Processing, microstructure and mechanical behavior of bulk nanostructured Mg alloy

Due to lightweight and high specific strength, the research and development of magnesium-based alloys has been widely expanded. New magnesium alloys and novel processing technology also have been developed to satisfy the need for applications in the automotive, communications and aerospace industries. In particular, ultrafine grain sized (UFG) and even nanostructured (NS) magnesium alloys fabricated via severe plastic deformation have attracted most of researchers’ attention because of impressive mechanical properties compared to micro sized Mg alloys. With a UFG, NS or a mixed microstructure, exceptional high strength with good ductility could be achieved. A combination of cryomilling and spark plasma sintering (SPS) was employed in this project to fabricate a NS magnesium alloy. Nanocrystalline (NC) AZ31 powders were produced by cryomilling. A minimum average grain size of approximately 26 nm was obtained when the cryomilling time was extended to 6 hours or longer. Cold welding played a dominant role in the early stage of cryomilling, while fracture took place in the late stage and surpassed cold welding. The former increased while the latter decreased the particle size. The highest hardness of around 160 HV was obtained after 6 hours cryomilling. This cryomilled NC powder showed excellent thermal stability during annealing at elevated temperatures. There were two separate growth stages with a transition point around 400 °C. More specifically, between 350 and 400 °C, NC Mg grains stabilized around 32 nm, even after 1h heating. At 450 °C, the nano grains grew to 37 nm in the first 5 minutes and grew quickly to approximately 60 nm after 15 minutes. Nevertheless, the average grain size was still less than 100 nm even after 60 minutes annealing at 450 °C. Bulk nanostructured (NS) Mg AZ31 alloy was produced by spark plasma. The bulk samples consolidated at 400 °C with an average grain size of 45 nm showed exceptional average true compressive yield strength of 408.7 MPa and true ultimate compressive strength of 504.0 MPa. These values were superior to published results for most of conventional Mg alloys. Higher sintering temperature (425-450 °C) improved compressive strain at the sacrifice of strength, while samples consolidated at 350 °C displayed brittle behavior with low strength. However, true compressive strains of these four samples were all less than 0.06 at true ultimate compressive strength. To enhance the ductility of bulk NS Mg AZ31 alloy in this study, a facile strategy, in situ powder casting during SPS, was introduced. Different amounts of eutectic Mg-Zn alloy powders with low melting temperature approximately 350 °C were mixed with cryomilled powder. During SPS at 400 °C, the low melting temperature eutectic alloy particles melted, and flowed along cryomilled powder particle boundaries and partly dissolved into the Mg matrix. The compressive strain was improved by in situ powder casting during SPS without loss of strength, especially when 20 % (wt. %) of eutectic alloy powder was added. Compared to samples sintered by pure cryomilled powder, its compressive strain was extended from 3.6% to 6.6%. The reason for this was in situ powder casting can simultaneously significantly remove the artifacts such as porosity, enhance the inter-particle bounding between nanostructured Mg particles and introduce very small dense precipitates into bulk NS Mg alloy

Direct observation of precipitation along twin boundaries and dissolution in a magnesium alloy annealing at high temperature

Precipitation along twin boundaries and dissolution in a cold-rolled Mg-Y-Nd alloy was directly observed for the first time during annealing at 490 °C. Precipitation occurred concurrently with recrystallization and the combined effect of precipitation and solute segregated to twin boundaries modified the recrystallization behaviour. Precipitates later dissolved into the matrix at the point where full recrystallization was nearly complete. The precipitates and higher solute concentration along original twin boundaries hindered grain growth of newly formed recrystallized grains. Even where twin boundaries had been consumed by recrystallization, the size of recrystallized grains were still controlled by the pre-existing twin boundaries

Individual effect of recrystallisation nucleation sites on texture weakening in a magnesium alloy: Part 1- double twins

