AZ91D/TiB2 Nanocomposites Fabricated by Solidification Nanoprocessing

AZ91D, as one of the most widely used casting magnesium alloys, still suffers from inadequate mechanical performances for various applications. Nanoparticles could be used to form high‐performance magnesium matrix nanocomposites. Among all nanoparticles, TiB₂ has great potentials to enhance the mechanical property of AZ91D. This paper studies the microstructures and mechanical property of AZ91D‐TiB₂ nanocomposites fabricated through solidification nanoprocessing. TiB₂ nanoparticles with a diameter of 25 nm are effectively fed into the AZ91D melt through a newly developed automatic nanoparticle‐feeding system. Ultrasonic cavitation is used to disperse these nanoparticles in AZ91D melt for casting. With 2.7 wt% (about 1.0 vol%) of TiB₂ nanoparticles addition, the mechanical property of AZ91D is much enhanced (by 21, 16, and 48% for yield strength, tensile strength, and ductility, respectively). Microstructural analysis with optical microscope, SEM, and S/TEM show that α‐Mg grain and a network of massive brittle intermetallic phase (β‐Mg₁₇Al₁₂) are simultaneously refined and modified. Further study suggests that the enhancement of mechanical properties of AZ91D is attributed not only to primary phase grain refinement, but also to the modification of intermetallic β‐Mg₁₇Al₁₂ by TiB₂ nanoparticles.     

Characterization of hot extruded Mg/SiC nanocomposites fabricated by casting

Mg–1%SiC nanocomposites were fabricated using an ultrasonic cavitation based casting method, resulting in the dispersion of the reinforcing SiC nanoparticles to form Mg–metal matrix nanocomposite (Mg–MMNC) billets. The MMNC billets were then processed using hot extrusion at 350 °C. Micrographic observations illustrate a significant grain size reduction and the presence of microbands that align the SiC nanoparticles parallel to the direction of extrusion for Mg–MMNCs. Observations from the cross-section at 90° of the extrusion direction show uniform nanoparticles dispersion. Results from the extruded Mg–MMNCs tensile testing at different temperatures (25, 125 and 177 °C) reveal an increase of the yield strength, ultimate tensile strength, and ductility values as compared to the un-reinforced and extruded Mg-alloy; such increase was also observed from the microhardness testing results where an increase from 19 to 34% was measured.     

Mechanical and fracture properties of an AZ91 Magnesium alloy reinforced by Si and SiC particles

A commercial AZ91 magnesium alloy (nominal composition Mg–9%Al; 1%Zn; 0.3%Mn, balance Mg in weight percent) reinforced with SiC particles and modified by the addition of Si has been used in this study. Formation of an “in situ” composite (Mg–Mg₂Si) results in strong bonding between Mg₂Si and the matrix interface. Samples were deformed in compression in the temperature interval from room temperature up to 300 °C. Stress relaxation tests were performed with the aim to reveal the thermally activated processes. Reinforcing effect of SiC and Mg₂Si particles decreases with increasing temperature. The estimated values of the activation volume as well as the activation enthalpy indicate that the main thermally activated process is connected with a rapid decrease of the internal stress. Fracture properties were studied in impact tests at various temperatures. A ductility enhancement was found at 200 °C and temperatures above 200 °C.     

Enhanced mechanical response of hybrid alloy AZ31/AZ91 based on the addition of Si3N4 nanoparticles

The main aim of this study was to simultaneously increase tensile strength and ductility of AZ31/AZ91 hybrid magnesium alloy with Si₃N₄ nanoparticles. AZ31/AZ91 hybrid alloy nanocomposite containing Si₃N₄ nanoparticle reinforcement was fabricated using solidification processing followed by hot extrusion. The nanocomposite exhibited similar grain size to the monolithic hybrid alloy, reasonable Si₃N₄ nanoparticle distribution, non-dominant (0 0 0 2) texture in the longitudinal direction, and 13% higher hardness than the monolithic hybrid alloy. Compared to the monolithic hybrid alloy (in tension), the nanocomposite simultaneously exhibited higher yield strength, ultimate strength, failure strain and work of fracture (+12%, +5%, +64% and +71%, respectively). Compared to the monolithic hybrid alloy (in compression), the nanocomposite exhibited higher yield strength and ultimate strength, lower failure strain and higher work of fracture (+35%, +4%, −6% and +6%, respectively). The beneficial effects of Si₃N₄ nanoparticle addition on the enhancement of tensile and compressive properties of AZ31/AZ91 hybrid alloy are investigated in this paper.     

