Effect of Ag additions on the microstructure and phase transformations of Zn-22Al-2Cu (wt.%) alloy

The relationship between microstructure and mechanical properties is studied for eutectoid Zn-22Al (wt.%) alloys modified with Cu and Ag. Three alloys with a Cu content of 2 wt.% and varying amounts of Ag were cast and hot-extruded. Different microstructural characteristics were induced by heat treatments: natural aging, artificial aging and furnace cooling. Structural and microstructural characterizations were carried out with X-ray diffraction and scanning electron microscopy. Mechanical properties were determined by tensile testing. Dilatometry was used for determining the effects of composition on the transformation points. The addition of Ag increased the ε phase fraction and provided solid solution strengthening, improving the mechanical strength and reducing ductility. Ag additions also displaced the eutectoid reaction to higher temperatures. The microstructure of the matrix has proven to have a strong impact on mechanical properties. The naturally aged specimens presented the highest ductility and tensile strength; however, these properties are severely affected by aging. Lamellar microstructures present the lowest ductility and values of tensile strength between those of the natural and artificially-aged specimens.

To question about Fe5B3 boride formation in boron-rich Fe–В alloys

The structure of boron-rich iron alloys in the concentration range of 9.0–15.0 wt. % В, 0.01–0.17 wt. % C, Fe – the balance (with charge impurities of Si, Al, Mn) was investigated in this work. The methods of metallographic, X-ray, stop-quenching, scanning electron microscopic, energy dispersive, and fluorescent spectral analyses were applied. The FeB- and Fe2B-based solid solutions are proved to be the major constituents of the investigated alloys. No evidence is found for the possible formation of the Fe5B3 boride via peritectic reaction L+FeB→Fe5B3 at 1650 K and its further decomposition via eutectoid reaction Fe5B3→FeB+Fe2B at 1410 K. It is shown that the phase under consideration is iron hemiboride alloyed mainly by silicon which peritectically forms from primary crystals of iron monoboride and the rest of liquid at 1650 K. The thermal effect at 1410 K is assumed to be caused by a heat production connected with polymorphic transformation α-FeB→β-FeB in the presence of carbon.

The Li2SO4–Na2SO4 System for Thermal Energy Storage

In this paper, the system Li2SO4–Na2SO4 is proposed as a candidate material for thermal energy storage applications at high temperatures (450–550 °C). Depending on the composition, the thermal energy can be stored by using a eutectoid reaction and solid–solid phase transition. In these types of systems, all the components (reagent and products) are in the solid state. This work includes the theoretical analysis (based on the Calphad method) of the system selected obtaining all the theoretical parameters (for example, enthalpies of reaction, transition temperatures, volume expansion, and the heat capacities) necessary to determine the theoretical performance in terms of thermal energy storage. The theoretical analysis allowed to identify two compositions (Li2SO4/Na2SO4 79/21 and 50/50) in the phase diagram with the most promising theoretical enthalpy of transformation (270 J/g and 318 J/g, respectively) corresponding to a eutectoid reaction and a solid–solid phase transition (stoichiometric compound LiNaSO4). The experimental analysis carried out allowed to confirm the great potential of this system for TES application even if some discrepancies with the theoretical calculation have been observed experimentally (energy densities lower than expected). For the two compositions studied, 79/21 and 50/50, the enthalpies of reaction are 185 J/g and 160 J/g, respectively. The reactivity of the system was tested under different experimental conditions preparing materials with a different degree of nanocrystallization to favor the diffusion in the solid state, testing the reactivity of the materials under controlled atmosphere and under air, and performing preliminary durability analysis (cycling behavior up to 20 cycles) to test the stability and reversibility.

