Effects of Zr,Ti and Sc additions on the microstructure and mechanical properties of Al-0.4Cu-0.14Si-0.05Mg-0.2Fe alloys

The evolution of microstructure and mechanical properties of Al-0.4Cu-0.14Si-0.05Mg-0.2Fe (wt.%) alloys, micro-alloyed with Zr, Ti and Sc, were investigated. The addition of 0.2%Zr to base alloy accelerates the precipitation of Si-rich nano-phase in α-Al matrix, which plays an important role in improving the mechanical properties of an alloy. The tensile strength increases from 102 MPa for the base alloy to 113 MPa for the Zr-modified alloy. Adding 0.2%Zr + 0.2%Ti to base alloy effectively refines α-Al grain size and accelerates the precipitation of Si and Cu elements, leading to heavy segregation at grain boundary. By further adding 0.2%Sc to Zr + Ti modified alloy, the segregation of Si and Cu elements is suppressed and more Si and Cu precipitates appeared in α-Al matrix. Accompanied with the formation of coherent Al₃Sc phase, the tensile strength increases from 108 MPa for the Zr + Ti modified alloy to 152 MPa for the Sc-modified alloy. Due to excellent thermal stability of Al₃Sc phase, the Sc-modified alloy exhibits obvious precipitation hardening behavior at 350 °C, and the tensile strength increases to 203 MPa after holding at 350 °C for 200 h.     

Effect of vanadium addition on the microstructure and properties of AlCoCrFeNi high entropy alloy

The purpose of this study is to investigate the effects of vanadium addition on the microstructure and mechanical properties of AlCoCrFeNiV_x (x values in molar ratio, x = 0, 0.2, 0.5, 0.8, 1.0) alloys. All the alloys were found to display a crystalline structure of simple body centered cubic (BCC). For AlCoCrFeNi and AlCoCrFeNiV_0.2 alloys, Cr and Fe elements segregated to the center of grain while Al and Ni elements segregated to the rest areas. With the increase of V content exceeding to x = 0.5, the homogenized polycrystalline grain can be obtained. For AlCoCrFeNiV_0.2 alloy, the compressive strength and plastic strain were as high as 3297.8 MPa and 26.8%, respectively, which were rare in high entropy alloys to date. The fine nanoscale spinodal decomposition microstructure was a key factor for the high fracture strength of AlCoCrFeNiV_0.2 alloy. The values of Vickers hardness increased from HV534 to HV648.8 with the increase of V content. The solid-solution strengthening of the body centered cubic matrix was found as the main factor that strengthened the alloys. With the increase of V contents from x = 0 to x = 1.0, the transformation of ferromagnetic behavior to paramagnetic behavior takes place.     

A high strength and ductility Mg–Zn–Al–Cu–Mn magnesium alloy

A high strength Mg–8.0Zn–1.0Al–0.5Cu–0.5Mn (wt.%) magnesium alloy with outstanding ductility was developed using a common casting technique and heat treatment. The microstructure of the as-cast alloy is composed of α-Mg, MgZn, MgZnCu and Al–Mn phases. After the solution treatment and subsequent two-step aging treatment, the yield strength (YS), ultimate tensile strength (UTS) and elongation of the alloy at peak hardness reach 228 MPa, 328 MPa and 16.0% at room temperature, respectively. The comprehensive mechanical properties of the alloy are superior to almost all other high performance casting Mg alloys.     

Microstructures,mechanical and cytocompatibility of degradable Mg–Zn based orthopedic biomaterials

The Mg–1Zn–xSr (x = 0.2, 0.5, 0.8 and 1 wt.%) alloys have been prepared by zone purifying solidification followed by backward extrusion (BE). The grain size was reduced and the hardness was improved with the increased concentration of strontium (Sr) after backward extrusion. The BE–Mg–1Zn–0.8Sr alloy was mostly composed of fine precipitates (MgZn and Mg₁₇Sr₂) and Mg matrix. At the same time, the mechanical properties of BE–Mg–Zn–Sr alloys were increased with the increment of strontium, which were strongly associated with fine average grain size and homogeneous secondary precipitates. The degradation rate is significantly increased when Sr content is over 0.8 wt.%. The homogenous degradation rate is achieved. The degradation products show good biocompatibility evaluated by MTT method using L929 cell line. It is demonstrated that the micro-alloying element of Sr is a potential approach to develop novel Mg–Zn based biomaterials.     

