Hot Deformation and Work Hardening Behavior of an Extruded Mg–Zn–Mn–Y Alloy

The aim of this study was to evaluate the strain hardening and hot deformation behavior of asextruded Mg-Zn-Mn(ZM31) magnesium alloy with varying Y contents(0.3,3.2,and 6 wt%) via compression testing along the extrusion direction at room temperature,200 °C and 300 ℃.Texture and phases were identified by X-ray diffraction.Alloy ZM31 + 0.3Y consisted of a mixture of fine equiaxed grains and elongated grains with I-phase(Mg_3YZn_6);alloy ZM31 + 3.2Y contained I-phase and W-phase(Mg_3Y_2Zn_3);alloy ZM31 + 6Y had long-period stacking-ordered(LPSO) X-phase(Mg_(12)YZn) and Mg_(24)Y_5 particles.With increasing Y content the basal texture became weakened significantly.While alloys ZM31 + 0.3Y and ZM31 + 3.2Y exhibited a skewed true stress-true stain curve with a three-stage strain hardening feature caused by the occurrence of {1012} extension twinning,the true stress-true stain curve of alloy ZM31 + 6Y was normal due to the dislocation slip during compression.With increasing temperature the extent of skewness decreased.While the compressive yield stress,ultimate compressive stress,strain hardening exponent,and hardening capacity all decreased as the temperature increased,the retention of the hightemperature deformation resistance increased with increasing Y content mainly due to the presence of thermally-stable LPSO X-phase.     

Effect of Nd on microstructure and mechanical properties of as-extruded Mg-Y-Zr-Nd alloy

The microstructure and mechanical properties of Mg-Y-Zr-x Nd alloys with 0–2.63 wt% Nd were investigated using optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and tensile testing test. Results indicated that more Mg_(24)Y_5 particles and Mg_(14)Nd_2Y(β) phases were dispersed in the matrix when Nd content increased from 0 wt% to 2.63 wt% in the extruded alloys.Consequently, the nucleation of dynamic recrystallization and the volume fraction of recrystallized grains were promoted obviously. The average grain size can be refined in the range of 4.6–1.3 μm after the addition of 2.63 wt% Nd. The tensile strength of extruded alloys increased with increasing Nd content, and elongation exhibited an opposite change tendency. The extruded alloy sheet with 1.01 wt% Nd demonstrates optimal combination of strength and plasticity, i.e., the ultimate tensile strength, yield strength,and elongation were 273 MPa, 214 MPa, and 24.2%, respectively. Variations in mechanical properties are discussed on the basis of microstructure observations.     

Strengthening FCC-CoCrFeMnNi high entropy alloys by Mo addition

In order to strengthen the face-centered-cubic(FCC) type CoCrFeMnNi high entropy alloys(HEAs), different contents of Mo(0–16 at.%, similarly hereinafter) were alloyed. Phase evolution, microstructure,mechanical properties and related mechanism of these HEAs were systematically studied. The results show that sigma phase is appeared with addition of Mo, and the volume fraction of it increases gradually from 0 to 66% with increasing Mo content. It is found that Mo is enriched in sigma phase, which indicates that Mo element is beneficial to form sigma phase. Compressive testing shows that the yield strength of the alloys increases gradually from 216 to 765 MPa, while the fracture strain decreases from 50%(no fracture) to 19% with increasing of Mo. The alloy exhibits the best compressive performance when Mo content reaches 11%, the yield strength, fracture strength and fracture strain are 547 MPa, 2672 MPa and44% respectively. The increased volume fraction of sigma phase plays an important role in improving the compressive strength of(CoCrFeMnNi)_(100-x)Mo_xHEAs.     

