Growth Behaviour Of Primary-crystalline Phases And Mechanical Properties Of Directionally Solidified Al-Mn-(Be) Alloys

In this study, Al-6wt.%Mn（-2.5wt.%Be） alloys have been selected forinvestigation. The growth morphology and evolution process of primary Al₆Mnphase during the directional solidification have been investigated, thereby revealingthe effect of solidification condition on the growth behavior of Al₆Mn phase.Meanwhile, effects of minor Be addition on the solidification property andformation of icosahedral quasicrystal have been investigated by adding the thirdelement Be into the hypereutectic Al-Mn alloy. The morphological feature andgrowth mechanism of the intermetallic compound and icosahedral quasicrystal afterBe addition have been discussed. Based upon those work, the mechanical propertiesof directionally solidified Al-Mn-（Be） alloy with different solidification parametershave been investigated. In addition, the fracture behavior and strengtheningmechanisms of two alloys have been analyzed. A technology for preparing a high-strength in-situ composite with finely dispersed intermetallic or icosahedralquasicrystal has been developed. This work provides a theoretical foundation andpractical guidance for the active control of the morphologies of intermetalliccompounds and the development of advanced in-situ intermetallic or quasicrystalparticles-reinforced metal matrix composite materials.During directional solidification, at a low growth rate （V=1m/s）, Al₆Mnprecipitates in a faceted growth with sharp edges and corners and exhibits elongatedmorphology. EBSD results and crystal structural analysis indicate that Al₆Mncrystal has a preferred growth direction along the crystallographic [001] directionduring directional solidification. Comparing with other low-index crystal plane,（011） and （101） is more closely packed. Therefore, at a low growth rate, Al₆Mntends to form an octahedron morphology enclosed by four {011} planes and four{101} planes. A nearly equilibrium growth model of Al₆Mn has been establishedbased on the analysis of actual3-D morphologies. Theoretically, the crystalmorphology is determined by the competition between various important crystalplanes. Based on the lattice parameter of orthorhombic Al₆Mn and combined withmorphological analysis, a relationship between morphologies and growth-rate ratiosof （001）,（011） and （101） planes different planes has been built.As the growth rate increases, the morphologies of Al₆Mn phase transit fromsolid truncated polyhedral or prism to hollow prism, and further to a groovemorphology and ultimately developed dendrites （V=1000m/s）, and also the growthpattern transits from faceted to non-faceted. Increasing growth rates lead to an accelerated angular growth mechanism mainly determined by volume diffusion, andhollow prism and groove morphologies form. Meanwhile, increasing growth rateslead to a large undercooling ahead of the S/L interface, which favors the continuousgrowth of Al₆Mn and formation of dendritic Al₆Mn. This paper proposes a melt-cluster model during the faceted-non-faceted transition when the cooling rateincreases. This model can be used to illustrate the transition of growth pattern ofAl₆Mn.A relation between the surface energy of （001） and （101） crystal planes and thecritical nucleus morphology in the complex orthorhombic Al₆Mn has been builtusing Helmholtz formula. The growth of Al₆Mn is controlled by the2-D nucleusand screw-dislocation layer growth mechanisms, which are conducted on theclosely-packed （101） and （011） planes. The frequent renucleation and growth modelhas also been proposed. The nucleation of a new crystal terminates the growth of aparent crystal, and sequent growth of the new crystal also may be terminated by anew nucleation event.The addition of Be results in the shift of binary phase diagram toward the Mn-rich side and the appearance of three intermetallic compounds, namely λ-Al₄Mn、H1and Be4AlMn, and significantly fined the microstructure of the as-cast anddirectionally solidified samples. Meanwhile, the addition of Be effectivelypromotes the formation of icosahedral quasicrystal, evidenced by the reduced Mnconcentration and cooling rate required for the formation of icosahedral phase. Inthe directionally solidified sample, the critical cooling rate determined is within1.5².5K/s.During directionally solidification of Al-6wt.%Mn-2.5wt.%Be alloy, with theincrease of growth rates, competitive growth of intermetallic compounds betweenprimary and eutectic phases takes place. The primary phase transits from λ-Al₄Mnto the H1phase, and ultimately icosahedral quasicrystal. The matrix transits from（-Al+λ-Al₄Mn） to （-Al+H1）, and ultimately （-Al+Icosahedral） eutectic.Meanwhile, the3-D morphology concurrently transits: Hexagonal prism λ-Al₄Mn�'Hexagonal prism H1�'Six-petal H1phase�'Dendritic H1phase�'Facetedicosahedral phase�'Flower-like icosahedral phase�'Dendritic icosahedral phase.Based on the pseudo I13cluster model and high resolution transmission electronmicroscopy of H1phase, the structural model of H1phase has been proposed. Suchtransition is well demonstrated based on the similarity of the structure. The3-Dmorphology is closely related to the crystal structure of λ-Al₄Mn, Be4AlMn and H1phase. Their growth patterns are the2-D nucleation layer growth on the closely-packed planes. As the growth rate increases, the morphology of primary icosahedralquasicrystal transits from faceted pentagonal dodecahedral cluster to flower-likedendrites, and ultimately non-faceted dendrites. Based on the actual morphologiesof icosahedral quasicrystal, the icosahedral symmetric model of the pentagonaldodecahedral cluster has been established along2-fold,3-fold and5-fold direction.The growth model of icosahedral quasicrystal has also been proposed, andrevealing that the icosahedral quasicrystal is grown on the5-fold plane by2-Dnucleation-layer growth mechanism.The mechanical properties of directionally solidified Al-6wt.%Mn and Al-6wt.%Mn-2.5wt.%Be alloys have been investigated. It is indicated that room-temperature mechanical properties of both alloys increase with the increase ofgrowth rates. But the elongation reduces first and then increases as the growth ratesincrease, which can be attributed to the properties of strengthening phases. At agrowth rate of1000μm/s, both two alloys have a highest ultimate tensile strengthand a relative large elongation. The ultimate tensile strength and elongation are188and244MPa,16.66%and12.01%, respectively. The fracture mode of two alloystransits from brittle fracture to ductile fracture as the growth rates increase.Compared with binary phase, the addition of Be significantly enhances themechanical properties of the alloy, especially the alloy with a high growth rate.This is mainly due to the interfacial strengthening effect of icosahedral quasicrystalexpected for the particle strengthening and matrix strengthening effects in the Al-6wt.%Mn-2.5wt.%Be alloy. Due to the strengthening effect of icosahedralquasicrystal, the Al-6wt.%Mn-2.5wt.%Be alloy with a large cooling rate exhibitsexcellent mechanical properties under high temperature. At200℃，the alloy has astrength closed to that under room temperature and a large elongation of35%.