Fabrication of a porous material with a porosity gradient by a pulsed electric current sintering process

A porous structure with a porosity gradient can be applied to the preparation of continuous FGM, where liquid or chemical vapor of the second phase is infiltrated into the graded pores. It also has applications in skeletal implant materials and ultrafiltration media. An attempt was made to fabricate a porous material with a porosity gradient by means of a pulsed electric current sintering (PECS) process. The present work describes not only the measured value of the temperature difference between the upper and lower part of the specimen, which brings about a gradual change in pore distribution, but also the sintering characteristics of the porous structure obtained by the pressureless PECS process.     

Amorphous Ti–Cu–Ni–Al alloys prepared by mechanical alloying

Ti _x (CuNi)_90–x Al₁₀ (x = 50, 55, 60) amorphous powder alloys were synthesized by mechanical alloying technique. The evolution of amorphization during milling and subsequent heat treatment was investigated by scanning electron microscopy, X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. The fully amorphous powders were obtained in the Ti₅₀Cu₂₀Ni₂₀Al₁₀, Ti₅₅Cu_17.5Ni_17.5Al₁₀ and Ti₆₀Cu₁₅Ni₁₅Al₁₀ alloys after milling for 30, 20 and 15 h, respectively. Differential scanning calorimetry revealed that thermal stability increased with the increasing (CuNi) content: Ti₆₀Cu₁₅Ni₁₅Al₁₀, Ti₅₅Cu_17.5Ni_17.5Al₁₀ and Ti₅₀Cu₂₀Ni₂₀Al₁₀. Heating of the three amorphous alloys at 800 K for 10 min results in the formation of the NiTi, NiTi₂ and CuTi₂ intermetallic phases.     

Mechanically driven decomposition of intermetallics

Under mechanical milling (MM) in a planetary ball mill the FeSn intermetallic decomposes with the formation of the Fe₅Sn₃ and FeSn₂ phases, and the Fe₂Ge₃ decomposes into FeGe (B20) and FeGe₂. These processes were investigated using X-ray diffraction (XRD), and magnetization measurements in order to assess whether or not local melting upon impact by milling balls, or low-temperature solid-state processes are responsible for decomposition. Fe₂Ge₃ decomposes completely after a relatively short milling period, while the amount FeSn decomposed reaches a steady state, depending on milling conditions, after prolonged milling. Particle sizes of the decomposition products (Fe₅Sn₃ and FeGe) determined from the XRD and magnetization data remain approximately constant during MM. These peculiarities are in good accordance with local melting as a mechanism of the mechanical decomposition of FeSn and Fe₂Ge₃.     

W-Particle shape in liquid Cu during heat treatment of Cu-W composite powder prepared by mechanical alloying

The equilibrium morphology of W-particles in liquid Cu was observed to be spherical (nonfaceted). When W and Cu were subjected to mechanical alloying treatment, the morphology of the W-particle in contact with liquid Cu was faceted. With prolonged heat treatment, the W-particle gradually became nonfaceted in morphology. The time needed to attain the equilibrium nonfaceted morphology depends on the duration of the mechanical alloying process. The present paper describes the morphological variation in the W-particle in liquid Cu that occurs during the heat treatment of Cu-5wt.%W composite powder prepared by a mechanical alloying process. The effect of mechanical alloying treatment on the morphology of the W-particle in contact with liquid Cu is discussed.     

Transmission electron microscopy and atom probe specimen preparation from mechanically alloyed powder using the focused ion-beam lift-out technique

The preparation of transmission electron microscopy (TEM) and atom probe-field ion microscopy (AP-FIM) specimens from mechanically alloyed Ti-Cu-Ni-Sn powder has been explored. Applying the focused ion beam (FIB) based in situ lift-out technique, it has been demonstrated that specimen preparation can be carried on single micrometre-sized powder particles without the use of any embedding media. Since the particles did not incorporate any micropores, as revealed by cross-sectioning, the standard procedure known for bulk samples could be simply implemented to the powder material. A sequence of rectangular cuts and annular milling was found to be a highly efficient way of forming a tip-shaped AP-FIM specimen from a square cross-section blank. A Ga level < or =1 at.% was detected if a low beam current of 10 pA was chosen for the final ion-milling stages. Implanted Ga ions were mostly confined to a zone of about 2 nm in thickness and indicated that ion-induced structural transformations were negligible.