However, the addition of more than 0.2 wt% induced a detrimental effect on the mechanical properties due to precipitation of a high volume fraction of insoluble intermetallic compounds containing RE. With regard to the mechanical properties, it was found that the highest strength of the casting alloys was found with 0–0.2 wt% addition of Ce/La. The microstructural observations show that the RE elements have a greater impact on the microstructure of cast alloys, resulting in the formation of new types of intermetallic phases containing RE, Al–Si–Cu–RE and Al–Ti–RE. The volume fraction, aspect ratio, and roundness of second phase constituents were determined by ImageJ analysis software. The microstructures were investigated using optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA).
#AL ATOMIC RADIUS SERIES#
In the current study, a series of forty-two cast Al–Si–Cu–Mg -based alloys with additions of transition metals (V, Cr, Ni) and rare earth (RE) metals (La, Ce) were prepared and mechanically tested in the as-cast, T5, T6 and T7 heat treatment conditions. The formed brittle phases deteriorate the tensile properties of the alloy and the fracture mode of Al- 7Si-0.2/0.4 La changes to mixed ductile-brittle fracture mode. However, when La element content reaches a certain value of 0.4wt.%, serious segregation takes place during the solidification process. The fracture mode evolves from the ductile-brittle mixed fracture to ductile fracture mode. After T6 heat treatment, compared with the without La, the ultimate tensile strength of the alloy with 0.1wt.% La is enhanced by 5.2% from 333 MPa to 350.2 MPa and the elongation increases by 73% from 7.37% to 12.75%, correspondingly. Higher undercooling degree suggests that nucleation is accelerated and subsequent growth is restrained.
Because when 0.1wt.% La is added, a relatively higher nucleation undercooling of 37.47 ☌ is observed. La addition can refine the secondary dendrite arm spacing (SDAS) and eutectic Si particles, which are decreased by 7.9% and 7%, respectively, with the optimal La content of 0.1wt.%. It is found that the La appears in the Al-7Si alloy in the form of Al4La and Al2Si2La phases. The effect of La addition (0, 0.1, 0.2, 0.4, wt.%) on the microstructure, tensile properties and fracture behavior of Al-7Si alloy was investigated systematically. Finally, failure envelopes or the Kitagawa-Takahashi diagram was prepared for different temperatures to facilitate a damage tolerant design approach.
#AL ATOMIC RADIUS CRACK#
Using the microstructural observations of the transformed zone, an estimate of the crack closure due to SIMT is provided. SIMT induces dilatation and shear, which enhances crack closure and retards FCG rates at RT compared to that at 150 ☌. Microstructural investigations of the fatigue crack reveal that SIMT occurs at the crack tip of alloys tested at room temperature, whereas the same was negligible at 150 ☌. FCG rates were found to be broadly independent of the build orientation and microstructure, although it is reduced in shorter builds due to the presence of higher compressive residual stress in them. Effect of build orientation, microstructure, and temperature-considering that the alloy undergoes temperature-dependent stress-induced martensitic transformation (SIMT)-were determined. The fatigue crack growth (FCG) behavior and fatigue strength of 304L stainless steel (SS) manufactured by the laser powder bed fusion (LB-PBF) process were investigated. Moreover, the formation of twinned Si particles was attributed to the adsorption of La-rich clusters along the ⟨112⟩Si\documentclass, because this eutectic reaction reduced the undercooling and produced the needle-like La-rich particles. This refinement effect of La on α-Al was further promoted by the squeeze pressure during solidification. These results indicate that the added La increased constitutional supercooling and heterogeneous nucleation cores ahead of the melt/α-Al interface and thus La promoted the nucleation of α-Al. The refinement mechanism of α-Al grains with the addition of La has been discussed based on the classical nucleation theory, interdependence model and thermodynamics. The results demonstrate that La can refine α-Al grains, modify eutectic Si and improve tensile properties of the Al7Si4CuMg alloys. The effects of rare-earth lanthanum (La) on the microstructure and tensile properties of Al7Si4CuMg alloys prepared by squeeze casting were studied by microscopy technologies, thermal analysis and tensile tests.
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