Victor M. Lopez-Hirata

Characterization of CoCu mechanical alloys by linear sweep voltammetry

Abstract The mechanical alloying process has shown that it is possible to obtain supersaturated solid solutions at any composition in immiscible metallic systems by ball milling of elemental powder mixtures. The mechanically alloyed CoCu powders are chemically homogeneous supersaturated solid solutions and exhibit crystallite sizes of a nanometer order. In the present work, the mechanically alloyed CoCu powders have been characterized by linear sweep voltammetry in 0.5 M NaOH and 0.15 M Na2B4O7 aqueous solutions. The electrochemical behavior has been related to the characteristics of enhanced solubility found in CoCu mechanical alloys. In order to perform a systematic study of this system, the voltammograms of pure Cu, pure Co, CoCu original powder mixtures and CoCu alloys, obtained by melting and casting, were also analyzed for comparison. The results showed that the voltammograms of mechanically alloyed CoCu powders presented peaks of dissolution at potentials intermediate between those exhibited in pure Cu and Co samples. Moreover, a third dissolution peak occurred at more noble potentials in mechanical alloys, but not in as-cast alloys. This peak is associated with the formation of the solid solution, in mechanical alloys. This suggests that both elements are dissolved simultaneously, which confirms that CoCu mechanical alloys are true supersaturated solid solutions. Additionally, it was found that the MA CoCu alloys showed corrosion rates higher than those in as-cast alloys, which is attributed to the high value of stored energy presented in MA alloys as a result of the severe plastic deformation during ball milling.

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 Active and Nonactive Fluxes on the Mechanical Properties and Microstructure in Submerged-Arc Welds of A-36 Steel Plates

A study of the effect of active and nonactive fluxes on the mechanical properties and microstructure of submerged-arc welds for steel plates was carried out for the Submerged-Arc Welding (SAW) of A-36 Steel Plates. The nonactive flux promoted the formation of pearlite and ferrite in the weld having the highest toughness and ductility. In contrast, the active fluxes with Cr and Mo promoted the formation of acicular ferrite and fine carbides in the welds showing the highest tensile strength and hardness.

Numerical analysis of phase decomposition in A-B binary alloys using Cahn-Hilliard equations

The analysis of phase decomposition was carried out using the nonlinear and linear Cahn-Hilliard equations in a hypothetical A-B alloy system with a miscibility gap. These equations were solved by the explicit finite difference method assuming a regular solution model. The supersaturated solid solution and decomposed phases were considered to have an fcc structure. Different aging temperatures and thermodynamic interaction parameters ΩA-B were used to simulate different alloy systems. The numerical simulation results showed that the growth kinetics of phase decomposition in the alloy with 30at.% A was slower than that of 50 at.% A. Additionally, the start time and modulation wavelength of phase decomposition are strongly affected by the thermodynamic interaction parameter ΩA-B value. The numerical simulation results showed that the growth kinetics of phase decomposition with the linear equation is slower than that with the nonlinear one.

Chemical and Physical Properties of Fluxes for SAW of Low-Carbon Steels

The submerged-arc welding of steels has been used since 1930. It is well known that the mechanical properties of steel weldments depend on the chemical compositions of electrodes and fluxes. The development of welding electrodes has been based on practical experiences. The study of welding deposits by means of physical metallurgy permitted to develop electrodes and fluxes for SAW process of steels. In contrast, the use of chemical and physical properties of fluxes for the development of welding process started in the 70 s. (Shah, 1986). Most of the works concerning with the fluxes for SAW process have been focused on its effect on microstructure and mechanical properties. Likewise, it has been interesting the study of the thermochemical and electrochemical reactions that occur at the welding pool which are very important for the transferring of metallic elements to the welds. In addition to the stated above, the electrode coverings is also a very important aspect to obtain weld metal with good mechanical properties. The covering materials are fluxes which are composed of different mineral chemical compounds such as, oxides, fluorides and carbonates (Singer & Singer, 1979). The firing and sintering process of fluxes during welding electrode processing promotes chemical reactions and phase transformations of these minerals. All these factors determine the valence or electric charge of the different elements which are deposited to the weld metal. All the oxides from flux may contribute to the dissolution process of different metallic elements and oxygen in the welding pool. These metallic elements can react with oxygen to form oxide inclusions which can serve as nucleation sites for the formation of some benefit phases such as acicular ferrite during the welding process (Davis & Bailey 1991). These events may improve the mechanical strength and ductility of weld metal. Thus, the purpose of this chapter is to show the effect of different fluxes on the chemical composition, microstructure and mechanical properties of weld metals by the submerged-arc welding procedure.

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 Precipitation on Cryogenic Toughness in N-Containing Austenitic Stainless Steels

Three types of austenitic stainless steels JK2, JJ1 and JN1 were isothermally aged at temperatures from 600 to 900°C for 10 to 1000 minutes in order to study the microstructural evolution and its effect on the fracture toughness at cryogenic temperatures. The Charpy V-Notch fracture energy at 77 K showed a significant decrease with aging time in JJ1 and JN1 steels because of their higher contents of C and N. In contrast, the fracture energy corresponding to the aged JK2 steel decreased gradually with aging time. The abundant intergranular precipitation of carbides and nitrides seems to be the responsible for the fracture toughness deterioration in the aged JJ1 and JN1 steels. On the other hand, the intergranular precipitation of carbides was less abundant in the aged JK2 steel. The scanning electron microscope fractographs of the CVN test specimens corresponding to the aged JJ1 and JN1 steels showed mainly an intergranular brittle fracture and its fraction increased with aging time and temperature. In general, the presence of a more abundant intergranular precipitation resulted in a more rapid decrease in toughness with aging time.

Microstructural and Mechanical Characterization of Nitinol GTAW and FB Welds of Titanium

Microstructural and mechanical characterization of Nitinol gas tungsten arc weld (GTAW) and furnace brazing (FB) welds for grade 1 titanium plates are carried out in order to study the microstructure developed after welding and its effect on the mechanical properties of welds. The GTAW process yields the highest hardness weld. The constituents for this weld consist of a dendritic structure of NiTi and NiTi2 intermetallic phases. The FB process promotes a change of the welds chemical composition due to atomic diffusion of Ti. The weld microconstituents consist of a mixture of a Ti-rich and NiTi2 eutectic and a proeutectic Ti-rich phase.