Enhancement of mechanical properties of AZ91 magnesium alloy based on the structural refinement induced by a newly developed Al–5Ti–2B master alloy and subsequent hot working through the extrusion ...process was studied. The addition of Al–5Ti–2B master alloy resulted in the grain refinement of α-Mg grains mainly due to the inoculating effect of TiB2 particles, where the optimal amount of the grain refiner was found to be ~0.3 wt%. The hot extrusion process led to a remarkable grain refinement and the fracturing and distribution of intermetallics along the extrusion direction. The enhancement of mechanical properties was successfully rationalized based on the grain size, and as a result, Hall-Petch relationships were developed for the yield stress, tensile strength, and hardness. However, the ductility of the extruded alloys was larger than those predicted by the grain size effects alone, which resulted in a superior strength-ductility trade-off compared to the as-cast counterparts. This was related to the disappearance of the grain boundary β-Mg17Al12 phase, closure of casting defects, and formation of a recrystallized and homogeneous microstructure in the extruded alloys.
In this study, the graphene reinforced aluminium matrix nanocomposite is fabricated through the combination of high energy ball milling (HEBM) and molecular level mixing (MLM) processes followed by ...spark plasma sintering (SPS) method. FTIR, XRD and FESEM analyses revealed that Cu/RGO nanocomposite was well synthesized by the MLM method with a uniform distribution of Cu nanoparticles on RGO surface. XRD and Raman spectroscopy showed that no Al4C3 phase was formed during the manufacturing process, but Al2Cu fine particles precipitated during the SPS treatment which were smaller in size and more uniform in distribution when RGO was used as reinforcement according to TEM analysis. Grain refinement of the nanocomposites was also investigated in the presence of RGO by EBSD method. The results showed that with increasing RGO content from 0 to 1 wt %, the coarse grained microstructure changed to bimodal microstructure. In addition, the mean grain size decreased from 3.9 to 1.6 μm, and dislocation density increased significantly with the increased RGO content. The SPS process also resulted in a nearly fully dense nanocomposite with a relative density higher than 99%. The investigation of mechanical properties of the sintered nanocomposite indicated an improvement of 79, 49 and 44% of yield strength, ultimate strength, and Vickers hardness, respectively, for only 1 wt % graphene containing nanocomposite in comparison to the unreinforced Al–4Cu alloy.
In this paper, the crystallographic orientation relationship between TiB2 particles and α-Mg phase is identified by TEM to clarify the grain refinement mechanism of TiB2 particulate reinforced AZ91 ...composites. The influence of the content of TiB2 particles on the grain size and mechanical properties of TiB2/AZ91 composites is investigated. The experimental result reveals that the TiB2 particles can refine α-Mg grains in the AZ91 matrix, and with TiB2 particles increased, the size of α-Mg grains in the composite decreases significantly. There exists a well-defined crystallographic orientation relationship between TiB2 and α-Mg matrix, which is (0001)TiB2∕∕(101¯1)α−Mg, 1¯21¯0TiB2∕∕1¯21¯0α−Mg in the present study. In the meanwhile, the lattice disregistry along the specific orientation relationship is 8.30% based on the theoretical calculation, confirming that TiB2 particles can act as the effective heterogeneous nucleation site of α-Mg grains in TiB2/AZ91 composites. Furthermore, the Brinell hardness, ultimate tensile strength and elongation of the composite are increased simultaneously with the increase of TiB2 particles, which are attributed to the uniform TiB2 particles dispersion, grain refinement strengthening and particles strengthening.
•α-Mg grains size decreases with increasing TiB2 particles.•The OR between TiB2 particles and α-Mg phase is identified experimentally.•Nucleation mechanism of TiB2/AZ91 composites is clarified.•The hardness, strength and elongation of composites are improved simultaneously.
Stationary shoulder friction stir processing (SSFSP) in thick AZ31B magnesium alloy was performed to refine the microstructure followed by evaluating corrosion behavior. The use of stationary ...shoulder exhibited low heat input and small temperature gradient across the thickness of stir zone (SZ). Moreover, smooth surface morphology with little flash was obtained. The probe-dominated SZ developed fine equiaxed uniform grain structure across the thickness of SZ, which in turn increased the corrosion resistance of SSFSPed alloy as compared to BM. SSFSPed alloy surface confirm uniform corrosion behavior with mud cracking and intergranual corrosion patterns instead of pitting corrosion in BM. This improvement in corrosion was attributed to homogenization of magnesium alloy microstructure by using low-heat-input stationary shoulder tool.
Supercooled CoCrFeNi high entropy alloy (HEA) with undercooling up to 300 K and mass above 100 g are fabricated using glass fluxing method. Evolution of non-equilibrium solidification microstructure ...with different undercoolings is described in detail by SEM, XRD and TEM. Results show that with the increasing undercooling, a typical grain refinement effect and existence of BCC structured Cr-rich phase are observed. Based on the bulk mass supercooled specimens, micro-hardness and compressive measurement at room temperature are carried out to investigate the influence of supercooling on the CoCrFeNi HEA. A strong enhancement of hardness as well as strength for the non-equilibrium solidified samples is evidenced. More importantly, even small undercooling (50 K in this paper) can enhance CoCrFeNi HEAs’ mechanical properties remarkably. The present study shows that using supercooling technique to make non-equilibrium solidified sample is an efficient method to improve the mechanical properties of HEAs.
•CoCrFeNi HEA weighing about 100 g is undercooled up to 300 K.•BCC structured Cr-rich phase is observed in undercooled CoCrFeNi HEA.•Strong enhancement of hardness and strength of undercooled CoCrFeNi HEA is proved.•Supercooling method is turned to be useful to improve HEAs’ mechanical properties.