Influence of extrusion temperature and process parameter on microstructures and tensile properties of a particulate reinforced magnesium matrix nanocomposite

Particulate reinforced magnesium matrix nanocomposites fabricated by semisolid stirring assisted ultrasonic vibration were subjected to extrusion. The results showed that grains of matrix in the SiCp/AZ91 nanocomposites were gradually refined while the amount of SiC nanoparticle bands was decreased with the extrusion temperature increasing from 250 to 350 °C. Under the same extrusion conditions, the grain size of the matrix was gradually decreased while the distribution of SiC nanoparticles was improved in the extruded nanocomposites fabricated by decreasing the stirring time. The yield strength and ultimate tensile strength of the nanocomposites were gradually enhanced with increasing the extrusion temperature. Significant improvement of tensile strength was obtained in the nanocomposites fabricated by decreasing the stirring time.     

Microstructure and mechanical properties of SiC nanoparticles reinforced magnesium matrix composites fabricated by ultrasonic vibration

A particulate reinforced magnesium matrix nanocomposite was fabricated by ultrasonic vibration. Compared with as-cast AZ91 alloy, grain size of matrix in the SiCp/AZ91 nanocomposite decreased and morphology of phase Mg₁₇Al₁₂ varied from coarse plates to lamellar precipitates. Although there were still some SiC microclusters in the nanocomposite, most of the SiC nanoparticles were dispersed well outside the microclusters. The ultimate tensile strength, yield strength and elongation to fracture of the SiCp/AZ91 nanocomposite were simultaneously enhanced compared with that of the AZ91 alloy. The study of interface between the SiC nanoparticles and the matrix in the nanocomposite suggested that SiC nanoparticles bonded well with the matrix without interfacial activity.     

Serrated flow behavior in a near alpha titanium alloy IMI 834

Serrated behavior of near alpha titanium alloy IMI 834 has been studied at elevated temperature from 400 °C to 475 °C. Serrations morphology was found as A type of locking serrations followed by B type serrations at 400 °C. E type of serrations has been observed at higher strains at 425 °C. B type and unlocking serrations of C_A type at 450 °C and again A and C_B type serrations were found at 475 °C. In strength parameters, anomalous tensile behavior was found in the variation of tensile strength and yield strength with test temperature in the temperature range between 400 °C and 475 °C. However, the variation of normalized flow stress showed regions I–III with test temperature. Regions I and III correspond to normal tensile behavior and region II corresponds to anomalous tensile behavior. Blue brittle temperature of IMI 834 was attributed at 450 °C by confirming minimum ductility of 8.2%. In present study, a different approach has been adopted to show the change in deformation behavior during serrated region called as abrupt change in strain path. Maximum irregularity in flow behavior has been observed at 450 °C and 475 °C. Room temperature fractographic features showed transgranular features whereas mixed ductile and cleavage fracture has been observed in the temperature range between 400 °C and 475 °C. However, reverse slope behavior has been observed in the plot of critical strain versus test temperature at 450 °C, which could be due to silicide precipitation. In the present study, interaction of dislocations with interstitial/substitutional solutes is responsible for dynamic strain aging in IMI 834.     

Hot deformation behavior of SiCp/AZ91 magnesium matrix composite fabricated by stir casting

The uniaxial compressive deformation behavior of a 10 vol.% SiC particulate reinforced AZ91 magnesium matrix composite (SiCp/AZ91) fabricated by stir casting is investigated at elevated temperature (250–400 °C). Peak stresses and flow stresses decrease as temperatures increase and strain rates decrease. The extent of dynamic recrystallization (DRX) becomes less as temperatures decrease at 250–350 °C or strain rates increase, and recrystallization occurs mainly within the intergranular regions rich of particles. Dynamic recrystallization accomplishes at 400 °C even at the strain rate of 1 s⁻¹. An analysis of the effective stress dependence on strain rate and temperature gives a stress exponent of n = 5 and a true activation energy of Q = 99 kJ/kJ. The value of Q is close to the value for grain boundary diffusion in Mg. It is concluded that the deformation mechanism of SiCp/AZ91 composite during hot compression is controlled by the dislocation climb.     