Dynamic Recrystallization and Precipitation Behavior of Mn-Cu-V Weathering Steel

The hot deformation behavior of a Mn-Cu-V weathering steel was investigated at temperatures ranging from 850 to 1050℃ and strain rates ranging from 0.01 to 5 s-1 using MMS-300 thermal-mechanical simulator. The activation energy for dynamic recrystallization and stress exponent were calculated to be 551 kJ/mol and 7.73, respectively. The accurate values of critical strain were determined by the relationship between work hardening rate and flow stress (θ-σ) curves. The hyperbolic sine constitutive equation was employed to describe the relationship between the peak stress and Zener-Hollomon parameter during hot deformation. The interaction between dynamic recrystallization and dynamic precipitation of V(C,N) at a low strain rate was analyzed. The results showed that precipitation particles size of weathering steel increased with increasing strain at deformation temperature 950℃ and strain rate 0.1 s-1. The calculation results of the recrystallization driving force and pinning force showed that dynamic precipitation could retard the progress of dynamic recrystallization but not prevent it while the pinning forces is less than driving force. On the contrary, dynamic precipitation can effectively prevent the progress of dynamic recrystallization.     

Corrosion Behavior of Mg-Zn-Y Alloy with Long-period Stacking Ordered Structures

Mg-Zn-Y alloys with long-period stacking ordered structures were prepared by an ingot casting method. The corrosion performance of Mg-Zn-Y alloys was studied by combining gas-collecting test, immersion test and electrochemical measurements in order to determine the corrosion rate and mechanism of the alloys. The results showed that the volume fraction of Mg(12)YZn phase increased and the shape of the Mg(12)YZn phase changed from discontinuous to continuous net-like with increasing Zn and Y content. The corrosion rate of the alloys greatly depended on the distribution and volume fraction of the Mg(12)YZn phase. Corrosion products appeared at the junction of Mg phase and Mg(12)YZn phase, indicating that the Mg(12)YZn phase accelerated galvanic corrosion of Mg matrix. Mg(97)Zn1Y2 alloy shows the lowest corrosion rate due to the continuous distribution of Mg(12)YZn phase.     

Microstructure,Mechanical Properties,Corrosion Behavior and Biocompatibility of As-Extruded Biodegradable Mg–3Sn–1Zn–0.5Mn Alloy

The microstructure evolution and mechanical properties of biodegradable Mg–3Sn–1Zn–0.5Mn alloys were investigated by the optical microscopy, X-ray diffractometer and a universal material testing machine.The corrosion and degradation behaviors were studied by potentiodynamic polarization method and immersion test in a simulated body fluid(SBF). It was found that the as-extruded Mg–3Sn–1Zn–0.5Mn alloy has the fine equiaxed grains which underwent complete dynamic recrystallization during the hot extrusion process, with the second phase particles of Mg_2Sn precipitated on the grain boundaries and inside the grains. The tensile strength and elongation of as-extruded Mg–3Sn–1Zn–0.5Mn alloys were 244 ± 3.7 MPa and 19.3% ± 1.7%, respectively. The potentiodynamic polarization curves in SBF solution indicated the better corrosion resistance of the as-extruded Mg–3Sn–1Zn–0.5Mn alloy in the SBF solution. Immersion test in the SBF solution for 720 h revealed that the corrosion rate of as-extruded Mg–3Sn–1Zn–0.5Mn alloy was nearly 4 ± 0.33 mm/year. The hemolysis rate of as-extruded Mg–3Sn–1Zn–0.5Mn alloy was lower than the safe value of 5% according to ISO 10993-4. As-extruded Mg–3Sn–1Zn–0.5Mn alloy showed good biocompatibility after being implanted into the dorsal muscle and the femoral shaft of the rabbit, and no abnormalities were found after short-term implantation. It was revealed that the as-extruded Mg–3Sn–1Zn–0.5Mn alloy is a promising material for biodegradable implants,which possesses an interesting combination of preferred mechanical properties, better corrosion resistance and biocompatibility.     

