Hector J. Dorantes-Rosales

Microstructure characterization of phase transformations in a Zn–22 wt%Al–2 wt%Cu alloy by XRD, SEM, TEM and FIM

Abstract The microstructure evolution in a Zn–22 wt%Al–2 wt%Cu alloy during aging was followed, using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and field ion microscopy. Samples were solution treated at 623 K for 1 week and then some of them annealed and the others quenched. Both annealed and quenched samples were aged at 373, 423 and 473 K for times from 3.6 (0.04 days) to 1800 ks (20.8 days). The X-ray patterns showed that the η phase decomposed according to the following reaction η→η+α during aging. This was also confirmed by TEM and SEM results, which showed first the spinodal decomposition of the η phase and then the formation of a coarse lamellar structure, composed of the α and η phases. The X-ray analysis of aged samples also showed the occurrence of a four-phase reaction α+ϵ→τ+η. The τ phase was widely recognized in SEM, TEM and FIM results. Additionally, the XRD and TEM results indicated that the formation of the equilibrium ordered τ′ phase is preceded by that of the disordered τ phase.

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.

Decomposition process in a Zn-22wt.%Al-2wt.%Cu alloy

Abstract The decomposition process in a Zn-22wt.%Al-2wt.%Cu alloy was investigated during aging at 373, 423 and 473 K, using XRD, SEM and TEM techniques. The results of X-ray diffraction of aged samples showed that two kinds of phase transformation occurred. One was the decomposition of metastable ηFC phase, as revealed by progressive shifting of the (0002) X-ray diffraction peak. The activation energy for the thermally activated process was determined to be ∼71.5 kJ mol−1. The other transformation was detected by the disappearance of the e phase diffraction peaks, accompanied by the appearance of the (110) and (110) X-ray diffraction peaks of the τ and τ′ phases, respectively. These two events seem to be related to the four-phase reaction: α+e→η+τ′.

Effect of the Aging Time on the Microstructure of Al-2%Li-1%Hf Alloys

In this article, the effect of aging time in Al–Li–Hf alloy was studied. The samples were homogenized at 600°C and aged from 0.166 to 100 h at 190°C in thermal oil. According to scanning electron microscope (SEM) images, the effect of aging in the intermetalic precipitates changes its morphology as well as composition. For as cast alloys small Hf–Li precipitates with high Hf content exist. When considering aging samples the size precipitates increases, as does Hf content, from 30.74% for 1 h to 59.85% for 100 h. For an aging time of 100 h, the microhardness Vickers show better values which can be attributed to the Hf–Li solubility into the Al matrix to the point where their precipitation of intermetalic precipites and further cause changes in the dendritic arm spacing.

Phase transformations induced by mechanical alloying of Ag–28 at.-%Al alloy

Abstract A Ag–28 at.-%Al alloy was obtained by mechanical alloying in a SPEX 8000D mill from elemental Ag and Al powders. Two vials with different internal volume and two ball powder weight ratios were used to analyse the phase evolution. Microstructural characterisation was carried out by X-ray diffraction, SEM and TEM. The sequence of phase transformation during the milling process was as follows: Ag+Al (powders)→α→α+ζ→α+ζ+μ→μ+ζ→μ. The formation of the α and ζ phases is favoured by the energy of the impacts in the early stage of milling, while the formation kinetics rate of the μ phase was faster for the higher vial volume and ball/powder weight ratio. TEM images confirm the presence of the μ phase with a nanometric grain size.

Effect of phase transformations on hardness in Zn-Al-Cu Alloys

Zn-Al-Cu alloys were prepared by melting of pure elements. The as-cast alloys were homogenized at 350 °C for 180 h. Both the as-cast and homogenized alloys were analyzed with X-ray diffraction and scanning electron microscope with Energy Dispersive X-Ray analysis. The hardness of alloys was determined using Rockwell “B” hardness. The X-ray diffractograms and scanning electron micrographs indicated the presence of several phases in the as-cast alloys. Some of them do not correspond to those shown in the equilibrium Zn-Al-Cu phase diagram. On the other hand, the homogenized alloys showed most of the phases present in the equilibrium diagram. The hardness of alloys increases with the increase in Cu content because of the presence of Cu-containing phases such as, the θ and τ’ phases.

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.

Iron nanoparticles on colloidal substrates

Iron nanoparticles are of interest for possible applications such as remediation of soil and water, and for their magnetic properties. In the present work, a simple method for producing iron nanoparticles on a colloidal substrate is presented. These composites have potential applications related to their surface and magnetic properties.

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.

Ostwald Ripening Process of Coherent β′ Precipitates during Aging in Fe0.75Ni0.10Al0.15 and Fe0.74Ni0.10Al0.15Cr0.01 Alloys

The Ostwald ripening process was studied in Fe0.75Ni0.10Al0.15 and Fe0.74Ni0.10Al0.15Cr0.01 alloys after aging at 750, 850, and 950°C for different times. The microstructural evolution shows a rounded cube morphology (Fe, Ni)Al β′ precipitates aligned in the ferrite matrix, which changes to elongated plates after prolonged aging. The variation of the equivalent radii of precipitates with time follows the modified Lifshitz-Slyozov-Wagner theory for diffusion-controlled coarsening. Thermo-Calc analysis shows that the chromium content is richer in the matrix than in the precipitates which causes higher hardness and coarsening resistance in the aged Fe0.74Ni0.10Al0.15Cr0.01 alloy.