Felipe Hernandez-Santiago

Electrocatalytic properties of mechanically alloyed Co-20wt%Ni-10wt%Mo and Co-70wt%Ni-10wt%Mo alloy powders

Mechanically alloyed Co-20wt%Ni-10wt%Mo and Co-70wt%Ni-10wt%Mo (nominal compositions) alloy powders were produced by milling of pure elemental powders. Mechanically alloyed powders were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. MA powder specimens were tested electrollitically in a 30% KOH aqueous solution at 298 K. X-ray diffraction analysis and transmission electron microscopy of milled powders showed the presence of two phases, an fcc solid solution and intermetallic compounds of Ni or Co with Mo. These phases showed a nanometric size. The linear sweep voltammograms confirmed also the presence of two phases in both mechanically alloyed alloy powders. The Co-20wt%Ni-10wt%Mo alloy powders showed the best electrocatalytic activity for hydrogen evolution reaction.

Coarsening process of coherent β′ precipitates in Fe–10 wt-%Ni–15 wt-%Al and Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cu alloys

Abstract The coarsening process of the NiAl ordered β′ precipitates was studied in the isothermally aged Fe–10 wt-%Ni–15 wt-%Al and Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cu alloys. The aging treatments at 750, 850 and 950°C caused the precipitation of the β′phase in a ferritic matrix. In general, the coarsening process of Cu containing alloy was slower with an activation energy of 248 kJ mol−1 compared to 194 kJ mol−1 for the other alloy. However, the coarsening kinetics for the former alloy was faster at 950°C. The precipitation hardening response was also better in the Cu containing alloy, with a slower overaging stage. The coarsening kinetics followed the behaviour predicted by the modified Lifshitz–Slyozov–Wagner theories for coarsening controlled by volume diffusion.

Phase Decomposition in Isothermally Aged MA Cu-Ni-Fe and Cu-Ni-Cr Alloys

Supersaturated solid solutions of Cu-44.5at.%Ni-22.5at.%Fe and Cu-37at.%Ni-6at.%Cr alloy were produced by ball milling of a pure chemical elemental mixture for 1080 ks. Two fcc supersaturated solid solutions with a grain size of about 20 and 50 nm, respectively, were obtained after milling. These alloys were subsequently aged at temperatures between 800 and 1003 K for different times. The aging promoted the phase decomposition of the supersaturated solid solution into a mixture of Cu-rich and Ni- phases in both the aged MA alloy powders. The growth kinetics of the modulation wavelength was determined from the X-ray diffraction results and followed the Lifshitz-Slyozov- Wagner theory for a diffusion-controlled coarsening in the mechanically-alloyed Cu-Ni-Fe alloy after aging. However, the sidebands intensity seems to be very low and overlapped with the peaks corresponding to the Cu-rich phase in the aged mechanically-alloyed Cu-Ni-Cr alloy. The growth kinetics of composition modulation wavelength for the aged MA Cu-Ni-Fe alloy was faster at 803 and 898 K than that for the same alloy composition obtained by a conventional processing and then aged at the same temperatures.

Analysis of β´ Cu 4 Ti Precipitation in Cu-Ti Alloys by Conventional and Diffusion-Couple Methods

An analysis of the Cu4Ti precipitation process was carried out in binary Cu-Ti alloys using the conventional method and subsequently employing a diffusion-couple based method, known as the Macroscopic-Gradient Method. The phase separation took place via the mechanism of spinodal decomposition at early stages of aging. The precipitation of the β´ (Cu4Ti) phase preceded to that of the β (Cu3Ti) phase, which is formed by cellular precipitation. The highest hardness and fastest precipitation kinetics occurred at aging of 400 and 600 °C, respectively. In the case of the macroscopic gradient method, a diffusion couple of Cu-4 wt. Ti alloys and pure copper was prepared by a diffusion annealing. This analysis enabled to determine the equilibrium and coherent lines in this alloy system. The Gibbs-Thomson equation was verified for this type of precipitation.

