Abstract:Native MgO particles in Mg-alloy melts have been recently exploited as potential substrates for heterogeneous nucleation during solidification, leading to significant grain refinement of various Mg-alloys. However, our current knowledge of the nature of the native MgO particles is still limited. In this work, we study both the physical and chemical nature of the native MgO in commercial purity Mg and Mg-9Al alloy by means of advanced electron microscopy. We found that as oxidation products MgO aggregates exist… Show more
“…The second phases which are stable and resistant to coarsening at the processing temperatures are paramount for operations involving high temperatures [26,3,8,13]. The secondary phases containing transition elements and/or rare earth elements and oxide particles were found to be better suited for this purpose [27,8,3,26,28,13]. Figure 6 (a) Compression stress–strain curves of the extruded alloys at room temperature and tested at 10 −3 s −1 and (b) compressive strength comparison of the current work with the literature [29–36].…”
Section: Resultsmentioning
confidence: 99%
“…The strength of the material depends upon various microstructural features such as grain size, size, shape and distribution of second-phase particles and other defects [5,19,[21][22][23][24][25]. The second phases which are stable and resistant to coarsening at the processing temperatures are paramount for operations involving high temperatures [26,3,8,13]. The secondary phases containing transition elements and/ or rare earth elements and oxide particles were found to be better suited for this purpose [27,8,3,26,28,13].…”
Section: Compression Testmentioning
confidence: 99%
“…The second phases which are stable and resistant to coarsening at the processing temperatures are paramount for operations involving high temperatures [26,3,8,13]. The secondary phases containing transition elements and/ or rare earth elements and oxide particles were found to be better suited for this purpose [27,8,3,26,28,13].…”
The hexagonal close-packed structure renders magnesium weak at room and high temperatures for structural applications despite its low density. Inducing thermally stable and coherent second phases would enhance/retain the strength of Mg-based alloys even at high temperatures. This paper aims to develop a high-strength Mg-based nanocomposite. A master alloy composed of Ni and Gd was cast and the composition of Mg 97.56 Ni 1.22 Gd 1.22 (at.-%) was prepared using ball milling for 150 h. XRD plots of the as-milled powder having nano-size crystallites confirm the partial dissolution of the master alloy. Consolidation through sintering with 5, 7 and 9 h of exposure at 550°C and extrusion at 500°C resulted in the formation of Mg 5 Gd, Mg 2 Ni, Gd 2 O 3 and MgO phases. The extruded samples possessed a high strength of 804 MPa, which can be attributed to ultra-fine grains and dispersoid strengthening by homogeneously distributed second-phase particles in the 100-200 nm range.
“…The second phases which are stable and resistant to coarsening at the processing temperatures are paramount for operations involving high temperatures [26,3,8,13]. The secondary phases containing transition elements and/or rare earth elements and oxide particles were found to be better suited for this purpose [27,8,3,26,28,13]. Figure 6 (a) Compression stress–strain curves of the extruded alloys at room temperature and tested at 10 −3 s −1 and (b) compressive strength comparison of the current work with the literature [29–36].…”
Section: Resultsmentioning
confidence: 99%
“…The strength of the material depends upon various microstructural features such as grain size, size, shape and distribution of second-phase particles and other defects [5,19,[21][22][23][24][25]. The second phases which are stable and resistant to coarsening at the processing temperatures are paramount for operations involving high temperatures [26,3,8,13]. The secondary phases containing transition elements and/ or rare earth elements and oxide particles were found to be better suited for this purpose [27,8,3,26,28,13].…”
Section: Compression Testmentioning
confidence: 99%
“…The second phases which are stable and resistant to coarsening at the processing temperatures are paramount for operations involving high temperatures [26,3,8,13]. The secondary phases containing transition elements and/ or rare earth elements and oxide particles were found to be better suited for this purpose [27,8,3,26,28,13].…”
The hexagonal close-packed structure renders magnesium weak at room and high temperatures for structural applications despite its low density. Inducing thermally stable and coherent second phases would enhance/retain the strength of Mg-based alloys even at high temperatures. This paper aims to develop a high-strength Mg-based nanocomposite. A master alloy composed of Ni and Gd was cast and the composition of Mg 97.56 Ni 1.22 Gd 1.22 (at.-%) was prepared using ball milling for 150 h. XRD plots of the as-milled powder having nano-size crystallites confirm the partial dissolution of the master alloy. Consolidation through sintering with 5, 7 and 9 h of exposure at 550°C and extrusion at 500°C resulted in the formation of Mg 5 Gd, Mg 2 Ni, Gd 2 O 3 and MgO phases. The extruded samples possessed a high strength of 804 MPa, which can be attributed to ultra-fine grains and dispersoid strengthening by homogeneously distributed second-phase particles in the 100-200 nm range.
“…This has been confirmed by microstructural observation, such as TiB 2 particles in Al-5Ti-1B master alloys, [43] Zr particles in Mg-33Zr mater alloy [44] and MgO particles in Mg alloys. [45] In this work, we use H (Figure 1(a)) to denote the nucleant particle separation (i.e., the nearest particle distance). For a given population of nucleant particles with a total number density of N 0 , the particle separation is usually described mathematically by a Schulz distribution or log-normal distribution.…”
Section: Numerical Modelling Of Solidification Processmentioning
Solute accumulation/depletion in the liquid around a growing solid particle during the solidification of metallic melts creates a constitutionally supercooled (CS) zone that has a significant effect on the final solidified grain structure. In this paper, we introduce two mechanisms related to the CS zone that affect grain size: one is the grain initiation free zone (GIFZ) that describes the inability of nucleant particles located in the CS zone for grain initiation and the other is re-melting (RM) of solid particles due to overlap of CS zones. Based on these two mechanisms, we have systematically analysed the effect of nucleant particle agglomeration on grain size. We found that nucleant particle agglomeration has a significant effect on grain size and is responsible for the discrepancy between theoretically predicted grain size and the experimental data. In addition, our numerical analysis suggests that under normal solidification conditions relevant to industrial practice solid particle re-melting has little effect on grain size and thus may be ignored during theoretical analysis. A practical implication from this work is that significant grain refinement can be achieved by dispersing the nucleant particles in the melt prior to solidification.
“…Additionally, in the study of grain refinements of MgO particles in Mg-0.5Ca alloys, Wang, et al observed adsorption layers of impurity Al, Ca and N atoms on the MgO{1 1 1} facets [24]. Ma, et al observed Cu segregation at the Al/α-Al 2 O 3 interfaces [25].…”
Segregation of foreign atoms on a solid substrate in a liquid metal modifies the geometry and chemistry of the substrate surface and, correspondingly, its potency to nucleate a solid metal. We here investigate the effects of the segregation of alkaline earth (AE) atoms, Mg, Sr, and Ba, at the interfaces between liquid Al and γ-Al2O3{1 1 1} substrates using an ab initio molecular dynamics method. This study reveals the high stability and localized nature of the segregated AE atoms at the oxide substrates. The segregation of the AE atoms induces reconstruction of the metal atoms terminating the oxide substrates, and causes atomic roughness of the substrate surfaces. The content of the induced atomic roughness relates to the ionic size of the AE atoms. Correspondingly, the potency of the oxide substrates is modified. This indicates the possibility of manipulating the substrate potency via segregation of selected impure atoms, which would help to control solidification processes.
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