Highly Improved Gaseous Hydrogen Storage Characteristics of the Nanocrystalline and Amorphous Nd-Cu-added Mg2Ni-type Alloys by Melt Spinning

The nanocrystalline and amorphous Mg-Nd-Ni-Cu quaternary alloys with a composition of （Mg24Ni10Cu2）loo-xNdx （x = 0-20） were prepared by melt spinning. The X-ray diffraction and transmission electron microscopy inspections reveal that, by varying the spinning rate and the Nd content, different microstructures could be obtained by melt spinning. Particularly, the as-spun Nd-free alloy holds an entire nanocrystalline structure but the as-spun Nd-added alloy has a nanocrystalline and amorphous structure, which implies that the addition of Nd facilitates the glass forming in the Mg2Ni-type alloy. Also, the degree of the amorphization in the as-spun Nd-added alloys clearly increases with increasing the spinning rate and the Nd content. The H-storage capacity and the hydrogenation kinetics of amorphous, partially and completely nanocrystalline alloys were investigated and it was found that they are dependent on the microstructure and the phase composition of the alloys. Specially, enhancing the spinning rate from 0 （the as-cast was defined as the spinning rate of 0 m/s） to 40 m/s makes the hydrogen absorption saturation ratio （R5a） （a ratio of the hydrogen absorption quantity in 5 min to the saturated hydrogen absorption capacity） increase from 35.2% to 90.3% and the hydrogen desorption ratio （R10d） （a ratio of the hydrogen desorption quantity in 10 min to the saturated hydrogen absorption capacity） rise from 12.7% to 44.9% for the （x = 5） alloy. And the growing of the Nd content from 0 to 20 gives rise to the R5a and R10d values rising from 85.7% to 94.5% and from 36.7% to 54.8% for the as-spun （30 m/s） alloys, respectively.     

Hot Deformation Behavior of 2124 Al Alloy

The mechanical behavior of 2124 Al alloy produced by powder metallurgy was investigated with compression test at different temperatures and strain rates. The tests were performed in the temperature range of 300℃～500℃ and at strain rates from 0.001 s^-1 to 1.0 s^-1. The compression flow curves exhibited an initial sharp increase with strain, followed by monotonous hardening. The maximum stress decreased with decreasing strain rate and increasing temperature. The hot deformation characteristics of the material were studied using processing maps. The domain of safety and unsafe regime were identified and validated through microstructural examination.     

Unraveling precipitation evolution and strengthening function of the Al-Zn-Mg-Cu alloys with various Zn contents: Multiple experiments and integrated internal-state-variable modeling

Zn content is one of the most concerned factors in the development of next generation ultra-strength Al-Zn-Mg-Cu alloys owing to its essential role in precipitation strengthening. In the present work, the underlying functions of Zn content in precipitation evolution and strengthening function of Al-Zn-Mg-Cu alloys were systematically investigated by combining multiple experiments and an integrated internalstate-variable model. The experimental results indicated that the increased Zn content in A...     

Effect of Yttrium Addition on Microstructural Characteristics and Superplastic Behavior of Friction Stir Processed ZK60 Alloy

As-extruded ZK60 and ZK60-Y magnesium alloy plates were successfully processed via friction stir processing （FSP） at a tool rotation rate of 1600 r/rain and a traverse speed of 200 mm/min. FSP resulted in the formation of equiaxed recrystallized microstructures with the average grain sizes of ,-8.5 and -4.7 μm in the ZK60 and ZK60-Y alloys, respectively. Moreover, FSP broke and dispersed the MgZn2 and W-phase （Mg3Zn3Y2） particles and dissolved MgZn2 phase in the FSP ZK60 alloy. With the addition of rare earth element yttrium （Y） into the ZK60 alloy, the ratio of the high angle grain boundaries （HAGBs） in the FSP alloys increased from 64% to 90%, and a certain amount of twins appeared in the FSP ZK60-Y alloy. The maximum elongation of 1200% and optimum strain rate of 3 X 10-3 s-1 achieved at 450 °C in the FSP ZK60-Y alloy were substantially higher than those of the FSP ZK60 alloy. This is attributed to the fine grains with high ratio of HAGBs and the distribution of a large number of dispersed second phase particles with high thermal stability in the FSP ZK60-Y alloy. Grain boundary sliding was identified as the primary deformation mechanism in the FSP ZK60 and ZK60-Y alloys from the superplastic data analyses and surficial morphology observations.     