Microstructure,mechanical properties and shape memory effect of Cu–Hf–Al–Ni alloys

The influence of hafnium element’s incorporation on a Cu–xHf–13.0Al–4.0Ni (wt-%) (x?=?0.5, 1.0 and 2.0) high-temperature shape memory alloy was investigated systematically. The results show that the matrix of Cu–xHf–13.0Al–4.0Ni (x?=?0.5, 1.0 and 2.0) alloys is 18R martensite, and an orthorhombic-structured Cu₈Hf₃ phase is formed and distributed at the grain boundaries. The grain size is significantly reduced with increasing Hf content. The mechanical properties of Cu–xHf–13.0Al–4.0Ni (x?=?0.5, 1.0 and 2.0) alloys are improved by Hf doping due to the combination of refinement strengthening, solid solution strengthening and second phase strengthening. After heating under pre-strain of 10%, the shape memory effect of the Cu–1.0Hf–13.0Al–4.0Ni alloy reaches 5.6%, which is obviously higher than that of the Cu–13.0Al–4.0Ni alloy.     

Transformation behavior and shape memory characteristics of thermo-mechanically treated Ti–(45−x)Ni–5Cu–xV (at%) alloys

Transformation behavior, shape memory characteristics and superelasticity of thermo-mechanically treated Ti–(45?x)Ni–5Cu–xV (at%) (x = 0.5–2.0) alloys were investigated by means of differential scanning calorimetry, transmission electron microscopy, X-ray diffractions, thermal cycling tests under constant load and tensile tests. The B2–B19′ transformation occurred when V content was 0.5 at%, above which the B2–B19–B19′ transformation occurred. The B2–B19 transformation was not separated clearly from the B19–B19′ transformation. Thermo-mechanically treated Ti–(45?x)Ni–5Cu–xV alloys showed perfect shape memory effect and transformation hysteresis(ΔT) of Ti–43.5Ni–5.0Cu–1.5V and Ti–43.0Ni–5.0Cu–2.0V alloys was about 9 K which was much smaller than that of a Ti–44.5Ni–5.0Cu–0.5V alloy(23.3 K). More than 90% of superelastic recovery ratio was observed in all specimens and transformation hysteresis (Δσ) of a Ti–44.5Ni–5.0Cu–0.5V alloy was about 70 MPa, which was much larger than that of a Ti–43.0Ni–5.0Cu–2.0V alloy (35 MPa).     

Influence of size and distribution of W phase on strength and ductility of high strength Mg-5.1Zn-3.2Y-0.4Zr-0.4Ca alloy processed by indirect extrusion

A high strength Mg-5.1Zn-3.2Y-0.4Zr-0.4Ca(wt%) alloy containing W phase(Mg_3Y_2Zn_3) prepared by permanent mold direct-chill casting is indirectly extruded at 350?C and 400?C, respectively. The extruded alloys show bimodal grain structure consisting of fine dynamic recrystallized(DRXed) grains and unrecrystallized coarse regions containing fine W phase and β2' precipitates. The fragmented W phase particles induced by extrusion stimulate nucleation of DRXed grains, leading to the formation of fine DRXed grains, which are mainly distributed near the W particle bands along the extrusion direction. The alloy extruded at 350?C exhibits yield strength of 373 MPa, ultimate tensile strength of 403 MPa and elongation to failure of 5.1%. While the alloy extruded at 400?C shows lower yield strength of 332 MPa,ultimate tensile strength of 352 MPa and higher elongation to failure of 12%. The mechanical properties of the as-extruded alloys vary with the distribution and size of W phase. A higher fraction of DRXed grains is obtained due to the homogeneous distribution of micron-scale broken W phase particles in the alloy extruded at 400?C, which can lead to higher ductility. In addition, the nano-scale dynamic W phase precipitates distributed in the un DRXed regions are refined at lower extrusion temperature. The smaller size of nano-scale W phase precipitates leads to a higher fraction of un DRXed regions which contributes to higher strength of the alloy extruded at 350?C.     

