ABSTRACT
The effects of Mn addition on the room temperature tensile strength and deformation mechanisms of as-cast Mg-8Al-1Nd-1.5Gd-xMn alloys (x = 0, 0.3, 0.5, 1.0 wt.%) are investigated in this paper. The results indicate that the addition of Mn contributes to the precipitation of Al-Mn-RE intermetallics and the refinement of α-Mg matrices, thereby improving the tensile strength of the 1.0 Mn alloy at 190 MPa. The fracture mechanism of Mn-containing alloys transforms from a cleavage fracture to a ductile fracture as the Mn content increases from 0.3 to 1.0 wt.%. The presence of intermetallic particles in the dimples confirms the hindrance effect of Al10Mn2 (Nd,Gd) on dislocation slips. The novel technology of in-grain misorientation axes (IGMAs) is used to identify activated slip modes and deformation twins. It can be concluded that the activated pyramidal slip during tensile deformation significantly promotes the ductility of the 1.0 Mn alloy with an elongation rate of 9.8%. It is worth noting that reducing the coarse 101¯2 tensile twins and enhancing the proportion of 101¯1 compressive twins and 101¯1-101¯2 double twins contributes to maintaining the continuous plastic deformation of Mg alloy.
ABSTRACT
Coarse primary and eutectic Mg2Si phases were generally precipitated in Mg-Al-Si alloys during solidification at a low cooling rate, which tends to deteriorate the strength and ductility of magnesium alloys due to stress concentration. Different volume fractions of TiB2 nanoparticles (1%, 3%, and 5%) were added to an Mg-4Al-1.5Si alloy to refine the coarse Mg2Si phases based on a heterogeneous nucleation mechanism. The nanoparticles were incorporated and dispersed in the molten Mg alloys and by using semi-solid stirring followed by ultrasonic treatment (SSUT), and TiB2/Mg-4Al-1.5Si composites were obtained. The effect of TiB2 content on the microstructure and mechanical properties of the composites was studied. The results showed that the average size of primary Mg2Si phases and α-Mg grains decreased as the TiB2 content raised, the dendritic primary Mg2Si phases were refined into polygonal shapes with smaller sizes, and the refined primary Mg2Si phases were uniformly distributed in the alloys after adding 1 vol.% or 3 vol.% TiB2 nanoparticles. As the TiB2 content increased, the morphology of the eutectic Mg2Si phases was modified from coarse Chinese characters to short rod or fine dot shapes. Vickers hardness and yield strength of the composites reached a maximum (153 HV and 90.9 MPa, respectively) when TiB2 content was 5 vol.%, while the most superior ultimate tensile strength (142.4 MPa) and elongation (9.2%) were obtained when TiB2 content was 3 vol.%, which were improved by 173.2%, 31.5%, 69.8%, and 187.5%, respectively compared with the Mg-4Al-1.5Si alloys.
ABSTRACT
Ni-CNTs/AZ91 magnesium matrix composites were fabricated by ultrasound treatment combined with a semi-solid stirred method for the first time. The agglomerated spherical Ni-CNTs transferred from spherical shape to clear tubular shape after pre-dispersion treatment. For the Ni-CNTs/AZ91 magnesium matrix composite prepared by semi-solid stirring followed by ultrasonic treatment, Ni-CNTs were evenly distributed in the magnesium matrix or wrapped on the ß (Mg17Al12) phase. Mg2Ni were formed at the interface of the magnesium matrix and CNTs by in-situ reaction, which significantly improved the interface bonding strength of CNTs and the Mg matrix. The tensile strength and elongation of 1.0wt.% Ni-CNTs/AZ91 magnesium matrix composites were improved by 36% and 86%, respectively, compared with those of AZ91 matrix alloy. After Ni-CNTs were added to AZ91 matrix alloy, more dimples were observed at the fracture surface. The fracture behavior of Ni-CNTs/AZ91 composite was transformed from a cleavage fracture of AZ91 matrix alloy to a quasi-cleavage fracture. Meanwhile, the CNTs dispersed near the fracture showed a "pull-out" state, which would effectively bear and transfer loads. The strengthening mechanism of CNTs was also discussed.
