Microscopic kinetics of isothermal sintering of Fe-20 % (mаs.) Mo alloy

Purpose of work. To investigate the features of microscopic kinetics of peritectoid transformation in Fe-Mo system alloys in an isothermal mode. Experimental part. Microscopic analysis of samples on light (Jenaphot 2000, K. Zeiss) and scanning electron (REM 106I, Selmi) microscopes, X-ray spectral microanalysis of the component’s concentrations distribution between the phases, X-ray phase analysis (Rigaku Ultima IV diffractometer). Results. Microstructure changes, phase composition and crystal lattices parameters of the phase constituents of the powder alloy during sintering at 920 °C were investigated. Variation in the phase constituents mass fraction during 7 hours of the isothermal exposure is analyzed. The formation of anomalous diffusion porosity at the beginning of the process, the nonmonotonic change in the phase constituents fraction and formation of intermediate phases with an unstable component’s concentration are the main features of the microscopic kinetics. The sintering mechanism is proposed. Scientific novelty. A local peritectoid transformation existence at the Fe/Mo interface was established by analyzing the local diffusion flows of components atoms. This transformation occurs upon isothermal supply of Mo atoms with the formation of a cooperative peritectoid structural constituents according to the α- Fe + Mo → α + μ scheme with residual Mo crystals. Formulation of the problem. This work aims to clarify the phenomenological theory of peritectoid transformation during isothermal α-Fe grains enrichment with molybdenum by studying the features of microscopic kinetics in the Fe-Mo system alloys. Practical value. Peritectoid (α + μ) with branched phase соnstituents of cooperative genesis forms a developed system of local diffusion flows of Mo atoms in α -Fe. This increases the molybdenum peritectoid transformation rate at a relatively low sintering temperature for these alloys and reduces the energy consumption in the technological process.

Peritectoid carbide transformation based on ε-carbide Fe2C in Fe-C system alloys. Part 2. Metallographic studies

Original metallographic studies of annealed medium carbon steel (steel 45, steel 40X and steel 35XGA) are carried out in order to reveal leakages in pearlite of these steels of low-temperature carbide transformation of peritectoid type, in which solid solutions of ferrite and cement form a solid solution of wide area of homogeneity based on ε-carbide Fe2C. Most of the inclusions of pearlite cement 45 steel are almost entirely covered with a light grey “foam” shell of nanoglobular crystals ε-сarbide Fe2C. In the process of crystallization of ε-сarbide Fe2C on the cement plates of pearlite steel 45 three morphological types of structure of ε-сarbide Fe2C can be distinguished: “foam” globular shell, granular “outgrowths” and wrapping of particles of decomposed and partially dissolved cement plates. Chromium, which is a part of steel 40X, abruptly inhibits peritectoid transformation. On many cement plates pearlite surface is smooth. In areas where the concentration of chromium is low, the process of formation of ε-сarbide Fe2C is active, with the formation of individual sites with light gray “foam” shell of nanoglobular crystals ε-сarbide Fe2C. It can be expected that in high-alloy chrome steels, the peritectic transformation can be completely blocked through the stabilization of chrome cement or its transformation into thermodynamically stable high chrome carbides. In 30ХГСA steel a sharp intensification of the process of peritectoid transformation of solid solutions of ferrite and cement which are part of pearlite is fixed. The reason for acceleration of the disintegration process of pearlite cement into individual fragments and intensification of release of ε-сarbide Fe2C in the form of column-shaped crystals between the plates of pearlite cement is silicon and manganese, which are part of steel 30ХГСА. As a result of the acceleration of martensite decomposition, the morphology of the released crystals of ε-сarbide Fe2C has changed from “foam” nanocrystals of ε-сarbide Fe2C, typical for steel 45 and steel 40X, to granular. There was also intensive fragmentation or disintegration of cement plates with the appearance of plane-parallel boundaries between the fragments and the formation of large longitudinal flat inclusions of ε-сarbide Fe2C above 100 nm, whose axis is mainly perpendicular to the axis of the cement plate. On the basis of the performed experiments it is possible to consider as proved the presence of low-temperature carbide peritectoid phase transformation in the Fe-C alloy system as a result of interaction of solid solutions of ferrite and cement at 3820C with formation of solid solution on the basis of ε-сarbide Fe2C with wide area of homogeneity. The influence of the chemical composition of steel on the peritectoid transformation between ferrite and cement slurries opens up additional possibilities for regulating the microstructure of pearlite, such as the degree of dispersion of pearlite, which has a determining influence on a number of performance characteristics of steel, such as wear resistance, yield strength and others.