Laser Surface Hardening of 9CrSi Steel

The effects of laser hardening parameters such as beam power,beam diameter and scanning rate on microstructure and mardness of 9CrSi steel were investigated.The microstructure of the surface layer of 9CiSi steel was changed from pearlite to martensite,retained austenite and carbide by laser hardening .The depth of the hardened layer increased with increasing laser energy density and the surface hardeness increased by 3-5times as high as the untreated steel.The laser hardened surface had good wear resistance due to martensite and carbide in the surface layer.The wear mode at low speed was abrasive,while the wear mode at high speed was adhesive.     

Improvement of elevated-temperature strength and recrystallization resistance via Mn-containing dispersoid strengthening in Al-Mg-Si 6082 alloys

The precipitation behavior of Mn-containing dispersoids in Al-Mg-Si 6082 alloys with different Mn contents(0, 0.5 and 1.0 wt%) during various heat treatments(300–500℃) was investigated. The effects of dispersoids on elevated-temperature strength and recrystallization resistance during hot-rolling and post-rolling annealing were evaluated. The results showed that the dispersoids in the Mn-containing alloys(0.5 and 1.0%) began to precipitate at 350℃ and reached the optimum conditions after 2–4 h at 400℃. However, the dispersoids coarsened with increasing holding time at temperatures above450℃. After the peak precipitation treatment at 400℃ for 2 h, the yield strength at 300℃ increased from 28 MPa(base alloy free of Mn) to 55 MPa(alloy with 0.5% Mn) and 70 MPa(alloy with 1% Mn), respectively, demonstrating a significant dispersoid strengthening effect at elevated temperature. In addition,the dispersoids were thermally stable at 300℃ for up to 1000 h holding owing to its relative high precipitation temperature(350–400℃), leading to the superior constant mechanical performance at elevated temperature during the long service life. During hot rolling and post-rolling annealing, the presence of a large amount of dispersoids results in the higher Zener drag PZcompared with base alloy and then significantly improved the recrystallization resistance. The alloy containing 0.5% Mn exhibited the highest recrystallization resistance among three experimental alloys studied during the post-rolling process,likely resulted from the lower coarsening rate of dispersoids and the lower dispersoids free zone.     

NbMoTiVSi_x refractory high entropy alloys strengthened by forming BCC phase and silicide eutectic structure

In order to improve mechanical properties of refractory high entropy alloys,silicide was introduced and NbMoTiVSi_x(x=0,0.1,0.2,0.3,and 0.4,molar ratio) refractory high entropy alloys are prepared by vacuum arc melting.Phase composition,micro structure evolution and mechanical properties were systematically studied.Results show that the silicide phase is formed in the alloys with addition of silicon,and the volume fraction of silicide increases from 0 to 8.3 % with increasing of silicon.Microstructure observation shows that the morphology of dendrite changes from columnar to near equiaxed,eutectic structure is formed at grain boundaries and composed of secondary BCC phase and silicide phase.The average length of the primary and second dendrites decreases with the increasing of silicon.Whereas,the ratio of eutectic structure increases from 0 to 19.8 % with the increment of silicon.The refinement of microstructure is caused by heterogeneous nucleation from the silicide.Compressive tests show that the yield and ultimate strength of the alloys increases from 1141.5 MPa to 2093.1 MPa and from 1700.1 MPa to 2374.7 MPa with increasing silicon content.The fracture strain decreases from 24.7 %-11.0 %.Fracture mechanism is changed from ductile fracture to ductile and brittle mixed fracture.The improvement of the strength is caused by grain bounda ry strengthening,which includes more boundaries around primary BCC phase and eutectic structure in grain boundary,both of them is resulted from the formation of silicide.     