Influence of nanoparticle reinforcements on the strengthening mechanisms of an ultrafine-grained dual phase steel containing titanium

To reduce cost, optimise mechanical properties and improve process tolerance, a series of 1000 MPa grade ultrafine-grained dual phase (DP) steels with nanosized precipitates have been developed based on the C–Si–Mn–Ti alloy system. The grain size of ferrite in ultra-high strength DP steels ranges from 1.1 to 1.7 μm. The amount of precipitations in the annealed sheet is much more than that in the hot-rolled plates with the highest distribution frequency being between 5 and 10 nm. The grain refinement and precipitation strengthening interact significantly and have a considerable effect on yield strength. Therefore, the strengthening effects cannot be expressed as a simple linear relationship. A modified root-mean-square (RMS) relationship has been proposed to express the yield strength for dual phase steel with obvious grain refinement and precipitation strengthening.     

Role of microstructure on initiation and propagation of fatigue cracks in precipitate strengthened Cu–Ni–Si alloy

Fatigue tests were conducted on round-bar specimens to understand the fatigue behavior of precipitate-strengthened Cu–6Ni–1.5Si alloy. Aging at 500 °C for 0.5 h produced δ-Ni₂Si precipitates in the matrix, homogeneously and heterogeneously precipitated δ-Ni₂Si particles, and a precipitate-free zone around the grain boundaries. The cracks were initiated at the grain boundaries, followed by growth along the crystallographic slip planes in the adjacent grains. Crack propagation from the crack origin along the grain boundaries was occasionally observed. The physical background of fatigue damage is discussed in light of the role of microstructure on the behavior of fatigue cracks.     

Novel Cu-bearing economical 21Cr duplex stainless steels

A new family of 21Cr–2 Ni–1.0Mo–0.2 N–xCu (x = 0.5, 1.0, 1.5) economical duplex stainless steels have been developed by examining the effect of Cu on the microstructure and properties of solution-treated specimens. The results have shown that these alloys have a balanced ferrite–austenite duplex structure. The ferrite content increases with the solution treatment temperature, but decreases with an increase in Cu. Some precipitates such as sigma phase, ε-Cu and Cr₂N were found when solution-treated at 780 °C for 30 min. The yield strength, tensile strength and fracture elongation values of experimental alloys solution-treated at 1020 °C for 30 min were about 540 MPa, 1000 MPa, and 35%, respectively. The pitting corrosion potentials of the solution-treated alloys were all above 500 mV in 1 mol/L NaCl solution at room temperature and the pitting corrosions always occur in ferrite phase. The mechanical properties and corrosion resistance of the designed alloys with lower production cost are better than those of AISI 316L austenitic stainless steel.     

Effects of Si addition on mechanical properties and superelasticity of Ti–7.5Nb–4Mo–2Sn shape memory alloy

Effects of 0–2.1 at.% Si additions on microstructure and mechanical properties of a Ni-free biomedical superelastic β-Ti alloy, Ti–7.5 at.%Nb–4 at.%Mo–2 at.% Sn (Ti–7.5Nb–4Mo–2Sn), were investigated. The alloys after annealing at 973 K mainly contain β and α″. As the concentration of Si is higher than 1 at.%, Ti₅Si₃ particles can be found in the alloys, and the number density of the particles increases with the increasing of silicon’s concentration. The addition of Si promotes the strength of the Ti–7.5Nb–4Mo–2Sn due to the Si solid solution strengthening effect and fine Ti₅Si₃ precipitates. However, as the Si concentration reaches 2.1%, the alloy exhibits a brittle fracture. The 0.5–1.6 at.% Si additions improve the superelasticity of the Ti–7.5Nb–4Mo–2Sn alloy by increasing the critical stress for inducing martensite (σ_SIM).     

Gibbs–Thomson relationship for the precipitation in Cu–Ti alloys

A diffusion-couple Cu/Cu–Ti alloy was prepared in order to obtain a macroscopic composition gradient in the Cu–Ti alloy system. This couple was solution treated subsequently water quenched and aged at 837 K for 600 s. The precipitation process was analyzed with an EDS-TEM. The Gibbs–Thomson relation was used to analyze the precipitation of the metastable and semicoherent β′ phase in Cu–Ti alloys and this relationship enabled us to estimate the interfacial free energy between matrix and precipitates to be about 0.31 ± 0.02 J m^− 2.     

