Texture Evaluation of a Bi-Modal Structure During Static Recrystallization of Hot-Deformed Mg-Al-Sn Alloy

In this study, Mg-Al-Sn alloy was hot compressed at 523 K (250 °C) and annealed at 623 K (350 °C) for various times. The initial as-deformed microstructure was partially dynamic recrystallized with strain-induced precipitates on the recrystallized grain boundaries. After annealing at 623 K (350 °C), static recrystallization (SRX) of the bimodal microstructure took place where, at this temperature, no static precipitates formed. The goal of this work was to study the effect of dynamic precipitation on the texture evolution during the SRX process. Progressive texture evolution was studied during annealing by electron backscattered diffraction technique through a microstructure-tracking process. It was found that the grain-coarsening mechanism during the early stage of annealing is not totally controlled by the basal-oriented grains. Also, it was found that the dynamic precipitates may have significant influence in the early texture weakening during annealing of a bimodal structure.     

Influence of the Processing Temperature on the Microstructure, Texture, and Hardness of the 7075 Aluminum Alloy Fabricated by Accumulative Roll Bonding

The 7075 alloy is an Al-Zn-Mg-Cu wrought age-hardenable aluminum alloy widely used in the aeronautical industry. The alloy was accumulative roll bonded at 300 °C (573 K), 350 °C (623 K), and 400 °C (673 K), and the microstructure, texture, and hardness were investigated. Cell/(sub)grain size in the nanostructured range, typical β-fiber rolling texture, and homogeneous hardness through thickness were determined in all cases. Misorientation was different at each processing temperature. At 400 °C, the presence of elements in solid solution and the partial dissolution of the hardening precipitates lead to a poorly misoriented microstructure with a high dislocation density and a homogeneous β-fiber texture of low intensity, typical of intermediate degrees of rolling. At 350 °C and 300 °C, highly misoriented microstructures with smaller dislocation density and intense heterogeneous β-fiber rolling texture are observed, especially at 350 °C, wherein the degree of dynamic recovery (DRV) is higher. Hardness of the accumulative roll bonded samples is smaller than that of the starting material due to particle coarsening, and it is affected by solid solution and/or by fine precipitates produced by reprecipitation of the elements in solid solution.     

Experimental and Theoretical Measurements of the Evolution of Embryos Before and During the Nucleation Stage

The early precipitation of a Cu-Ni-P alloy during aging for 100 ks at 523 K and 623 K (250 °C and 350 °C) after solution treatment has been characterized using a three-dimensional atom probe (3DAP) and transmission electron microscopy (TEM). It is shown that the particles have a wide range of Ni/P ratios when they are relatively small, whereas larger ones exhibit a narrow distribution of the Ni/P ratio, approaching the ratio of approximately two. The threshold radii that show the steady Ni/P ratio are around 1.5 nm and 2.0 nm for the materials aged at 523 K and 623 K (250 °C and 350 °C), respectively. These values are in a reasonably good agreement with the critical nuclei radius estimated from classic nucleation theory. It is suggested that the particles with steady Ni/P ratios of approximately two are considered to be the equilibrium precipitates formed through nucleation, whereas the extremely fine particles with varying Ni/P ratios, detected by the 3DAP experiments, indicate subcritical clusters or embryos.     

A Study on the Combined Treatment of Cryorolling,Short-Annealing,and Aging for the Development of Ultrafine-Grained Al 6063 Alloy with Enhanced Strength and Ductility

High-strength ultrafine-grained (UFG) metals and alloys often show a reduced tensile ductility when compared with their coarse-grained counterparts. The earlier attempts in trying to improve their ductility usually have led to sacrificing its strength. Optimized process conditions are proposed to achieve both high strength and high ductility in the Al 6063 alloy in the current work. It involves solution treatment of the Al 6063 alloy to dissolve the second-phase particles, cryorolling (CR) to produce a high density of dislocations, short annealing (SA) treatment to recrystallize partially the microstructure without affecting the age-hardening effect, and finally aging treatment to generate highly dispersed nano precipitates. The solution treatment prior to CR combined with post-CR SA at 428 K (155 °C) for 5 minutes followed by aging treatment at 398 K (125 °C) for 12 hours are the optimum processing conditions to obtain the UFG microstructure with improved tensile strength (286 MPa) and good tensile ductility (14 pct) in the Al 6063 alloy. It is observed that the accumulation of dislocations and the formation of nanosized precipitates are responsible for improving the strength, whereas both a low dislocation density and a high density of nanosized precipitates contribute to the improvement in ductility of the CR Al 6063 alloy subjected to an optimized treatment of short annealing and aging.     

