Growth Morphology, Orientation And Mechanical Properties Of Intermetallic Al₂Cu Phase Under Directional Solidification

Intermetallic is an important component phase of engineering alloys, with highyield strength, appreciable creep resistance and reasonable corrosion resistance, has awide range of application, such as Ti3Al structural material, NiTi shape memorymaterial, LaNi6hydrogen storage material. In solidification process, intermetallic couldform the competition and transformation between the faceted phase and the non-facetedphase, which causes the growth morphology evolution with orientation selecting. Theorientation changing is particularly important for the growth morphology and thematerial properties. However, the study of orientation changing of intermetallic duringits morphology evolution has not been established. Moreover, the applications ofintermetallic alloys are hindered by the brittleness, especially, the room-temperaturebrittleness. To solve those problems, this work selected the intermetallic Al2Cu phase ofAl-40wt%Cu hypereutectic alloy as the research object. Basing on directionalsolidification, this thesis carried out systematic researches on the influences of thesolidification rate and the size effect on the intermetallic Al2Cu phase microstructureevolution and orientation changes. Meanwhile, the growth mechanism of Al2Cu phasewas investigated under different conditions. In addition, the influences of solidificationrate on mechanical properties of Al-40wt%Cu hypereutectic alloy were also discussed.The main conclusions of the present study can be drawn as follows.(1) On the basis of the maximum interface growth temperature assumption, thecritical solidification rate of Al2Cu phase dendrite obtained was calculated as2.67μm/s,and the rules of Al2Cu phase growth morphology changes were also analyzed andverified by the experiment. With the solidification rate increasing, the Al2Cu phaseturned from lamellar to faceted L-shaped morphology, irregular I-shaped morphologyand non-faceted smooth three lobation pattern successively. The three-dimensional (3D)microstructures were reconstructed using the serial sectioning technique.(2) Using the macro and micro orientation analysis technique, it observed that theprimary Al2Cu phase dendrites both grew along its [001] direction in axial direction atdifferent directional solidification rates. With the solidification rate increasing, Al2Cudendrites grew along its growth direction gradually deviated from the heat flowdirection, between which the maximum degrees of deviation was8.91°. At higher directional solidification rate, the non-faceted rough interface of Al2Cu dendrites inradial direction resulted from that the phase grew along simultaneously [110],[310] and[200] orientations in the lateral direction. Due to the two-dimensional growth theory, thegrowth mechanism of Al2Cu phase dendrites under directional solidification wasexplained. The growth mechanism of Al2Cu phase dendrite under directionalsolidification included coarsening process in the lateral direction and growth process inaxial direction.(3) After pulled100mm at directional solidification rate10μm/s, the primaryAl2Cu dendrite transited from regular faceted V-shaped morphology to the entirelycoupled eutectic at the solidification distance of80mm at10μm/s with solidificationdistance increasing. Then the alloy liquid composition ahead of the solid/liquid interfaceapproached the eutectic point. Its growth orientation changed from [110] direction to thenormal direction of (121) plane in axial direction. The [001] direction of Al2Cu phasewas along the direction of heat flux. The reason about solute front solid-liquid interfacedecreased and growth morphology evolution were explained by the Scheil model.(4) As the growth rate was changed abruptly from10μm/s to2μm/s, the alloyliquid composition ahead of the solid/liquid interface increased firstly and thendecreased to the eutectic point. It caused that the primary Al2Cu dendrite transited fromregular faceted V-shaped morphology to long smooth lath morphology in longitudinalsection, and finally disappeared to form the entirely coupled eutectic. In transversesection the primary Al2Cu dendrite transited from regular faceted L-shaped patternrod-shaped morphology. Moreover, the growth orientation of Al2Cu phase was [001]direction. As the distance increasing from the transmission interface, the deviation anglebetween [001] direction and the heat flow directional decreased. The relationshipbetween the liquid composition on dendrite interfaces and the Péclet number wasquantitatively analyzed, and Al2Cu phase dendrite morphology evolution aftersolidification rate changes was discussed.(5) To study the effect of Al2Cu phase morphology and orientation evolution bythe convection effects, this work had self-designed the device for preparing the0.45mmsample and done the directional solidification experiments in different size samples.With the sample size decreased, microstructures of the primary Al2Cu phase of the alloycan be observed with various growth patterns, L-shaped, E-shaped, and regularrectangular pattern in transverse section. The three-dimensional (3D) microstructure ofregular rectangular pattern was the tetrahedron morphology. Al2Cu dendrites all grewalong their [001] direction in axial direction. Moreover, with the sample size reduced, the deviation angle between the [001] direction and the heat flow direction wasdecreased to3.78°. The growth directions of Al2Cu phase were gradually transformedfrom [110] and [310] directions to [310] direction in radial growth. The L-shape andregular rectangular shape of Al2Cu phase are bounded by {110} facets by the HRTEManalysis. The growth mechanism for the regular faceted Al2Cu dendrite grew radialdirection was interpreted by the oriented attachment mechanism (OA). By analyzingconvection effects, the reasons of the growth direction of Al2Cu phase dendrite changedin radial direction and the degree of deviation in axial direction reduced were discussed.(6) In the0.45mm sample, the primary Al2Cu phase dendrite gradually displayedregular faceted square morphology with the solidification rate increasing. The growthorientation [001] direction in the axial direction was more protruding. In the purediffusion growth condition, through analyzing the effect of solidification rate on thesolute diffusion variation ahead the solid-liquid interface, the rule of crystal surfaceenergy changing was discussed. In addition, the growth mechanism of Al2Cu phasedendrite with faceted square morphology in smallest sample was given.(7) With the directional solidification rate increasing, the compressive strength,tensile strength and flexural strength of the directionally solidified Al-40%Cu alloywere reached to247.8MPa,277.2MPa and49.6MPa, which were significantly higherthan that of vacuum induction melting alloy with158MPa,152MPa and36.3MPa.Moreover, the room temperature fracture toughness of the directionally solidified alloywas reached to6.23MPa·m1/2, which was increased by37.3%compared with vacuuminduction melting alloy. Due to the fine-microstructure strengthening and the crackpropagation way changing caused by the orientation variation, the reason of themechanical properties of the directional solidification Al-40%Cu alloy had been greatlyimproved with the directional solidification rate increasing. In addition, the relationshipbetween solidification microstructure, orientation and mechanical properties wasdiscussed.