Enhanced Fracture Toughness of Pressureless-sintered SiC Ceramics by Addition of Graphene

To enhance the fracture toughness of pressureless-sintered Si C ceramic, graphene was introduced as an additive. The effects of graphene contents on the fracture toughness, bending strength, micro-hardness,phase compositions, and microstructure evolutions of the Si C ceramics were investigated in detail by scanning electron microscopy, energy dispersive X-ray spectroscopy, and metallographic microscopy. The fracture toughness, bending strength and micro-hardness increased initially, and then decreased with the graphene content increasing from 0 to 5.0 wt%. The highest fracture toughness of 5.65 MPa m1/2was obtained for sample with 1.0 wt% graphene sintered at 2130 °C for 1 h in Ar, which was about 22.6% higher than that of Si C sample without graphene. In addition, the highest bending strength and microhardness of 434.14 MPa and 29.21 GPa corresponded to the Si C samples with graphene content of 0.5wt% and 2.0 wt%, respectively.     

Ultrahigh strength Mg-Y-Ni alloys obtained by regulating second phases

Mg-Y-Ni alloys with different second phases were designed by changing Y/Ni atomic ratio from 1.5 to 0.5.The microstructure and mechanical properties of as-cast and as-extruded alloys were investigated. The as-cast Mg-Y-Ni alloy with Y/Ni ratio of 1.5 is composed of α-Mg and long period stacking ordered(LPSO)phase. When Y/Ni ratio is equal to 1, nanoscale lamellar γ' phase and eutectic Mg_2Ni phase are formed in addition to LPSO phase. As Y/Ni ratio decreases further, the amount of eutectic Mg_2Ni phase increases,while the amount of LPSO phase decreases. After extrusion, the LPSO and γ' phases are distributed along the extrusion direction, while eutectic Mg_2Ni phase is broken and dispersed in the as-extruded alloys.LPSO phase and Mg_2Ni phase in the alloys promote dynamic recrystallization(DRX) during extrusion,while γ' phase inhibits DRX. Consequently, the Mg96Y2Ni2(at.%) alloy with LPSO phase and γ' phase as the main second phases shows the strongest basal texture after extrusion. The tensile yield strength of the as-extruded Mg-Y-Ni alloys increases first and then decreases with decreasing Y/Ni ratio. The as-extruded Mg96Y2Ni2(at.%) alloy with Y/Ni = 1 exhibits excellent mechanical properties with tensile yield strength of 465 MPa, ultimate tensile strength of 510 MPa and elongation to failure of 7.2%, which is attributed to the synergistic effect of bulk LPSO phase and nanoscale γ' phase.     

Hot deformation behavior of an antibacterial Co–29Cr–6Mo–1.8Cu alloy and its effect on mechanical property and corrosion resistance

In order to optimize the deformation processing, the hot deformation behavior of Co–Cr–Mo–Cu(hereafter named as Co–Cu) alloy was studied in this paper at a deformation temperature range of 950–1150°C and a strain rate range of 0.008–5 s~(-1). Based on the true stress–true strain curves, a constitutive equation in hyperbolic sin function was established and a hot processing map was drawn. It was found that the flow stress of the Co–Cu alloy increased with the increase of the strain rate and decreased with the increase of the deforming temperature. The hot processing map indicated that there were two unstable regions and one well-processing region. The microstructure, the hardness distribution and the electrochemical properties of the hot deformed sample were investigated in order to reveal the influence of the hot deformation. Microstructure observation indicated that the grain size increased with the increase of the deformation temperature but decreased with the increase of the strain rate. High temperature and low strain rate promoted the crystallization process but increased the grain size, which results in a reduction in the hardness. The hot deformation at high temperature(1100–1150°C) would reduce the corrosion resistance slightly. The final optimized deformation process was: a deformation temperature from 1050 to 1100°C, and a strain rate from 0.008 to 0.2 s~(-1), where a completely recrystallized and homogeneously distributed microstructure would be obtained.     