Study of standard heat treatment on mechanical properties of Inconel 718 using ball indentation technique

The effect of standard heat treatment on the microstructure and mechanical properties of Ni–Fe base super-alloy, Inconel 718 was studied by optical microscopy and ball indentation technique (BIT) using small amount of specimen. In order to get good ductility, good formability, yield, tensile and creep rupture, as-received material was given the standard heat treatment, viz solution treatment at two temperatures 940 °C and 1040 °C for1 h and water quenched (WQ) followed by aging treatment at 720 °C for 8 h. and furnace cooling (FC). The BIT has revealed that the strengths for as-received material are maximum compared to other heat-treated materials. After solution treatment there has been a radical decline in strength. But the ageing causes a significant enhancement of strength. Optical microscopy studies supported the obtained BIT results. γ″-phase is the basic strengthening phase in 718 alloys.     

Investigation of hot ductility in Al-killed boron steels

The influence of boron to nitrogen ratio, strain rate and cooling rate on hot ductility of aluminium-killed, low carbon, boron microalloyed steel was investigated. Hot tensile testing was performed on steel samples reheated in argon to 1300 °C, cooled at rates of 0.3, 1.2 and 3.0 °C s⁻¹ to temperatures in the range 750–1050 °C, and then strained to failure at initial strain rates of 1 × 10⁻⁴ or 1 × 10⁻³ s⁻¹. It was found that the steel with a B:N ratio of 0.19 showed deep hot ductility troughs for all tested conditions; the steel with a B:N ratio of 0.47 showed a deep ductility trough for a high cooling rate of 3.0 °C s⁻¹ and the steel with a near-stoichiometric B:N ratio of 0.75 showed no ductility troughs for the tested conditions. The ductility troughs extended from 900 °C (near the Ae₃ temperature) to 1000 or 1050 °C in the single-phase austenite region. The proposed mechanism of hot ductility improvement with increase in B:N ratio in these steels is that the B removes N from solution, thus reducing the strain-induced precipitation of AlN. Additionally, BN co-precipitates with sulphides, preventing precipitation of fine MnS, CuS and FeS, and forming large, complex precipitates that have no effect on hot ductility.     

Fabrication of SiC particles-reinforced magnesium matrix composite by ultrasonic vibration

Magnesium matrix composites reinforced with two volume fractions (1 and 3%) of SiC particles (1 μm) were successfully fabricated by ultrasonic vibration. Compared with as-cast AZ91 alloy, with the addition of the SiC particles grain size of matrix decreased, while most of the phase Mg₁₇Al₁₂ varied from coarse plates to lamellar precipitates in the SiCp/AZ91 composites. With increasing volume fraction of the SiC particles, grains of matrix in the SiCp/AZ91 composites were gradually refined. The SiC particles were located mainly at grain boundaries in both 1 vol% SiCp/AZ91 composite and 3 vol% SiCp/AZ91 composite. SiC particles inside the particle clusters may be still separated by magnesium. The study of the interface between the SiC particle and the alloy matrix suggested that SiC particles bonded well with the alloy matrix without interfacial reaction. The ultimate tensile strength, yield strength, and elongation to fracture of the SiCp/AZ91 composites were simultaneously improved compared with that of the as-cast AZ91 alloy.     

Prediction of crack depth during quenching test for an ultra high temperature ceramic

Quenching test was used to characterize thermal shock properties of ZrB₂–20%SiC_p–10%AlN. It showed that critical temperature difference was 400 °C, and residual strength was a constant while quenching temperature was higher than 400 °C. Inertial stress was investigated under different temperatures, as shown, a higher temperature led to a lower inertial stress. Crack resistance under room temperature was compared with that under 600 °C, as can be seen, a higher temperature led to a higher crack resistance. Dynamic thermal stress intensity factor was investigated at the quenching temperature of 600 °C, and it indicated that stress intensity factor ascended first and descended afterwards, farther crack propagation would not occur when Biot value is in a certain range, such as Biot = 5. Crack would not propagate when Biot = 1, and specimen would be destroyed when Biot = 10.     