Influence of second phases on mechanical properties of spray-deposited Al–Zn–Mg–Cu alloy

Al–10.66Zn–2.48Mg–1.41Cu–0.17Zr–0.17Sc (wt.%) alloy prepared by spray deposition was processed with different hot deformation followed by heat treatment. The mechanical properties and microstructure evolution were investigated. The results indicate that uniform ultimate tensile strength of 774 MPa, yield strength of 734 MPa and elongation of 13.7% are obtained with two-step hot deformation, which increase by 2.7%, 3.82% and 95% compared with one-step hot deformation. Microstructural observations show that increase of elongation is mainly ascribed to high volume fraction of smaller precipitates and reduction of stress concentration areas as a result of disappearance of the coarse second phases. The fractured tensile specimens with two-step hot deformation exhibit dimple fractographic features. Improvement of strength is attributed to the precipitation strengthening and dispersed strengthening.     

Heating aging behavior of Al–8.35Zn–2.5Mg–2.25Cu alloy

In the present work, the influence of heating aging treatment (HAT) on the microstructure and mechanical properties of Al–Zn–Mg–Cu alloy was investigated. When the final aging temperature (FAT) was lower than 180 °C, the hardness increased with the decreasing of heating rate, however, in the case of the FAT was higher than 180 °C, the variation of hardness was opposite. The electrical conductivity of Al–Zn–Mg–Cu alloy increased with the decrease of heating rate regardless of FAT. The tensile strength, yield strength and conductivity of the Al alloy after (100–180 °C, 20 °C/h) HAT increased by 1.6%, 4.5% and 14.1% than that after T6 treatment, respectively. The precipitates sequence of HAT was coincident with that of isothermal aging, which is SSS → GP zone → η′ → η. With the increase of FAT and the decrease of heating rate, the fine precipitates became larger and the continuous η phase at grain boundary grew to be individual large precipitates. The HAT time was decreased about 80% than that for T6 treatment, indicating HAT could improve the mechanical properties, corrosion resistance and production efficiency with less energy consumption.     

Microstructures and mechanical properties of spark plasma sintered Cu–Cr composites prepared by mechanical milling and alloying

Nanograined Cu–8 at.% Cr composite was produced by a combination of mechanical milling (MM), mechanical alloying (MA) and spark plasma sintering (SPS). Commercial Cu and Cr powders were pre-milled separately by MM. The milled Cu and Cr powders were then mechanically alloyed with as-received Cr and Cu powders respectively. After milling, the powder mixtures were separately subjected to SPS. It was found that pre-milling Cr can efficiently decrease the size of grain and reinforcement, resulting in remarkable strengthening. The grain size of Cu matrix was about 82 nm after SPS. The Vickers hardness, compressive yield strength and compression ratio of the composite were 327 HV, 1049 MPa and 10.4%, respectively. The excellent mechanical properties were primarily attributed to dispersion strengthening of the Cr particles and fine grain strengthening of the Cu matrix. The strong Cu/Cr interface and dissolved Cr atoms can also contribute to strengthening of the composite.     

Enhancement of creep resistance and thermal behavior of eutectic Sn–Cu lead-free solder alloy by Ag and In-additions

The eutectic Sn–0.7Cu solder alloy is widely used in electronic packaging in which the creep property of the solder joint is essential to meet the global demand for longer operating lifetime in their applications. In this study, the influence of Ag and In additions on tensile creep behavior and thermal properties of bulk eutectic Sn–Cu solder alloy is reported. Results show that addition of Ag and In resulted not only in the formation of new Ag₃Sn and γ-SnIn₄ intermetallic compounds (IMCs), but also in the refinement of grain size of Sn–0.7Cu solder from ∼0.50 to ∼0.15 μm. Accordingly, the creep properties of the Ag or In-containing solder alloys are notably improved. The creep strain rate increases and creep lifetime decreases as the applied stress level and temperature increase. Room and elevated-temperature creep rate of bulk Sn–Cu solder was reduced by 521.0% after Ag addition, but for In addition the reduction was about 200.7%. These differences are attributed to the presence of new Ag₃Sn and γ-SnIn₄ precipitates and their rules in classical dispersion strengthening as a separate phases. Moreover, the eutectic temperature of Sn–0.7Cu is decreased from 227.4 to 217.8 and 224.0 °C with the addition of Ag and In, respectively.     