Age hardening responses of as-extruded Mg-2.5Sn-1.5Ca alloys with a wide range of Al concentration

This article aims to explore the age hardening responses of both as-extruded and as-aged Mg-2.5 Sn-1.5 Ca-x Al alloys(x = 2.0, 4.0 and 9.0 wt%, termed TXA322, TXA324 and TXA329, respectively) through microstructural and mechanical characterization. Results indicate that grain size of as-extruded TXA322,TXA324 and TXA329 alloys were ～ 16 μm, ～ 10 μm and ～ 12 μm, respectively. A number of a and c+adislocations were observed in all the as-extruded samples. Guinier – Preston(GP) zones were evidently identified in TXA322 alloy, while only a small number of Mg17 Al12 phases existed in both TXA324 and TXA329 alloys. An aging treatment facilitated the precipitation of a high number density of GP zones within the matrix of TXA322 alloy. In contrast, no obvious nano-precipitates were in as-aged TXA324 alloy. Numerous nano-Mg17 Al12 phases were formed through a following aging treatment in TXA329 alloy. In terms of mechanical properties, it is apparent that an increment in ultimate tensile strength of ～46 MPa and ～ 40 MPa was yielded in peak-aged TXA322 and TXA329 alloys, while no obvious variations in UTS were present in peak-aged TXA324 alloy, in comparison with the as-extruded counterparts.     

On Temperature and Strain Rate Dependent Strain Localization Behavior in Ti-6.5Al-3.5Mo-1.5Zr-0.3Si Alloy

Deformation behaviors of Ti-6.5AI-3.5Mo- 1.5Zr-0.3Si alloys withα/βlamellar structure were investigated systematically at different temperatures from room temperature to 950℃and different strain rates.Results reveal that when the deformation temperature is higher than a critical temperature of 600℃,an evident transition of deformation behavior from localized shear banding toα/βlamella kinking,flow softening and temperature/strain rate-dependent peak flow stress occurred in the alloy.The critical conditions for the occurrence of internal cracking and strain localization behavior associated with temperature and strain rate were determined.     

Steady State Rheological Characteristic of Semisolid Magnesium Alloy

Isothermal compressive experiments at different temperatures, strain rates and holding time for semisolid AZ91D, Zr modified AZ91D and MB15 alloy with higher solid volume fraction were carried out by using Gleeble-1500D simulator and the true stress-strain curves were given directly. The relationship of apparent viscosity vs temperature, shear rate and holding time of the three kinds of semi-solid magnesium alloys, as well as isothermal steady state rheological characteristic and mechanical behavior were studied. The results show that the three magnesium alloys had the characteristic of shear-thinning. The rheological characteristic of the semi-solid MB15 is different from that of semi-solid AZ91D. The semi-solid MB15 has higher apparent viscosity and deformation resistance.     

Hot deformation behavior and processing map development of AZ110 alloy with and without addition of La-rich Mish Metal

In order to compare the workability of AZ110 alloy with and without addition of La-rich Mish Metal(MM), hot compression tests were performed on a Gleeble-3500 D thermo-mechanical simulator at the deformation temperature range of 473-623 K and strain rate range of 0.001-1 s-1. The flow stress, constitutive relation, DRX kinetic model, processing map and microstructure characterization of the alloys were investigated. The results show that the flow stress is very sensitive to deformation temperature and strain rate, and the peak stress of AZ110 LC(LC = La-rich MM) alloy is higher than that of AZ110 alloy.The hot deformation behavior of the alloys can be accurately predicted by the constitutive relations. The derived constitutive equations show that the calculated activation energy Q and stress exponent n for AZ110 alloy are higher than the calculated values of AZ110 LC alloy. The analysis of DRX kinetic models show that the development of DRX in AZ110 LC alloy is earlier than AZ110 alloy at the same deformation condition. The processing maps show that the workability of AZ110 LC alloy is significantly more excellent than AZ110 alloy and the microstructures are in good agreement with the calculated results.The AZ110 LC alloys can obtain complete DRX microstructure at high strain rate due to its higher stored energy and weak basal texture.