Carbon doping is studied in MgB2 pellets during one-step synthesis by solid-state reaction, employing both undoped and carbon-doped boron with and without the addition of nano-SiC. The phase formation during the synthesis as a function of time was followed using powder X-ray diffraction and Rietveld refinement. The superconducting properties were characterized with a magnetometer to investigate doping-induced changes. Mg(B1-xCx)2 is obtained with nano-precipitates and different compositions depending on the synthesis temperature. It is found that the addition of nano-SiC prevents the phase formation at low temperature (700°C). Nevertheless, the best superconducting properties are obtained for the sample treated at 900°C using simultaneously C and SiC, with a critical current density of 105 A cm-2 at 3 T and 20 K, named the 900-20-C-nanoSiC sample.

No uniform rolling deformation produces shear strains that give rise to textural and microstructural heterogeneities in processed metals and alloys. In this work, the authors investigate Fe–30Mn–4Si shape memory alloy sheets rolled in different conditions at 600 °C, in order to determine the process giving rise to the best structure and the strongest {100}<110> shear texture. This crystallographic orientation is the most favorable for the γ → ϵ martensitic transformation, which provides the shape memory effect in these alloys. In the current conditions, the authors find that unidirectional rolling produces a shear texture in sheet's surface layers. The authors compare the texture and microstructure from this process to those obtained from reverse rolling and single‐roller drive rolling.

A device was designed to apply the directional recrystallization method to Fe-based alloys in order to obtain bamboo-like microstructures. This microstructure is suitable for improving creep properties and resistance to fatigue in some alloys and for enhancing pseudoelastic properties in shape memory alloys. The design and construction of a flat coil are described in detail. In addition, we developed an electromechanical system to control the movement of a wire within the flat coil. The construction details and system performance are presented. Furthermore, metallographic studies taken from the directionally recrystallized low-carbon steel samples are shown. Nearly monocrystalline and bamboo-like microstructures were achieved in the steel wires.

Early stages of coherent precipitation have been studied in several Fe-Mn-Al-Ni samples aged at 200 °C for different time intervals in order to analyze the effect of nanoprecipitation on the phase stability during thermally induced martensitic transformations. The α-γ′ martensitic transformation was studied by means of dilatometry measurements. Transmission electron microscopy (TEM) observations were performed in order to characterize the microstructure of the samples. The size and volume fraction of nanoprecipitates were evaluated for the as-quenched material and samples aged for 10, 20 and 180 min. Considering these data and additional high resolution TEM results reported in the literature a phenomenological model is presented that enables understanding the effects of B2 precipitates on the relative phase stability between austenite and martensite, and their influence on the observed pseudoelasticity of this metallic system. With this model, the changes of the measured transformation temperatures can be predicted with considerable accuracy.

We present a detailed analysis of the effect of the sheath materials on the microstructure and superconducting properties of MgB2 wires produced by the powder-in-tube method (PIT). We reduced commercial MgB2 powder by attrition milling in nitrogen atmosphere using tungsten carbide balls and obtained powders with grain sizes lower than 150 nm and different strain states through this process. Several Ti, stainless steel, and copper monofilamentary wires were prepared using these powders by the PIT method. We investigated different thermal treatments and mechanical paths during the processing of the wires for the enhancement of the critical currents. The superconducting properties were determined by magnetization measurements in a SQUID magnetometer. The correlation between the thermal treatments, structure, and superconducting properties is discussed.

Samples of a Fe-Mn36-Al15-Ni7.5 shape memory alloy were subjected to different thermal treatments at 200ºC (namely for 0 min, 10 min, 20 min and 3 h) in order to evaluate the evolution of the coherent precipitates related to the pseudoelastic behavior. After performing the thermal treatments, samples were studied by means of electrical resistivity in experiments aimed at evaluating the effect of precipitation on the martensitic transformation temperatures and at determining the possible effects of thermal cycling. Mechanical tests were performed to measure the degree of pseudoelastic recovery for each thermally treated sample. Evidences of pseudoelastic behavior were found even in samples subjected to a rather short treatment such as 20 min after three thermal cycles. Transmission electron microscopy (TEM) observations were performed in order to identify the distribution and size of B2 nano-precipitates after the various thermal treatments.

In Cu-based SMA alloys, the grain size (d) effect on the martensitic transformation temperature was investigated for a wide range of d. Specimens were prepared by different heat treatments in order to create a range of grain sizes, from about 500nm (ribbons and tapes obtained by rapid solidification techniques) up to 6mm diameter single-crystals (grown by the Bridgman method). Information obtained from the literature was also included in the set of analyzed experimental data. The reduction of grain size shifts the forward transformation temperature downwards. These grain-size effects are observed in specimens with d below ∼ 100μm, and become more pronounced for d below ∼ 20μm. An empirical expression was obtained that describes the grain-size effect over the whole temperature range. The obtained curve differs considerably from the Hall-Petch behaviour reported in the literature by some other investigators.

MgB2 has become a commercially attractive material for technological applications for its particular superconducting properties. However, due to its brittleness to obtain wires, a metallic sheath is needed for drawing. In the present work, grade 2 titanium has been used as sheath material, and several multifilamentary wires have been prepared. The powder-in-tube method has been used to prepare the wires with the in situ variant, where the sheath is filled with the unreacted precursor powders (Mg and B). Different thermal treatments have been investigated including several intermediate treatments during the drawing process in addition to the final one. This last treatment is necessary to accomplish the synthesis and heal the cracks generated during the cold work. The superconducting properties were determined by magnetization measurements on a SQUID magnetometer, and the microstructure evolution was followed by TEM and SEM microscopy. Mechanical behavior is also evaluated.

A strong alpha-fiber I texture with components from {001} < 1 1 0 >(beta 1) to {1 1 2} < 110 >(beta 1) was developed in a polycrystalline Cu-13Al-5.5Ni-1Ti (wt%) shape memory alloy by hot extrusion at 800 degrees C (followed by recrystallization). Nearly fully recoverable strains up to the order of 6%, associated with cubic to monoclinic martensitic transformation beta(1)->beta(1)' have been measured by thermal cycling under constant load experiments. Such a degree of strain recovery is typical of single crystals or polycrystalline arrays with a low degree of grain constraint, such as highly textured, "bamboo" like, extruded materials. The reversible transformation strains reached about 68% of the upper bound predicted by a Sachs-type model which averages the most favorable martensite variants in each grain, taking into account their orientation and disregarding interactions between them. Due to the strong texture introduced in the material, such high values of recoverable transformation strains are obtained even for specimens having a grain size/thickness ratio of similar to 0.05, producing relatively high grain constraint. (C) 2011 Elsevier B.V. All rights reserved.