Electrochemical Polymorphic Phase Formation in Metals

Abstract

The phenomenon of electrochemical phase formation in metals and alloys via a supercooled liquid state stage was discussed. Assuming the electrodeposited metal to be a product of formation and ultrarapid solidification of supercooled metallic liquid, a possibility of metastable phase formation during electrodeposition of polymorphous metals was suggested. It was anticipated that the polymorphic transition of the metal’s metastable form to the stable one occurs by shear, as does the martensitic transformation. To enable revealing an orientation relationship between grains of the two phases, a method for X-ray texture analysis of metals was developed using a combination of direct pole figures. It was established that the phase formation during electrodeposition of polymorphous metals produces metastable modifications typical of entities that crystallized from a liquid state at extremely high rates. In regards polymorphic transitions in metal electrodeposition, certain orientation relationships were observed between grains of the stable and the metastable phase, which is typical of phase transformations proceeding at extremely high rates. The results obtained provided additional arguments in favor of the phenomenon under discussion.

Review—Electrochemical Phase Formation via a Supercooled Liquid State Stage: Metastable Structures and Intermediate Phases

“…Electrochemical amorphous phase formation in metals and alloys.-Idea one and its realization.-If a metal/alloy indeed passes a stage of supercooled liquid state in electrodeposition, then, in order to prove the existence of such intermediary state, one should prevent crystallization of the liquid phase, thus capturing the amorphous structure of the metallic liquid solidified on the cathode. 61,64,65 Obtaining electrodeposited metals or alloys in the form of amorphous phases by hindering crystallization of an intermediate liquid phase will serve as evidence of the phenomenon at hand. 64,65 Development of a method for X-ray diffractometry of amorphous materials.-To identify an amorphous state in materials, X-ray diffractometry is usually employed in combination with differential thermal analysis (DTA) data.…”

“…61,64,65 Obtaining electrodeposited metals or alloys in the form of amorphous phases by hindering crystallization of an intermediate liquid phase will serve as evidence of the phenomenon at hand. 64,65 Development of a method for X-ray diffractometry of amorphous materials.-To identify an amorphous state in materials, X-ray diffractometry is usually employed in combination with differential thermal analysis (DTA) data. As a rule, short-wave Mo-K α irradiation is used in X-ray diffractometry of amorphous materials.…”

“…With a view to improving reliability of amorphous component identification, an innovative method for X-ray diffractometry of amorphous materials was developed. 67 The inventive method includes irradiating a specimen with X-rays having a wavelength of λ 1 (typically, Mo-K α radiation is used), recording the diffraction spectrum and finding characteristics of the amorphous phase from 65 the intensity distribution in the spectrum recorded. To improve the analytical reliability in characterization of the material's amorphous component, the specimen is further irradiated with an X-ray beam of wavelength λ 2 , such that…”

“…Since atomic sizes of metals and nonmetals differ greatly and no nonmetallic element can deposit on the cathode in the shape of a distinct phase, the nonmetallic addition will at certain concentrations hinder metal crystal nucleation, leading to an amorphous structure of a solidified supercooled metallic liquid frozen on the cathode. 65 The anticipated suppression of crystal nucleation in the metallic liquid due to addition of nonmetallic elements, such as carbon or phosphorus, will be more pronounced in electrodeposition of transition metals like chromium or nickel.…”

“…Crystallization hindrance in the electrodepositing metal's liquid phase.-On the basis of the above, structural condition was investigated in rhenium electrodeposits obtained in a sulphate electrolyte at temperature 26 °C (ΔТ = 3160 K) while current density was varied stepwise in 5 A dm −2 increments from 15 to 45 A dm −2 . The X-ray diffractometry technique was as developed above, 65,67 involving Мо-K α and Fe-K α irradiation with a DRON-3M and a DRON-2 diffractometer. The diffraction data were complemented with DTA analyses of the deposits using a DuPont 990 thermal analyzer.…”