Phase Equilibria Among β/α/α2/γ Phases and Phase Transformations in Ti-Al-Cr System at Elevated Temperatures

Effects of Cr addition to TiAl alloy on the phase equilibria among β/α/α2/γ Phases and phase transformation pathways within the temperature range of 1523 K∼1073 K were investigated using several alloys in the composition of interest. In between 1473 K and 1373 K, The slope of β/α tie-line in the three-phase coexisting region of β+α+γ remains basically unchanged. However, this slope drastically rotates in a clockwise direction, in between 1373 K and 1273 K and below the eutectoid reaction temperature in the binary system (1400 K). This is a strong indication that α-phase exists even below the 1400 K, i.e. addition of Cr stabilizes α against α2 and the three-phase coexisting region of β+α2+γ at lower temperatures is formed through a ternary eutectoid reaction (α → β+α2+γ) with decreasing temperature. This three-phase tie-triangle moves towards lower Al content in phase diagram. This suggests that Cr addition results in increase of the volume fraction of γ-phase with decreasing temperature even in alloys with low Al content.

Effect of N/C Ratio on Precipitation Behavior of (Cr,Fe)23C6 Carbide in Novel Cast Austenitic Heat-Resistant Steels during Directional Solidification

The precipitation of (Cr,Fe)23C6 carbide could significantly degrade the mechanical properties of Nb-bearing cast austenitic heat-resistant steels, designed for exhaust components of automotive gasoline engines at 1000 °C. In the current research, the precipitation behavior of (Cr,Fe)23C6 carbide in these alloys, with great variations in N/C (Nitrogen/Carbon) ratio, was investigated through the liquid metal cooling directional solidification method, combined with thermodynamic calculations. Microstructural characterization suggested that the (Cr,Fe)23C6 carbide formed in the steady-state zone and the competitive zone, upon cooling to room temperature, after the solidification ended. It grew in the colony of the δ-ferrite, through the eutectoid reaction and showed different concentrations of C and Si from the δ-ferrite. Its precipitation temperature decreased significantly with increasing the N/C ratio, thereby retarding its precipitation. Therefore, the quantity of (Cr,Fe)23C6 carbide could be limited though increasing the N/C ratio of this type of alloys.

Isothermal Transformation Ar1 Product Morphology and Kinetics of EN-GJS-600-3 Cast Iron

This work presents the qualitative and quantitative changes in the products of isothermal transformation (reaction) in a ductile cast iron austenite after supercooling to the temperature range Ar1. The austenitizing temperature considered in this work was 900, 960 or 1020°C. The eutectoid reaction was investigated by metallographic examination at a holding temperature right below Ar11(820°C) or right below Ar12(760°C). The quantitative metallographic examination was carried out with a light microscope (LM). The initial transformation stage products were identified with a transmission electron microscope (TEM). The selected samples were studied for chemical microsegregation of manganese, silicon, phosphorus, and carbon with an X-ray microanalyser (MAR). The tested cast iron material was found to predominantly feature a eutectoid reaction in the metastable system the ratio of which was increasing with the austenitizing temperature. The austenitizing temperature was found to be conducive to the evolution kinetics of individual phases and to the graphitization kinetics of the eutectoid cementite that was formed during the contemplated reaction.

Eutectoid Reaction Ar1 of Cast Iron GJS-400-15 in Isothermal Conditions

This scientific paper presents the research on influence of austenitizing temperature on kinetics and evolution of the spheroidal plain cast iron during eutectoid reaction in isothermal conditions. The cast iron has been austenitized in temperatures of 900, 960 or 1020°C. There were two temperature values of isothermal holding taken into consideration: 760 or 820°C. The order of creation of reaction products and their morphology have been analyzed. The particular attention has been paid to the initial stage of transformation. The qualitative research has been executed using the transmission electron microscope (TEM), as well as quantitative research (LM). The influence of austenitizing temperature has also been determined on transformation kinetics and structural composition. It was found that the increase of austenitizing temperature is conductive to the initial release of structures by metastable system. A reduction of time was observed of the initial stage of transformation at temperature close to Ar12 with its simultaneous elongation at temperature close to Ar11, with an increase of austenitizing temperature. The dependences obtained by the metallographic method confirm the prior results of dilatometric research of eutectoid reaction.