Mechanical Behavior of Al-SiC Nanocomposites Produced by Ball Milling and Spark Plasma Sintering

Al-SiC nanocomposites were prepared by high energy ball milling of mixtures of pure Al and 50-nm-diameter SiC nanoparticles, followed by spark plasma sintering. The final composites had grains of approximately 100 nm dimensions, with SiC particles located mostly at grain boundaries. The samples were tested in uniaxial compression by nano- and microindentation in order to establish the effect of the SiC volume fraction, stearic acid addition to the powder, and the milling time on the mechanical properties. The results are compared with those obtained for pure Al processed under similar conditions and for AA1050 aluminum. The yield stress of the nanocomposite with 1 vol pct SiC is more than ten times larger than that of AA1050. The largest increase is due to grain size reduction; nanocrystalline Al without SiC and processed by the same method has a yield stress seven times larger than AA1050. Adding 0.5 vol pct SiC increases the yield stress by an additional 47 pct, while the addition of 1 vol pct SiC leads to 50 pct increase relative to the nanocrystalline Al without SiC. Increasing the milling time and adding stearic acid to the powder during milling lead to relatively small increases of the flow stress. The hardness measured in nano- and microindentation experiments confirms these trends, although the numerical values of the gains are different. The stability of the microstructure was tested by annealing samples to 423 K and 523 K (150 °C and 250 °C) for 2 hours, in separate experiments. The heat treatment had no effect on the mechanical properties, except when treating the material with 1 vol pct SiC at 523 K (250 °C), which led to a reduction of the yield stress by 13 pct. The data suggest that the main strengthening mechanism is associated with grain size reduction, while the role of the SiC particles is mostly that of stabilizing the nanograins.     

Spark Plasma Sintering of Nanocrystalline Cu and Cu-10?Wt?Pct Pb Alloy

For the first time, we report here that high purity nanocrystalline Cu and Cu-10 wt pct Pb alloys can be densified with more than 90 pct theoretical density at a low temperature of 623 K (350 °C) using spark plasma sintering (SPS) in argon atmosphere at a pressure of 100 MPa. Scanning electron microscopy (SEM) analysis indicates that molten Pb particles travel through Cu grain boundaries, delineating a “flowlike” pattern in the microstructure. An extensive transmission electron microscopy (TEM) analysis of the ultrafine scale microstructure reveals partial wetting of Cu by liquid Pb as well as entrapment of Pb particles within the Cu matrix. The sintering kinetics and microstructural evolution are discussed in reference to the intrinsic characteristics of SPS as well as phase equilibria in the Cu-Pb system. An important result is that high hardness of around 2 GPa is measured in the Cu-10 wt pct Pb nanostructured alloy, SPS at 573 K to 623 K (300 °C to 350 °C).     

Recovery Phenomenon During Annealing of an As-Rapidly Solidified Al Alloy

It has been well documented that recovery occurring in metals/alloys produced via solid-state quenching involves only annihilation of supersaturated vacancies. Interestingly, in the present study, we observed completely different mechanisms underlying recovery during annealing of an Al-Zn-Mg-Cu (7075 Al) alloy processed via liquid-state quenching, i.e., rapid solidification (specifically melt spinning herein). The as-melt-spun alloy consists of refined grains containing tangled dislocations inside the grains. Following annealing at 393 K (120 °C) for 24 hours, refined grain structure was still retained and grain sizes essentially remained unchanged, but subgrains separated by dense dislocation walls were generated at grain interiors, with a much lower density of dislocations at subgrain interiors than that in the as-melt-spun 7075 Al alloy and dislocation arrays inside some subgrains. The microstructural evolution suggests the absence of recrystallization and the occurrence of recovery primarily via the annihilation and rearrangement of dislocations and the formation of subgrains. Based on the stored energy in dislocations in, and the annealing temperature of, the as-melt-spun 7075 Al alloy, the recovery phenomenon was analyzed and discussed in detail.

     

Effect of Postweld Heat Treatment on the Microstructure and Cyclic Deformation Behavior of Laser-Welded NiTi-Shape Memory Wires

NiTi wires of 0.5 mm diameter were laser welded using a CW 100-W fiber laser in an argon shielding environment with or without postweld heat-treatment (PWHT). The microstructure and the phases present were studied by scanning-electron microscopy (SEM), transmission-electron microscopy (TEM), and X-ray diffractometry (XRD). The phase transformation behavior and the cyclic stress–strain behavior of the NiTi weldments were studied using differential scanning calorimetry (DSC) and cyclic tensile testing. TEM and XRD analyses reveal the presence of Ni₄Ti₃ particles after PWHT at or above 623 K (350 °C). In the cyclic tensile test, PWHT at 623 K (350 °C) improves the cyclic deformation behavior of the weldment by reducing the accumulated residual strain, whereas PWHT at 723 K (450 °C) provides no benefit to the cyclic deformation behavior. Welding also reduces the tensile strength and fracture elongation of NiTi wires, but the deterioration could be alleviated by PWHT.     

