Microstructure refinement mechanism of undercooled Cu55Ni45alloys

Different undercooling degrees of Cu55Ni45 alloy were obtained by the combination of molten glass purification and cyclic superheating, and the maximum undercooling degree reached 284 K. The microstructure of the alloy was observed by metallographic microscope, and the evolution of microstructure was studied systematically. There are two occasions of grain refinement in the solidification structure of the alloy: one occurs in the case of low undercooling, and the other occurs in the case of high undercooling. Electron backscatter diffraction (EBSD) technology was used to analyze the rapid solidification structure under high undercooling. The features of flat polygonal grain boundary, high proportion of twin boundary, and large proportion of large angle grain boundary indicate recrystallization. The change in microhardness of the alloy under different undercooling degrees was studied by microhardness tester. It was found that the average microhardness decreased sharply at high undercooling degrees, which further confirmed the recrystallization of the solidified structure at high undercooling degrees.

The Prediction of the Undercooling Degree of As-Cast Irons and Aluminum Alloys via Machine Learning

As-cast irons and aluminum alloys are used in various industrial fields and their phase and microstructure properties are strongly affected by the undercooling degree. However, existing studies regarding the undercooling degree are mostly limited to qualitative analyses. In this paper, a quantitative analysis of the undercooling degree is performed by collecting experimental data and employing machine learning. Nine machining learning models including Random Forest (RF), eXtreme Gradient Boosting (XGBOOST), Ridge Regression (RIDGE) and Gradient Boosting Regressor (GBDT) methods are used to predict the undercooling degree via six features, which include the cooling rate (CR), mean atomic covalence radius (MAR) and mismatch (MM). Four additional effective models of machine learning algorithms are then selected for a further analysis and cross-validation. Finally, the optimal machine learning model is selected for the dataset and the best combination of features is found by comparing the prediction accuracy of all possible feature combinations. It is found that RF model with CR and MAR features has the optimal performance results for predicting the undercooling degree.

Carbide to Graphite Transition Control by Thermal Analysis in Grey Cast Irons

The present work compared the solidification pattern of un-inoculated and inoculated hypoeutectic grey cast irons (3.7–3.8% CE), focused on carbide to graphite formation transition, by the use of an adequate experimental technique, able to measure real stable (Tst) and metastable (Tmst) eutectic temperatures. Have been used three ceramic cups for investigating thermal analysis: (i) for normal solidification; (ii) with addition of Te for Tmst measurement; (iii) with more inoculant addition for Tst measurement. As a general rule, measured values appear to be lower compared with calculated values (as chemical composition effects), with an average difference at 14.4 °C for Tst and 8.3 °C for Tmst. It is found a good relationship between the undercooling degree at the lowest eutectic temperature (ΔT1) and at the end of solidification (ΔT3), reported to measured Tmst. The free carbides formation (chill tendency) is in good relationship with the undercooling degree during the eutectic reaction, reported to measured Tmst, especially for thin and medium wall thickness castings. The real measured Tmst instead of calculated Tmst is compulsory for the thin wall castings production, very sensitive to carbides to graphite transition. In the present experimental conditions, no visible relationship appears to be between chill tendency and undercooling at the end of solidification (ΔT3).

The Study on TiAl Based Alloy Blade Casting Structure by Bottom Pouring Vacuum Suction Casting

In this paper, the bottom pouring vacuum suction casting is used, and the TiAl-based alloy blade with good face quality was obtained. Bottom pouring vacuum suction casting obtained Ti-47Al-5Nb-0.5Si alloy fine structure, the average grain size is 20um or less. Metal permanent forced cooling effect increases the undercooling degree, and the formation of Ti5Si3 can hinder the grain growth and the formation of nucleation particles, and Nb elements are conducive to the formation of B2 phase, and these three reasons refined the grain size.

Effects of Melt Superheating Holding Time on Solidification Structure and Mechanical Property of AZ31B Magnesium Alloy

Effects of melt superheating holding time on microstructure and mechanical properties of AZ31B magnesium alloy were studied. The results show that the grain size of AZ31B magnesium alloy is gradually increased with the elongation of holding time. The grain is the finest without insulation work, and the increase amplitude of grain size is the most when holding for 20 minutes in this experiment. The higher is the melt superheating temperature, the larger the increase amplitude of grain size when holding for 20 minutes is. The tensile strength, yield strength and elongation percentage of AZ31B magnesium alloy specimens are small declined with the elongation of holding time under lower melt temperature. At 850°C or 900°C, the mechanical properties are rapidly declined with the elongation of holding time when holding time is less than 20 minutes, whereas the mechanical properties are slight increased when holding time is more than 20 minutes. DSC analysis shows that the elongation of holding time causes the increase of solidification onset temperature and the augment of solidification range, so the undercooling degree and nucleation rate are decreased in the melt, which result in the grain coarsening and the mechanical properties decrease.

Influence of Liquid Structure Change on Microstructure and Properties of SnZnBi Solder Alloy

Currently, Pb-free is the primary trend of development for solder alloys, and the existing Pb-free solder alloys are still difficult to replace the traditional tin-lead solder alloys. How to further improve the welding properties of Pb-free solder alloys is the issue we currently faced. In this paper, through melt overheating treatment, the influence of liquid-liquid structure change (LLSC) on the structure and properties of SnZn8Bi3 Pb-free solder alloy has been studied. Experimental results show that the LLSC has obvious effects on the solidification process and solidified microstructure of SnZn8Bi3 alloy: bigger solidification undercooling degree in the solidification process, finer and more dispersed solidification structure, and more importantly, the mechanical and welding properties of the solder alloy have also been obviously improved.

Observation of Microstructural Refinement in Mg-3Sn-2Ca Magnesium Alloy Containing Minor Sr

The microstructures of the Mg-3Sn-2Ca magnesium alloys with and without adding minor Sr were investigated and compared. The results indicate that adding 0.1 wt.% Sr to the Mg-3Sn-2Ca alloy can effectively refine the primary CaMgSn phases and grains. In addition, in the alloy containing minor Sr the volume fraction of the primary and eutectic CaMgSn phases appears to be decreased, whereas the volume fraction of the Mg2Ca precipitates seems to be increased. The mechanism for the microstructural change in the alloy containing minor Sr is possibly related to the effects of Sr additions on the onset crystallizing temperature and undercooling degree.

Effects of Nd123/MgO Thin Film and MgO Single-crystal Seeds in Isothermal Solidification of YBaCuO/Ag

The seeding effects of (001) Nd123/MgO thin films and MgO single crystals were studied in isothermal solidification of YBa2Cu3Oy composite with the additions of 40 mol% Y2BaCuO5, 10 wt% Ag, and 0.5 wt% Pt. Seeding with the Nd123/MgO thin film resulted in single-domain growth of Y123 crystal with a stable growth along the “100”?direction, while seeding with MgO single crystal produced multidomain growth in which the dominant growth facet is rotated 45° about (100) plane of MgO. Multidomain growth in MgO seeded sample was suppressed by decreasing undercooling degree. The effects of undercooling degree and seed size on multidomain growth are discussed in view of classical nucleation and growth theory.