristic of the ferromagnetic austenite phase with 15<x<25. Consistent with the magnetization data, transmission electron microscopy examination confirms that the alloy decomposed into two phases with x=20 and 1. From this result one can conclude that the martensitic transformation occurs only in those compositions where the single phase L2(1) has been retained in a metastable state on cooling. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3067855]

The underlying relation between the magnetostriction (3/2) λ100 and Ga concentration for Fe–Ga alloys is an open question. This work presents a systematic structural study of the structure-property dependence of Fe–Ga alloys by transmission electron microscopy and X-ray diffraction. Results show that in the regions where monotonic increases in (3/2) λ100 are exhibited, single-phase A2 or D03 is found for the slow-cooled alloys. For the alloys in the range 18–21at.% Ga, quenching extends the single-phase A2 to higher Ga concentrations, thereby continuing to enhance magnetostriction. The sudden decrease in (3/2) λ100 near Fe–19at.% Ga as well as Fe–29at.% Ga was associated with the formation of phase mixtures. For quenched alloys between 25 and 29at.% Ga, a phase mixture of (A2+B2+D03) was found, but the presence of this phase mixture does not seem to have a large effect on magnetostriction.

Lorentz microscopy was applied to the observation of magnetic domains in iron-gallium (Fe–Ga) alloys. Results did not show any link between the magnetic domains and the magnetostriction enhancement by Ga addition, but did reveal that the drastic decrease in magnetostriction for Fe–31.2 at. % Ga was due to the presence of large scale precipitates. Magnetic domain features did not change in the alloys of A2, D03, A2+D03, A2+B2+D03, and A2+fine scale precipitates. Large scale precipitates within the slow-cooled Fe–31.2 at. % Ga affected both the distribution and wall motion of magnetic domains.

This work describes a rapid method to correct the two-fold astigmatism of transmission electron microscope (TEM) objective lens employing caustic curve when no objective aperture is inserted. The method makes use of rounding the caustic curve via the objective lens stigmators after the condenser lens astigmatism has been corrected. It has many advantages over other methods, it is fast, straightforward, and does not need holes or an amorphous material.

The addition of nonmagnetic Ga into body-centered cubic Fe enhances the magnetostriction constant λ100 over tenfold. Literature reports for substitution of Ge at low concentrations suggest that the addition of Ge also enhances the magnetostriction. In this work, the magnetostriction and microstructure of Fe–Ge were investigated to correlate magnetostriction with microstructure. The magnetostriction of Fe100−xGex single crystals with x between 0.05 and 0.18 varies with Ge concentration and correlates with phase changes. The value of (3∕2)λ100 increases with Ge additions in the A2 single phase region (up to x∼10), reaching a maximum of 94ppm at the solubility limit of the disordered A2 phase. Further increases in Ge in the A2+D03 two-phase region (1216, magnetostriction remains negative with an absolute value of strain of 129ppm at 18at.% Ge. This behavior is similar to that observed for Fe–Si alloys.