Microstructure Evolution and Mechanical Properties of FeCoCrNiCuTi0.8 High-Entropy Alloy Prepared by Directional Solidification

A CoCrCuFeNiTi0.8 high-entropy alloy was prepared using directional solidification techniques at different withdrawal rates (50 μm/s, 100 μm/s, 500 μm/s). The results showed that the microstructure was dendritic at all withdrawal rates. As the withdrawal rate increased, the dendrite orientation become uniform. Additionally, the accumulation of Cr and Ti elements at the solid/liquid interface caused the formation of dendrites. Through the measurement of the primary dendrite spacing (λ1) and the secondary dendrite spacing (λ2), it was concluded that the dendrite structure was obviously refined with the increase in the withdrawal rate to 500 μm/s. The maximum compressive strength reached 1449.8 MPa, and the maximum hardness was 520 HV. Moreover, the plastic strain of the alloy without directional solidification was 2.11%, while the plastic strain of directional solidification was 12.57% at 500 μm/s. It has been proved that directional solidification technology can effectively improve the mechanical properties of the CoCrCuFeNiTi0.8 high-entropy alloy.

Phase Field Modeling of β(Ti) Solidification in Ti-45at.%Al: Columnar Dendrite Growth at Various Gravity Levels

At present, our understanding of the interaction between melt flow and solidification patterns is still incomplete. In columnar dendritic growth buoyancy driven flow may alter the dendrite tip and spacing selection and consequently the microsegregation of alloying elements. With the aim of supporting directional solidification experiments under hyper-gravity using a large diameter centrifuge (LDC), phase field simulations of β (Ti) dendrite growth have been performed under various gravity conditions for the binary alloy Ti-45at.%Al. The results show that Al segregation at the growth front causes convection rolls around the dendrite tips. The direction of the gravity vector is an essential parameter. When g is opposite to the direction of dendrite growth, increasing gravity leads to a marked decrease of the primary dendrite spacing and to a decrease of the mushy zone length. When g is aligned parallel to the direction of dendrite growth, the primary dendrite spacing and mushy zone length are almost unchanged, however the secondary dendrite arms grow more prominently as the magnitude of g increases.

Influence of Withdrawing Rates on Elements Segregation and Creep Properties of A Single Crystal Nickel-Based Superalloy

The single crystal nickel-based superalloys with different dendrite spacings were prepared by using various withdrawing rates, and the influence of withdrawing rates on elements segregation and creep properties of them is investigated through creep property measurements and microstructure observation. Results show that relatively bigger withdrawing rate leads to the smaller dendrite spacing and lower segregation extent of elements between the dendrite/interdendrite regions, which further causes the better creep resistance of the alloy. Dislocations climbing over the rafted

Crystal Growth during the Solidification Process of Continuous Casting Slab

Under the condition that the solid-liquid interface bends periodically in continuous casting, the expression of solid-phase growth rate adapting to continuous casting was set up, and then the growth rates were calculated. On this basis, the morphologic of crystal growth and the variation of primary dendrite spacing during continuous casting slabs were studied. The results show that the growth rate is the fastest when solid-phase moves to wave crest within a deformation periodicity, whereas the growth rate is the slowest when the crystal moves to wave hollow. The bigger the bulge size is, the greater the variation amplitude of the growth rate will become. The variation of the growth rate results in the S/L interface to develop towards a planar surface. Because the value is much smaller than the critical value of the transformation from cells to dendrites, and the crystals only grow in the fashion of dendrites. The primary dendrite spacing at wave crest is bigger than the primary dendrite spacing at wave hollow in early stage of columnar crystal growth, and the dendrite spacing at wave crest is basically equal with the dendrite spacing at wave hollow in the late stage of solidification, and they quickly simultaneous increase. Good correlation is obtained between the experimental results and the calculation results of the dendrite arm spacing.

Effect of Calcium and Strontium on Microstructure of AZ31 Wrought Magnesium Alloys

AZ31 wrought alloys at as-cast state with different microcontent calcium and strontium was studied by optical microscopy, X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis. The study shows that the primary dendrite spacing and the secondary dendrite arm spacing can be refined significantly by Ca or Sr element. At 0.5wt.% Sr and 1.8wt.% Ca, the best refinement effect is fulfilled, its primary dendrite spacing and secondary dendrite arm spacing decreased from 292μm to 87μm. The Al4Sr intermetallic compound is observed at grain boundaries When Sr was added. The Al4Sr disappears after Ca added, a new ternary intermetallic compound (Mg, Al)2Ca presents. The addition of Sr and Ca can cause microhardness increasing.