Oxidation Behavior of a Pd<Subscript>43</Subscript>Cu<Subscript>27</Subscript>Ni<Subscript>10</Subscript>P<Subscript>20</Subscript> Bulk Metallic Glass and Foam in Dry Air

The oxidation behavior of both Pd₄₃Cu₂₇Ni₁₀P₂₀ bulk metallic glass (Pd4-BMG) and its amorphous foam containing 45 pct porosity (Pd4-AF) was investigated over the temperature range of 343 K (70 °C) to 623 K (350 °C) in dry air. The results showed that virtually no oxidation occurred in the Pd4-BMG at T < 523 K (250 °C), revealing the alloy’s favorable oxidation resistance in this temperature range. In addition, the oxidation kinetics at T ≥ 523 K (250 °C) followed a parabolic-rate law, and the parabolic-rate constants (k _p values) generally increased with temperature. It was found that the oxidation k _p values of the Pd4-AF are slightly lower than those of the Pd4-BMG, indicating that the porous structure contributes to improving the overall oxidation resistance. The scale formed on the alloys was composed exclusively of CuO at T ≥ 548 K (275 °C), whose thickness gradually increased with increasing temperature. In addition, the amorphous structure remained unchanged at T ≤ 548 K (275 °C), while a triplex-phase structure developed after the oxidation at higher temperatures, consisting of Pd₂Ni₂P, Cu₃P, and Pd₃P.     

Thermal-Treated Pitches as Binders for TiB2/C Composite Cathodes

The effect of thermal treatment on the microstructure and properties of pitches and thermal-treated, pitch-based TiB₂/C composite cathodes were investigated. Thermal treatments were performed at 473 K, 523 K, 573 K, 623 K, and 673 K (200 °C, 250 °C, 300 °C, 350 °C, and 400 °C), respectively. The results show that the aromaticity of the treated pitches increases with an increasing thermal treatment temperature, and subsequently, the coking value and quinoline-insoluble (QI) content increase from 60.62 wt pct to 79.09 wt. pct and from 8.97 wt pct to 32.54 wt pct when the treatment temperature increases from 473 K to 623 K (200 °C to 350 °C). The volume fraction of coalesced mesophase in semicoke decreases with an increasing thermal treatment temperature, and after 673 K (400 °C) is reached, the coalesced mesophase is almost invisible. The bulk density and compressive strength of modified pitch-based cathodes increase with an increasing thermal treatment temperature from 2.24 g cm⁻³ to 2.39 g cm⁻³ and from 24.21 MPa to 54.85 MPa, whereas open porosity decreases from 34.62 pct to 27.06 pct. Both electrical resistivity and electrolysis expansion ratio first decrease and then increase with an increasing thermal treatment temperature, and the lowest values (45.63 μΩ m and 0.65 pct) are achieved at 573 K (300 °C). Compared with those of the parent pitch-based cathode, the properties of the modified pitch-based cathodes had improved significantly. The mechanisms of the improvements are discussed in the text.     

Effect of Annealing on Hardness and the Modulus of Elasticity in Bulk Nanocrystalline Nickel

Experiments on hardness and the modulus of elasticity were conducted at room temperature on samples of electrodeposited (ED) nanocrystalline (nc) Ni that were annealed at temperatures ranging from 323 to 693 K (50 to 420 °C). The results showed the presence of three regions: I, II, and III. In region I (300 K (27 °C) < T < 350 K (77 °C)), the hardness and the elastic modulus remained essentially constant. In region II (350 K (77 °C) < T < 500 K (227 °C)), both the hardness and the elastic modulus increased. In region III (T > 500 K (227 °C)), the hardness dropped and then decreased with increasing grain size, whereas the modulus of elasticity approached a maximum plateau of ~240 GPa. It is suggested that while the increase in hardness in region II can be attributed in part to the formation of annealing twins, which serve as a source of strengthening, the decrease in hardness above 500 K (227 °C) is due to the occurrence of significant grain growth. The increase in the modulus of elasticity with increasing temperature in region II was attributed to the preferred orientation along (200) that was observed in the as-received samples and that continuously diminished with increasing temperature. In region III (T > 500 K (227 °C)), the preferred orientation disappeared and, a result, the modulus of elasticity approached a constant value of approximately 240 GPa.     