Recrystallised grain nucleation, grain growth and corresponding texture evolution in a cold-rolled rare earth containing WE43 Mg alloy during annealing at 490 �C was fully tracked using a quasi-in-situ electron backscatter diffraction method. The results show nucleation sites, such as double twins, can weaken the deformed texture and for the first time provide direct evidence that recrystallised grains originating from double twins can form the rare earth texture during annealing. Precipitation and recrystallisation occurred concurrently during most of the annealing period, with precipitates forming preferentially along prior grain and twin boundaries. These precipitates effectively retard the recrystallisation due to particle pinning leading to an excessively long time for the completion of recrystallisation. A large portion of recrystallised grains were observed to have 〈0001〉 poles tilted 20e45� away from the normal direction. The RE texture emerges during the nucleation of recrystallised grains and is maintained during subsequent uniform grain growth, which results in a stable RE texture being developed as recrystallisation progresses. The uniform grain growth could be attributed to solute drag suppressing the grain boundary mobility of those grains that had recrystallised with a basal texture and precipitate pinning restricting potential orientated grain growth

The relation between heat treatment and corrosion behavior of Mg-Gd-Y-Zr alloy

The corrosion behavior of Mg–7Gd–3Y–0.4Zr (GW73K) was investigated in as-cast (F), solution-treated (T4) and peak-aged (T6) conditions using immersion tests and electrochemical measurements in NaCl solution (5 wt.%). Microstructure analyses were carried out on GW73K after different heat treatments by optical microscope (OM), field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM) and X-ray diffraction. It is found that GW73K alloy exhibits higher corrosion resistance in T4 than in F and T6 conditions due to the fully dissolution of cathodic coarse (Gd + Y) rich eutectic compound. The corrosion products of GW73K have different morphologies for F, T4 and T6 conditions. The product for F is less uniform and compact than T4 and T6, and it has been founded that GW73K-T6 had two different morphologies owing to the presence of β′. The results of polarization curves also confirm that proper heat treatment is beneficial to improve the corrosion resistance of GW73K alloy by transforming the microstructures

Effect of pass deformation on microstructure, corrosion and electrochemical properties of aluminum alloy anodes for alkaline aluminum fuel cell applications

The effect of pass deformation in rolling processes on the microstructure, corrosion resistance and electrochemical activity of Al-Mg-Bi-Sn-Ga-In alloy anodes operated in an alkaline solution (85 °C, 5 mol·L−1 NaOH with addition of NaSnO3) with current density of 800 mA·cm−2 was investigated. To analyze the microstructure of aluminum alloy anodes, we used scanning electron microscope, transmission electron microscope and electron backscatter diffraction techniques. We also used the hydrogen evolution method and electrochemical testing techniques to investigate the corrosion and electrochemical behavior of aluminum alloy anodes. The results showed that uniform microstructures with homogeneous distribution of fine active segregated phases as well as an excellent electrochemical performance of the aluminum alloy anodes were achieved by the dynamic recrystallization under pass deformation of 40%. We found that the aluminum alloy anodes (under pass deformation of 40%) had the lowest hydrogen evolution rate (0.092 mL·min−1·cm−2) and the most negative electrode potential (−1.585 V)

Enhancing ductility and strength of nanostructured Mg alloy by in-situ powder casting during spark plasma sintering

Due to internal processing defects, bulk nanostructured Mg alloys have high strength but extremely poor ductility. A novel and facile process was designed and in-situ powder casting was initiated during spark plasma sintering. This process significantly reduced processing induced defects, enhanced inter-particle bonding and introduced significant precipitation without extra ageing treatment, leading to improvement of the compressive strength and ductility. The compressive strain of bulk sample consisting of pure cryomilled powder was 3.6% with an ultimate strength of 500 MPa, while cryomilled powder mixed with eutectic Mg-Zn alloy powder obtained a compressive strain of 6.6% and ultimate strength of 506 MPa. The ductility of the sample with mixed powder was increased by 83% without any sacrifice of strength compared to the sample consisting of only pure cryomilled powder

A novel approach for producing AZ31B mg alloy wire with a promising combination of strength and ductility using CoreFlow™

