Influence Of Surface Tension Anisotropy On Microstructure And Mechanical Properties Of Directionally Solidified Al-4.5%Cu Alloy

The dendritic morphology plays a crucial role in determining the performance of cast products.Therefore,the selection of crystal growth patterns and the evolution of morphological transition are of particular importance.A significance understanding of including the anisotropy of the solid-liquid surface tension in the dendritic morphology selection has been well characterized.Further,the surface tension anisotropy relies on the crystallographic orientation with respect to the crystal lattice.By accurately controlling the orientation of crystals,a variety of morphologies,including dendrite,degenerate pattern and seaweed,have been found during directionally solidified transparent organic alloys.However,the authors are not aware of the degenerate pattern and seaweed in the solidification experiments on metallic alloys.So,experimental investigation of the pattern formation was presented in directionally solidified Al-4.5 wt.%Cu alloy by using single-crystal seed in a planar front to control the surface tension anisotropy.During the experiment,many kinds of morphologies were fabricated firstly.The main contents and conclusions are as following:By modification of surface tension anisotropy with changing orientation,axial dendrite（AD）,tilted dendrite（TD）and degenerate pattern（DP）were identified during directional solidification of Al-4.5 wt.%Cu alloy in the（100）plane.Crystals presenting a<100>axis parallel to the temperature gradient would produce stable dendrites and deflected in{100}plane tilted dendrites grew.When two<100>axes were symmetrically titled with respect to the temperature gradient direction,degenerate pattern emerged.Moreover,different types of seaweeds,including the axial,the tilted and the degenerate seaweeds were observed in the（111）plane.The surface tension in the（111）plane of Al-4.5wt.%Cu alloy is anisotropic.The axial seaweed（AS）existed in the crystal presenting a<110>axis nearly parallel to the temperature gradient.When the<110>direction deviated from the temperature gradient,the seaweed was emerged with an inclination feature.In crystals presenting two<110>axes symmetrically arranged with respect to the temperature gradient direction,the degenerate seaweed（DS）was observed.Among these different dendritic morphologise,we focused on a complex dendrite,degenerate dendrite and the growth dynamics of degenerate pattern was studied in detail.Under a high temperature gradient（G,around 200 K/cm）,the degenerate pattern can grow steadily at the growth velocities less than 50μm/s.At a high growth velocity（>50μm/s）,the pattern transition from the degenerate to regular dendrite happened.During the degenerate pattern growth,its tip-splitting spacing followed a power law λ∞ V^-0.5,agreeing with the M-S instability and the frequency of the tip splitting was related to the growth velocity as f∞ V^1.5.In addition,the dimensionless growth direction,θ/θ₀,increased monotonously with the dimensionless growth velocity V/V_C and followed θ/θ₀=1-1/(1+0.1186（V/V_C）^0.673).Meanwhile,by employing step-increasing experiments,the influence of growth velocity on the microstructural evolution of degenerate pattern in Al-4.5 wt.%Cu alloy was systematically studied.After abruptly increasing the growth velocity from 15μm/s to 100μm/s,the texture intensity I₁₀₀/I₁₁₀ of the degenerate pattern was found to remain unchanged,indicating the degenerate pattern could be growing till the end of solidification.The necessary velocity condition for the degenerate pattern growth could be extended to 100V_cs,two orders of magnitude higher than the planar-cellular transition velocity.By comparing the step-increasing experiments with the constant growth velocity ones,it was also found that the tip-splitting spacing （λ） and the frequency （f） were strongly dependent on the growth velocity and fitted to the power law as λ=134.4×V^-0.46 and f=0.005×V^1.45.Furthermore,bicrystal assembled experiments were carried out to investigate the competitive growth between the degenerate pattern and the regular dendrites.The experimental results for the competitive growth showed that the dendrites could overgrow the degenerate pattern completely at V=15μm/s and 25μm/s.The grain boundary in between these two morphologies was not smooth,and its inclination angleθ_GB was slightly increased with an increase in the growth velocity.For the converging grain boundaries（GBs）,we found that the dendrites overgrew the degenerate pattern either by generating new primary arm dendrites through tertiary branching or being blocked by the growth of the existing primary arm dendrites,depending on the misorientation arrangement between these two morphologies.Specimens with different microstructures that identified during directional solidified Al-4.5 wt.%Cu alloy were tensile tested to explore their corresponding mechanical properties.The ultimate tensile strength decreased slightly from 174 in AD,157 in TD to 152 MPa in DP specimens,respectively.The yield strength also exhibited the same trend.However,the uniform elongation appreciably increased from 15.5%in AD,26.1%in TD to 52.2%in DP specimens.Remarkable elongation to failure（?_f=62.8%）was obtained in the DP specimen,increasing by 211%and 191%in comparison with AD and TD specimens.This remarkable increase of uniform elongation and elongation to failure indicated that the degenerate morphology has a favorable effect on the ductility.The toughness of 85.4 MJ·m^-3and work hardening effect were achieved for DP specimen,which were much higher than those for AD(45.6 MJ·m^-3)and TD(44.9 MJ·m^-3)specimens.Moreover,for alloy with seaweed morphologies the yield strength and ultimate tensile strength reached 88.8 and 186.1 MPa for AS,84.6 and 151.6 MPa for TS,77.6 and 164.3 MPa for DS specimen,respectively.In addition,the uniform elongation increased from 43.2%in AS to 44.4%and 55.7%in TS and DS specimens,respectively.Among these three kinds of seaweed morphologies,AS specimen gets a higher strength and DS specimen has a remarkable ductility.Field emission scanning electron microscopy（FESEM）and transmission electron microscope（TEM）characterization have uncovered underlying mechanisms associated with this enhancement.