Despite extensive use of ER2319 welding wires for additive manufacturing of aluminum alloys in the aerospace and automotive field, the internal denseness, grain coarseness, and non-uniformity of ...additive-manufactured aluminum alloy parts are yet to be appropriately addressed. Thus, this study attempted to overcome this gap by using an ER2319 aluminum alloy welding wire to produce a uniform and fine-grained aluminum alloy using deformation-based friction rolling additive manufacturing (FRAM). The findings demonstrated that the FRAM-prepared materials were free of voids and cracks, with approximately equiaxed grains of only 4–6 μm. Further, the ultimate tensile strength (UTS) and yield strength (YS) of the deposited material in all directions differed only slightly, with the strengths in the longitudinal direction being 9.5 and 9.6% higher than that in the vertical direction, respectively. Furthermore, the combined UTS and elongation of the FRAM-prepared material are, to the best of the authors’ knowledge, the best results reported for printed aluminum alloys using ER2319 welding wire. The interesting research results presented in this study are expected to have significant practical application implications and usher in further research in this field.
A crack-free, strong and ductile Al-Cu-Mg-Ag alloy with TiB2 particles was successfully fabricated with laser powder bed fusion (L-PBF). Subsequent solution and aging were designed to modify the ...microstructure and strengthen the alloy. The as-fabricated alloy achieved an isotropic microstructure consisting of random-orientated and equiaxed grains with an average grain size of about 1.15 µm. The role of TiB2 particles in reducing metallurgical defects (such as hot cracking) and enhancing mechanical performance of the alloy was revealed by multi-scale advanced microstructural characterizations. This benefits from the formation of two-dimensional compounds of Al3Ti on the terminated (0001)TiB2 plane, which could reduce the lattice mismatch, alter the potency of TiB2 particles, and refine the grains. The as-built alloy shows an excellent combination of strength and ductility with a yield strength (YS) of 289 ± 5.7 MPa, ultimate tensile strength (UTS) of 390 ± 6.5 MPa, and elongation to failure (El) of 14.8 ± 1.8%. After the heat treatment, high-density of θ', Ω', and Ω phases were precipitated, significantly improving the YS to 405 ± 3.2 MPa and UTS to 470 ± 3.5 MPa, while slightly decreasing the El to 11.6 ± 0.9%. The strengthening mechanisms are quantitatively evaluated and the plastic instability phenomenon (Lüders band and Portevin-Le-Chatelier effect) of the as-built alloy was also analyzed.
•A crack-free, strong and ductile TiB2/Al-Cu-Mg-Ag composite was fabricated with L-PBF.•Monolayer Al3Ti 2DC on terminated (0001)TiB2 plane, altering the potency of TiB2 particles.•Solute segregation at Al/TiB2 interface is unraveled by atomic-scale observation and EDS.•Strengthening mechanisms of L-PBF fabricated alloy are quantitatively evaluated.•Plastic instability stems from the interaction of dislocations with atomic clusters/G.P. zones.
Surface integrity of machined components has a critical impact on their performance. Magnesium alloys are lightweight materials used in the transportation industry and are also emerging as a ...potential material for biodegradable medical implants. Surface integrity factors, such as grain size, crystallographic orientation and residual stress, have been proved to remarkably influence the functional performance of magnesium alloys, including corrosion/wear resistance and fatigue life. In this study, the influence of dry and cryogenic machining (liquid nitrogen was sprayed on the machined surface during machining) using different cutting edge radius tools on surface integrity was investigated. Compared with the initial material, cryogenic machining when using a large edge radius tool led to enhanced surface integrity in terms of the following: (1) improved surface finish; (2) significant grain refinement from 12μm to 31nm in the featureless surface layer; (3) large intensity of (0002) basal plane on the machined surface; (4) 10 times larger compressive areas in residual stress profiles; these changes should notably improve the functional performance of machined AZ31B Mg alloy. In addition to the frequently reported benefits on tool life, this study suggests that cryogenic machining may also enhance the surface integrity of the workpiece and improve the performance of machined components.
Display omitted
► Spraying liquid nitrogen onto the machined surface led to a 60% decrease of its temperature during machining. ► Nanocrystallined grain structures about 30nm were found in the “featureless layer” after cryogenic machining. ► The featureless layer thickness increased with larger cutting edge radius. ► Strong basal texture was created on the machined surface, which may significantly improve its corrosion resistance. ► Cryogenic machining resulted in 10 times larger compressive areas in the residual stress profiles.
In this work, TiP/VW94 composites were prepared by semi-solid stirring assisted ultrasonic vibration followed by hot extrusion. The results indicate that the presence of Ti particles can promote the ...precipitation of lamellar long period stacking ordered structure (LPSO) while inhibiting the precipitation of block-shaped LPSO during heat treatment due to the dislocations introduced by Ti particles. Besides, the content and size of RE-rich phase decrease with the increase of the Ti particle content. The lamellar LPSO can hinder recrystallization and RE-rich phase can hinder grain growth. The addition of Ti particles decreased the grain size, recrystallization fraction and texture intensity of as-extruded TiP/VW94 composites compared with VW94 matrix alloy. The 5%TiP/VW94 composite presents the best tensile properties, with a YS, UTS and elongation of 288 MPa, 371 MPa and 9.4%, respectively. The enhanced tensile properties of TiP/VW94 composites are attributed to the load transfer, grain refinement, dislocation strengthening, dispersion strengthening, and an increase in lamellar LPSO and RE-rich phase with appropriate content and size.
PDF