Low-temperature pearlite dissociation in iron-carbon alloys by peritectoid transformation reaction

Metallographic investigations of thermally treated iron-carbon alloys have shown that in pearlite of the alloys mentioned passes a low-temperature carbide transformation of the peritectoid type at which solid ferrite and cementite solutions form a solid solution of a wide area of homogeneity based on ε-carbide of Fe2C. The analysis of peritectoid transformation opens new techniques of thermal treatment and manufacturing natural nano-strengthened composite alloys of the carbide-carbide class

Peritectoid carbide transformation based on ε-carbide Fe2C in Fe-C-system alloys. Part 1. Basics of theory

In the present work low-temperature carbide phase transformations in the system of Fe-C alloys based on ε-сarbide Fe2C with consideration of identification of θ-Fe3C cement as a solid solution were studied. It has been proved that the θ-Fe3C cement slurry is colourfastonide, and the ε-Fe2C carbide slurry is bertollide. When tempering hardened steels, ε-сarbide Fe2C is emitted in the structure of hardened martensite, which is absent in the phase diagram of iron-carbon system alloys. It is believed that ε-сarbide Fe2C is not a stable phase, and since it is metastable, it is formed only in quenched steels under non-equilibrium conditions. The isolation and dissolution of ε-сarbide Fe2C is a classic phase transformation and the absence of this transformation on the diagram is not caused by the metastable phase of ε-сarbide Fe2C, but by the incomplete iron-carbon diagram. The martensite decomposition phase transformation is based on the formation of carbon enriched zones. The processes of carbon segregation on dislocation structures and grid planes of martensite create zones with excess energy. Beginning approximately with temperature 100 °С in structure of martensite begins to allocate ε-сarbide Fe2C, finishing a stage of two-phase segregational disintegration of martensite. At rather small concentrations of carbon in cluster zones the fastest and most effective way of relaxation of redundant energy in these zones, as well as in the tetragonal lattice of martensite is the formation of phases with low value of work of nucleation, first of all ε-сarbide Fe2C and α-Fe(C) or ferrite. The main stages of phase transformations in the peritectoid reaction of martensite decomposition are considered. It is proposed to introduce the peritectoid transformation horizontal at 382 °C and the peritectic transformation horizontal of cement at 1650 °C into the Fe-C alloy state diagram.

Silver Matrix Composite Reinforced by Aluminium-Silver Intermetallic Phases

Silver and aluminum powders (82 mass % Ag and 18 mass % Al) were mixed and hot extruded at 673 K with extrusion ratio λ = 25. Performed X-ray diffraction analysis of as extruded rod revealed the development of Ag3Al and Ag2Al-type intermetallic phases. Structural observations and both chemical and diffraction analysis of structural components confirmed the growth of mentioned phases in the vicinity of elementary Al and Ag granules. No pores or voids were observed in the material. Mechanical properties of the composite, UTS = 490MPa, YS = 440 MPa, HV2 = 136, were relatively high if compared to commercial Ag and Cu products. Hot compression tests pointed to the good hot workability of the composite at deformation temperature range 473 K - 773 K.The differential scanning calorimetry tests were performed in order to estimate structural processes during heating of Ag/Al composite that lead to thermodynamically stable liquid state. It was found that characteristic temperature of three endothermic peaks correspond to (1) peritectoid transformation μ-Ag3Al → ζ-Ag2Al + (Ag), (2) the eutectic melting ζ-Ag2Al + (Al) → L, (3) melting of the ζ-Ag2Al phase.The Vickers hardness of the samples annealed at 673 K, for the time range up to 6900 minutes, was also determined. It was concluded that mutual diffusion of elements between Ag and Al granules and the growth of μ-Ag3Al and ζ-Ag2Al grains during annealing at 673 K result in a slight hardening of the composite.

Decomposition of Ni-23 at.% Si and formation of Ni3Si

The purpose of the paper is to outline the formation of Ni3Si from Ni-23 (at. %) Si. The Ni-23Si was annealed at 1130°C for 24 h and quenched, after which it consisted of ∼10μm particles in an otherwise single phase matrix. To produce the peritectoid transformation to Ni3Si, samples were annealed at 725°C.Thin foils of Ni-23Si were prepared by electropolishing 250μm thick discs with 13% hydrochloric acid in methanol in a Struers Tenupol at -40°C, 40V (200 mA) and a medium flow rate. After perforation, the thin foils were rinsed in ethanol before washing in hydrofluoric acid for 30-40s. The foils were examined in a JEOL 2000FX TEM, furnished with a Tracer Northern TN5500 EDS system.