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Register a new userIcosahedral quasicrystal-enhanced nucleation in Al alloys fabricated by selective laser melting
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Selective laser melting (SLM) is rapidly evolving to become a mainstream technology. However, the fundamental mechanisms of solidification and microstructure development inherent to the non-equilibrium conditions of this additive manufacturing method, which differ largely from those typical of conventional processing techniques, remain widely unknown. In this work, an in-depth characterization of the microstructure of Al7075 SLM processed samples, built from powder mixtures containing ZrH2 microparticles, demonstrates the occurrence of icosahedral quasicrystal-enhanced nucleation during laser fabrication. This solidification mechanism, only observed to date in cast Al-Zn and yellow gold alloys containing minute additions of Cr (Kurtuldu et al., 2013) or Ti (Chen et al. 2018), and Ir (Kurtuldu et al., 2014), is evidenced by the presence of an abnormally high fraction of twin boundaries and of five-fold orientation symmetry between twinned nearest neighbors lying within a matrix of equiaxed, randomly textured, ultrafine grains. This research attests to the wide range of possibilities offered by additive manufacturing methods for the investigation of novel physical metallurgy phenomena as well as for the design of advanced metals.
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Microstructure modifications induced by sliding a WC-Co indenter in scratch tests on the surface of a single phase AlCuFe icosahedral quasicrystal (IQC) was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The scratch track was shown tocomprise many smaller tracks. Dislocations were discovered to emerge from the edges of the smaller scratch tracks. Along a small track where shear stress is concentrated, a phase transition from IQC to a body-centered cubic (b.c.c.) phase with lattice parameter a=0.29 nm was pointed out. A modulated quasicrystal state as well as a deformation twin of IQC were determined in the region beneath the scratch.
We report on the fabrication of fractal dendrites by laser induced melting of aluminum alloys. We target boron carbide (B4C) that is one of the most effective radiation-absorbing materials which is characterised by a low coefficient of thermal expansion. Due to the high fragility of B4C crystals we were able to introduce its nanoparticles into a stabilization aluminum matrix of AA385.0. The high intensity laser field action led to the formation of composite dendrite structures under the effect of local surface melting. The modelling of the dendrite cluster growth confirms its fractal nature and sheds light on the pattern behavior of the resulting quasicrystal structure.
With important application prospects, eutectic high entropy alloys have received extensive attention for their excellent strength and ductility in a large temperature range. The excellent casting characteristics of eutectic high entropy alloys make it possible to achieve well manufacturability of selective laser melting. For the first time, we have achieved crack-free eutectic high entropy alloy fabricated by selective laser melting, which has excellent mechanical properties in a wide temperature range of -196 degrees Celsius~760 degrees Celsius due to ultra-fine eutectic lamellar spacing of 150 ~ 200nm and lamellar colony of 2 ~ 6 {mu}m. Specifically, the room temperature tensile strength exceeds 1400MPa and the elongation is more than 20%, significantly improved compared with those manufactured by other techniques with lower cooling rate.
We report the discovery of a new binary icosahedral phase in a Sc-Zn alloy obtained through solution-growth, producing millimeter-sized, facetted, single grain, quasicrystals that exhibit different growth morphologies, pentagonal dodecahedra and rhombic triacontahedra, under only marginally different growth conditions. These two morphologies manifest different degrees of quasicrystalline order, or phason strain. The discovery of i-Sc$_12$Zn$_88$ suggests that a reexamination of binary phase diagrams at compositions close to crystalline approximant structures may reveal other, new binary quasicrystalline phases.
The effect of Al atoms in solid solution on the critical resolved shear stress for twin nucleation and growth was analyzed by means of the combination of diffusion couples with compression tests in micropillars oriented for twinning. The critical resolved shear stress for twin nucleation was higher than that for twin growth and both increased by the same amount with the Al content. Nevertheless, the increase was small ( approx. 10 MPa) for 4 at.%Al but large (up to 60-70 MPa) for 9 at.%Al. These results were in agreement with Labusch-models based on first principles calculations in the dilute regime (< 5 at.%Al) [51]. Comparison with recent data in the literature showed that Al atoms are more effective in increasing the critical resolved shear stresses for twin nucleation and growth than for basal slip [21]. Finally, compression tests in micropillars oriented along [0001] showed the critical shear stress for pyramidal slip increased rapidly with the Al content from 98 MPa in pure Mg to 250 MPa in Mg-9 at.%Al. Thus, the addition of Al increased the plastic anisotropy of Mg alloys.
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