Coarsening of Decomposed Phases in Cu-Ni-Cr Alloys

The coarsening process of the decomposed phases was studied in the Cu-34wt.%Ni-4wt.%Cr and Cu-45wt.%Ni-10wt.%Cr alloys aged at 600, 700 and 800 °C for different times by transmission electron microscopy. The morphology of the coherent decomposed Ni-rich phase changed from cuboids to platelets aligned in the <100= Cu-rich matrix directions as aging progressed. The variation of mean equivalent radius of the coherent decomposed phases with aging time followed the modified LSW theory for thermally activated growth in ternary alloy systems. The linear variation of the density number of precipitates and matrix supersaturation with aging time, also confirmed that the coarsening process followed the modified LSW theory in both alloys. The size distributions of precipitates in the Cu-Ni-Cr alloys were broader and more symmetric than that predicted by the LSW theory. The coarsening rate was faster in the symmetrical Cu- 45wt.%Ni-10wt.%Cr alloy due to its higher volume fraction of precipitates.

Influence of the Fluence and Substrate Temperature on the Structural and Optical Properties of CdSe Thin Films Prepared by Pulsed Laser Deposition

Cadmium Selenide (CdSe) thin films were prepared by pulsed laser deposition using a Nd:YAG laser (355 nm). Films were grown by ablating a sintered pure CdSe target with fluences from 0.1 to 1.5 J/cm2 on corning glass, silicon (100) and quartz substrates. Deposition chamber was maintained under vacuum pressure while substrate temperature was increased from room temperature to 500°C in order to control the crystalline phase. All the films show mirror-like surface morphology. Atomic force microscopy (AFM) images shown that films have very flat surfaces with RMS values around 0.7 and 5 nm for room temperature and 500°C respectively. The X-ray diffraction analysis proves the presence of the cubic zinc blend phase for the CdSe films deposited at low temperature, at 400°C and at higher substrate temperature the hexagonal phase is present. TEM analysis shows that at 100°C the films are constituted by particles with an average size of 30nm in diameter. The optical properties of the films were determined from the UV-transmission spectra. The estimated band gap values of the films deposited at room temperature and at 400°C (0.1 J/cm2) were 1.87 and 1.70 eV respectively.

Coarsening Process of Decomposed Phases in Cu-Ni-Cr Alloys

A study of the coarsening process of the decomposed phases was carried out in the Cu-34wt.%Ni-4wt.%Cr and Cu-45wt.%Ni-10wt.%Cr alloys using transmission electron microscopy. As aging progressed, the morphology of the coherent decomposed Ni-rich phase changed from cuboids to platelets aligned in the <100> Cu-rich matrix directions. Prolonged aging caused the loss of coherency between the decomposed phases and the morphology of the Ni-rich phase changed to ellipsoidal. The variation of mean radius of the coherent decomposed phases with aging time followed the modified LSW theory for thermally activated growth in ternary alloy systems. The coarsening rate was faster in the symmetrical Cu-45wt.%Ni-10wt.%Cr alloy due to its higher volume fraction of precipitates. The activation energy for thermally activated growth was determined to be about 182 and 102 kJ mol-1 in the Cu-34wt.%Ni-4wt.%Cr and Cu-45wt.%Ni-10wt.%Cr alloys, respectively. The size distributions of precipitates in the Cu-Ni-Cr alloys were broader and more symmetric than that predicted by the modified LSW theory for ternary alloys.

Effect of prior cold working on microstructural evolution of precipitation in Zn–22Al–2Cu alloy during aging

Abstract The microstructural evolution of precipitation was characterised by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and microhardness measurements during two different types of isothermal aging in a Zn–22Al–2Cu alloy. Solution treated samples were quenched and rolled at room temperature and then aged at 250°C for different times. The X-ray results showed that the decomposition of the β sss phase and the formation τ′ phase, during the aging treatment occurred by the following reactions β sss→α + η + ϵ and α + ϵ→η + τ′ respectively. Similar phase transformations were observed in the sample solution treated, quenched and then cold worked before aging condition, with a faster kinetics. Besides, the microstructure of the latter samples consists of equiaxial grains of the τ′, α and η phases. In contrast, the former sample showed a lamellar structure of the α and η phases. This difference of microstructure caused a slightly higher hardness in the samples without cold working. Furthermore, the aging process caused a decrease in hardness with aging time, which seems to be related to the coarsening process of the equilibrium phases for both heat treatments.