Tensile behaviors of fine-grained γ-TiAl based alloys synthesized by pulse current auxiliary sintering

Micro-grained γ-TiAl based alloy obtained via pulse current auxiliary sintering exhibits good room temperature ductility with the common influence of fine grain size and inner twinning microstructure. Superplastic behavior at relatively low temperatures is also observed. It is also noted that the tensile strength of the studied alloy manifests anomalous hardening from room temperature to approximately 600 °C as a result of the controlling of dislocations slip, and softening above 600 °C due to thermal activation. Based on calculation, the superplastic deformation mechanism in the present work is determined as the grain boundary sliding accommodated by grain boundary diffusion.     

Structure and mechanical properties of extruded Mg–Gd based alloy sheet

The Mg–8Gd–2Y–1Nd–0.3Zn–0.6Zr (wt.%) alloy sheet was prepared by hot extrusion technique, and the structure and mechanical properties of the extruded alloy were investigated. The results show that the alloy in different states is mainly composed of α-Mg solid solution and secondary phases of Mg₅RE and Mg₂₄RE₅ (RE = Gd, Y and Nd). At aging temperatures from 200 °C to 300 °C the alloy exhibits obvious age-hardening response. Great improvement of mechanical properties is observed in the peak-aged state alloy (aged at 200 °C for 60 h), the ultimate tensile strength (σ_b), tensile yield strength (σ_0.2) and elongation () are 376 MPa, 270 MPa and 14.2% at room temperature (RT), and 206 MPa, 153 MPa and 25.4% at 300 °C, respectively, the alloy exhibits high thermal stability.     

Influence of straining and ageing on the room temperature mechanical properties of dual phase steel

In this work, changes in mechanical properties in dual phase steel containing 20% martensite volume fractions were observed at various ageing temperatures. For example, ΔY (increase in yield strength due to strain ageing), YS and UTS exhibit maximum values at ageing temperature of 100 °C for the pre-strains of 2 and 4%. This is due to the formation of solute atom atmospheres around dislocation. When the ageing temperature increased to 200 °C, yield strength decreased due to overageing resulted from tempering that starts in martensite phase.     

Influence of aging treatment on mechanical properties of 6061 aluminum alloy

Aluminum–magnesium–silicon (Al–Mg–Si) alloys show medium strength, excellent formability, good corrosion resistance and are widely used in extruded products and automotive body panels. The major advantage of these alloys is their age hardening response during the paint baking process as well as the fact that they exhibit no yield point phenomenon and Lüdering. In this study, the mechanical properties of a commercially available AA6061 alloy aged to various levels were studied. Peak-aged conditions were reached in this particular alloy after a 2 h heat treatment at 200 °C. The variation of the yield stress, ultimate tensile strength, ductility and strain hardening rate with aging time is measured and discussed in relation to the microstructural changes induced by the heat treatment.     

SiCp/AZ91D镁基纳米复合材料的室温拉伸行为及塑性变形机理

为得到高强度和高塑性的镁基复合材料,通过高能超声分散法和金属型重力铸造工艺制备了SiC纳米颗粒分散均匀的SiCp/AZ91D镁基纳米复合材料,并进行T4固溶热处理和室温拉伸。利用扫描电子显微镜(SEM)、透射电子显微镜(TEM)对试样拉伸后的显微组织和塑性变形机理进行观察与研究。结果表明:T4态SiCp/AZ91D镁基纳米复合材料室温下抗拉强度达到296 MPa,伸长率达到17.3%。经室温拉伸变形后复合材料基体微观组织中出现了大量的孪晶和滑移,孪生和滑移是复合材料塑形变形的主要机制。在室温拉伸过程中,α-Mg基体中SiC纳米颗粒周围形成高应变场,高应变场内形成大量位错和堆垛层错,这些位错和堆垛层错在拉伸应变的作用下演变成大量的滑移带和孪晶,这是SiCp/AZ91D镁基纳米复合材料在室温下具有高塑性的微观塑性变形机理。     

Nanoparticle interactions with the magnesium alloy matrix during physical deformation: Tougher nanocomposites