AZ31B Mg alloy wires were successfully produced from commercial hot-rolled plates in one step using the CoreFlow™ process, a novel stationary shoulder friction stir extrusion manufacturing. CoreFlowed AZ31B wires exhibited fine grains with a heterogeneous grain size distribution of 6.5±4.2 μm along the transverse direction (TD) compared with the as-received material. A weakened texture was also obtained in CoreFlowed AZ31B, with basal poles aligned parallel to TD shift toward extrusion direction (ED) from wire centre to edge. Periodic needle-like regions with a distinctively different orientation from neighbouring regions were observed at the sample edge. The true ultimate tensile strength (UTS) and elongation (El) of CoreFlowed sample was 313±4 MPa and 20.1±0.4%. The El was significantly increased by 52% with improved UTS compared to the as-received material. Such a good combination of strength and ductility is attributed to the grain refinement with heterogeneity, texture weakening, and homogeneously redistributed second phase particles

Thermal Stability of Cryomilled Mg Alloy Powder

In this paper, the thermal stability of cryomilled nanocrystalline (NC) AZ31 powder was evaluated by annealing at elevated temperature ranging from 350 to 450 °C. The results show the NC AZ31 powder exhibited excellent thermal stability during short anneals at 350–450 °C, and the mechanisms were investigated in detail. There were two separate growth stages with a transition point at around 400 °C. More specifically, between 350 and 400 °C, NC Mg grains were stable at approximately 32 nm, even after 1 h annealing. At 450 °C, the nano grains grew to 37 nm in the first 5 min and grew quickly to approximately 60 nm after 15 min. However, the grain growth was limited when the annealing time was increased to 60 min. The average grain size remained stable less than approximately 60 nm even after long anneals at temperatures as high as 450 °C (0.78 T/TM), indicating an outstanding degree of grain size stability. This excellent thermal stability can be mainly attributed to solute drag and Zener pinning

Statistical analyses of the relationship between inclination angle and twin growth in uniaxial compression of Mg alloys

Inclination angle (IA), which is that between the c-axis of a twin and the global loading direction, satisfactorily captures trends observed in area fractions of both Schmid and non-Schmid twins. The detailed analyses also show that the non-Schmid twins form preferentially in smaller grains compared to that of Schmid ones.</p

Microstructure evolution and tensile behaviour of fine-grained 6082 Al wire with high ultimate strength and high work hardening by friction stir extrusion of bulk Al sheet

Driven by the highly enhanced demand for metallic materials with excellent strength-ductility synergy, fine-grained alloy fabricated by severe plastic deformation (SPD) is a long-lasting research interest. In this study, a fine-grained aluminium (Al) alloy wire with high ultimate tensile strength (316.8 MPa) and high work hardening capability (n = 0.34) was developed via a novel bulk-consolidation friction stir extrusion (FSE) enabled by CoreFlow™ process based on FSE processed 6082-T6 Al sheet. Grain refinement of 200 times occurred in CoreFlowed Al alloy, which is rarely observed in FSEed Al alloy counterpart based on milling metal chips. The undesirable feature of coarse and preferred secondary phase particles in the initial 6082-T6 Al billet was well tailored during CoreFlow™ process, which led to an attractive characteristic of dispersed and refined particle phases in CoreFlowed Al alloy. Texture components of S (V = 8.24%) and Goss (V = 6.86%) were observed, however, the overall micro-texture intensity of CoreFlowed Al wire was significantly weakened. Due to the cooperative deformation modes consisting of dislocation slip and grain boundary sliding, as well as the positive compatibility effect of fine-grains on deformation, CoreFlowed Al specimens demonstrated an excellent ductility (EI = 19.3%) and acceptable yield strength (YS = 182 MPa) compared to initial 6082-T6 Al sheet (EL = 10.4%, and YS = 225 MPa). Benefiting from the higher work hardening capability, the final ultimate tensile strength (UTS) of CoreFlowed Al (316.8 MPa) reached almost the same level as the 6082-T6 Al sheet (324.6 MPa). Furthermore, after one simple post-processing one-step heat treatment (175 °C for 10 h), the wire's yield strength was improved to 250 MPa, although only maintaining 9% elongation. This low elongation could be attributed to micro-cracks induced by coarsening of the second phases, while high yield strength resulted from fine grain size and precipitate strengthening. The wide range of variations in the mechanical properties of CoreFlowed Al wire under different conditions provides significant freedom in tailoring the mechanical properties of alloy wire in applications