Fabrication and characterization of novel biodegradable Zn-Mn-Cu alloys

Zn-Mn-Cu alloys with micro-alloying of Mn and Cu in Zn are developed as potential biodegradable metals. Although the as-cast alloys are very brittle, their ductilities are significantly improved through hot rolling. Among the as-cast and the as-hot-rolled alloys, as-hot-rolled Zn-0.35 Mn-0.41 Cu alloy has the best comprehensive property. It has yield strength of 198.4 ± 6.7 MPa, tensile strength of 292.4 ± 3.4 MPa,elongation of 29.6 ± 3.8% and corrosion rate of 0.050-0.062 mm a~(-1). A new ternary phase is characterized and determined to be MnCuZn18, which is embedded in MnZn13, resulting in a coarse cellular/dendritic MnZn13-MnCuZn18 compound structure in Zn-0.75 Mn-0.40 Cu alloy. Such a coarse compound structure is detrimental for wrought alloy properties, which guides future design of Zn-Mn-Cu based alloys.The preliminary research indicates that Zn-Mn-Cu alloy system is a promising candidate for potential cardiovascular stent applications.     

Effects of pre-treatments on aging precipitates and corrosion resistance of a creep-aged Al–Zn–Mg–Cu alloy

The effects of pre-treatments (solution and retrogression) on aging precipitates and corrosion resistance of a creep-aged Al–Zn–Mg–Cu alloy are investigated by means of transmission electron microscope (TEM), scanning electron microscope (SEM) and cyclic potentiodynamic polarization experiments. It is found that the aging precipitates and corrosion resistance are greatly affected by the pre-treatments. For the creep-aged alloy after solution pre-treatment, fine aging precipitates with high density are formed within grains. Meanwhile, large and continuously-distributed aging precipitates appear along grain boundaries. Also, this creep-aged alloy is strongly sensitive to the electrochemical corrosion, and the corrosion pits are easily induced in the 3.5 wt.% NaCl solution. For the creep-aged alloy after retrogression pre-treatment, when the retrogression pre-treatment time is increased, the density of intragranular aging precipitates first increases and then decreases, while the size of grain boundary precipitate and the width of precipitate free zone continuously increase. Compared with the creep-aged alloy after solution pre-treatment, the corrosion resistance of the creep-aged alloy after retrogression pre-treatment is greatly improved.     

Transient liquid phase (TLP) bonding of Al7075 to Ti–6Al–4V alloy

TLP diffusion bonding of two dissimilar aerospace alloys, Ti–6Al–4V and Al7075, was carried out at 500 °C using 22 μm thick Cu interlayers for various bonding times. Joint formation was attributed to the solid-state diffusion of Cu into the Ti alloy and Al7075 alloy followed by eutectic formation and isothermal solidification along the Cu/Al7075 interface. Examination of the joint region using SEM, EDS and XPS showed the formation of eutectic phases such as, ?(Al₂Cu), T(Al₂Mg₃Zn₃) and Al₁₃Fe along grain boundaries within the Al7075 matrix. At the Cu/Ti alloy bond interface a solid-state bond formed resulting in a Cu₃Ti₂ phase formation along this interface. The joint region homogenized with increasing bonding time and gave the highest bond strength of 19.5 MPa after a bonding time of 30 min.     

Effect of stacking fault energy on mechanical behavior of cold-forging Cu and Cu alloys

Cu, Cu–2.87 wt% Mn, Cu–4.40 wt% Mn and Cu-10.19 wt% Mn were prepared by cold-forging. The deformation behavior of Cu–Mn alloys is consistent with the Cu-Al alloys and Cu–Zn alloys but without lowering the stacking fault energy to simultaneously increase the strength and ductility. A series of analysis demonstrate that Cu–Mn alloys have a much smaller twin density than low stacking fault energy (SFE) metals, and dislocation strengthening is the major reason for the higher strength. The role of short range order (SRO) in promoting the mechanical properties has also been briefly discussed.