This study is aimed at understanding the toughness enhancing function of nanoparticles in magnesium nanocomposites, focussing on experimentally observed nanoparticle–matrix interactions during physical deformation. Al₂O₃ nanoparticles were selected for reinforcement purposes due to the well known affinity between magnesium and oxygen. AZ31/AZ91 (hybrid alloy) and ZK60A magnesium alloys were reinforced with Al₂O₃ nanoparticles using solidification processing followed by hot extrusion. In tension, each nanocomposite exhibited higher ultimate strength and ductility than the corresponding monolithic alloy. However, the increase in ductility exhibited by ZK60A/Al₂O₃ (+170%) was significantly higher than that exhibited by AZ31/AZ91/Al₂O₃ (+99%). The previously unreported and novel formation of high strain zones (HSZs, from nanoparticle surfaces inclusive) during tensile deformation is highlighted here as a significant mechanism supporting ductility enhancement in ZK60A/Al₂O₃ (+170% enhanced) and AZ31/AZ91/Al₂O₃ (+99% enhanced) nanocomposites. Also, ZK60A/Al₂O₃ exhibited lower and higher compressive strength and ductility (respectively) compared to ZK60A while AZ31/AZ91/Al₂O₃ exhibited higher and unchanged compressive strength and ductility (respectively) compared to AZ31/AZ91. Here, the previously unreported nanograin formation (recrystallization) during room temperature compressive deformation as a toughening mechanism in relation to nanoparticle stimulated nucleation (NSN) ability is also highlighted.     

Comparison of superplastic forming abilities of as‐cast AZ91 magnesium alloy prepared by twin roll casting and WE43 magnesium alloy

This paper describes and compares the superplastic behaviour and microstructural evolution of twin roll cast AZ91 and WE43 rolled sheet alloys. Tests were carried out in uniaxial tension on both alloys across a range of temperatures (300 °C–525 °C) and strain rates (1?10^‐4 s^‐1–1?10^‐1 s^‐1). In the case of WE43 gas bulge testing was employed at 400 °C and 0.6 MPa to offer a better analogy to superplastic forming than uniaxial tensile testing. Elongations of over 400 % were observed within WE43 when tested at 450 °C and 1?10^‐3 s^‐1 strain rate, and over 200 % within AZ91 when tested at 350 °C and 1?10^‐3 s^‐1 strain rate. A peak cone height of 41 mm was achieved with WE43 at a temperature of 400 °C and pressure of 0.6 MPa. Electron back scattered detection technique was employed to analyse the microstructural evolution of the two alloys during the forming process. Both WE43 and AZ91 were observed to undergo dynamic recrystallization during elevated temperature tensile testing and failed at low strain rates mainly by means of coalescence of cavitation, in the case of AZ91 at high strain rates cracking of Al₁₂Mg₁₇ intermetallic particles was the dominating failure mechanism. Both alloys were seen to achieve good levels of superplastic ductility over 200 % elongation, which would be industrially useful in niche vehicle and aerospace manufacturing.     

Effect of Mg17Al12 precipitates on the microstructural changes and mechanical properties of hot compressed AZ91 magnesium alloy

During hot compression, Mg₁₇Al₁₂ (β) precipitates show strong influence on the microstructural changes of 415 °C-24 h homogenized AZ91 alloy. When compressed at 300 °C and 350 °C, dynamic recrystallization (DRX) only occurs near grain boundaries with discontinuous β precipitate pinning at the newly DRXed grain boundaries. With increasing compression temperature and decreasing strain rate, the β-precipitating region expands; however, the amount of pinning precipitates decreases, resulting in increases in the DRX ratio and average DRXed grain size. With a compression ratio of only 50%, the specimen compressed at 350 °C and a strain rate of 0.2 s⁻¹ (designated 350 °C-0.2 s⁻¹ compressed specimen) shows an ultimate tensile strength (UTS) of 334 MPa, a 0.2% proof stress (PS) of 195 MPa and an enough elongation of 17.9%. After a subsequent aging treatment at 180 °C, due to the large number of β precipitates, the strength of the compressed specimens are further improved, and the specimen peak aged after compression at 400 °C and 0.2 s⁻¹ shows UTS of 364 MPa and PS of 248 MPa with a moderate elongation of 7.7%.