Mechanical Behaviors of Ultrafine-Grained 301 Austenitic Stainless Steel Produced by Equal-Channel Angular Pressing

The technique of equal-channel angular pressing (ECAP) was used to refine the microstructure of an AISI 301 austenitic stainless steel (SS). An ultrafine-grained (UFG) microstructure consisting mainly of austenite and a few martensite was achieved in 301 steel after ECAP processing for four passes at 523 K (250 °C). By submitting the as-ECAP rods to annealing treatment in the temperature range from 853 K to 893 K (580 °C to 620 °C) for 60 minutes, fully austenitic microstructures with grain sizes of 210 to 310 nm were obtained. The uniaxial tensile tests indicated that UFG 301 austenitic SS had an excellent combination of high yield strength (>1.0 GPa) and high elongation-to-fracture (>30 pct). The tensile stress–strain curves exhibited distinct yielding peak followed by obvious Lüders deformation. Measurements showed that Lüders elongation increased with an increase in strength as well as a decrease in grain size. The microstructural changes in ultrafine austenite grains during tensile deformation were tracked by X-ray diffraction and transmission electron microscope. It was found that the strain-induced phase transformation from austenite to martensite took place soon after plastic deformation. The transformation rate with strain and the maximum strain-induced martensite were promoted significantly by ultrafine austenite grains. The enhanced martensitic transformation provided extra strain-hardening ability to sustain the propagation of Lüders bands and large uniform plastic deformation. During tensile deformation, the Lüders bands and martensitic transformation interacted with each other and made great contribution to the excellent mechanical properties in UFG austenitic SS.     

Equal-channel angular pressing of commercial aluminum alloys: Grain refinement, thermal stability and tensile properties

Using equal-channel angular (ECA) pressing at room temperature, the grain sizes of six different commercial aluminum-based alloys (1100, 2024, 3004, 5083, 6061, and 7075) were reduced to within the submicrometer range. These grains were reasonably stable up to annealing temperatures of ∼200 °C and the submicrometer grains were retained in the 2024 and 7075 alloys to annealing temperatures of 300 °C. Tensile testing after ECA pressing through a single pass, equivalent to the introduction of a strain of ∼1, showed there is a significant increase in the values of the 0.2 pct proof stress and the ultimate tensile stress (UTS) for each alloy with a corresponding reduction in the elongations to failure. It is demonstrated that the magnitudes of these stresses scale with the square root of the Mg content in each alloy. Similar values for the proof stresses and the UTS were attained at the same equivalent strains in samples subjected to cold rolling, but the elongations to failure were higher after ECA pressing to equivalent strains >1 because of the introduction of a very small grain size. Detailed results for the 1100 and 3004 alloys show good agreement with the standard Hall-Petch relationship.     

Microstructure,Tensile Properties,and Hot-Working Characteristics of a Hot Isostatic-Pressed Powder Metallurgy Superalloy

A series of microstructure observation, tensile, and hot compression tests were conducted to investigate the variation of microstructure, tensile properties, and hot-working characteristics of a powder metallurgy (PM) superalloy with hot isostatic pressing (HIPing) temperature, to establish a basis for the parameter selection for PM superalloy preparation. The results show that the dendritic structure from the powder was not completely removed until the HIPing temperature is above the γ′ solvus; γ/γ′ eutectic formed when the powder particles were HIPed at 1533 K (1260 °C) or above. Prior particle boundaries (PPBs) were observed in alloys HIPed at 1513 K (1240 °C) and below; the PPB decoration is serious in alloys HIPed at 1483 K and 1513 K (1210 °C and 1240 °C), owing to melting and aggregation of the boride phase at the particle boundaries during HIPing; the PPBs were eliminated when the HIPing was done at 1533 K (1260 °C) or above. Tensile fracture mode of the alloy changes from inter-particle and transgranular mixed fracture to transgranular fracture with increasing HIPing temperature, which is in accordance with the change in precipitate distribution at the PPBs. The hot workability of alloy is poor for all combinations of HIPing/deformation conditions except for HIPing at sub-solvus temperature and deformation at low strain rates.

     

Diffusion of Zn in Nanostructured Aluminum Alloys Produced by Surface Mechanical Attrition Treatment

After surface nanocrystallization of pure Al and a cast Al-Si alloy through surface mechanical attrition treatment (SMAT), 200- to 300-??m-thick Zn coatings were deposited on the nanostructured surface using the clod spray technique. Subsequently, diffusion of Zn into the Al substrate was induced by postspray annealing treatment at various temperatures for different times. The diffusion kinetics of Zn in the nanostructured surface layers was studied in terms of the Zn concentration profile in the substrate by using scanning electron microscopy (SEM) and electron probe microscopy analysis (EPMA). Experimental results show that not only the diffusivity of Zn in the nanocrystalline grains is significantly increased compared with the diffusion in the coarse grained counterpart, but the temperature at which noticeable Zn diffusion in Al alloys occurs is also reduced from 573?K (300?°C) in coarse-grained Al alloys to 523?K (250?°C) in nanostructured alloys. In addition, because the nanocrystalline grains produced by SMAT in Al-Si alloys are much smaller than those in pure Al due to the effect of eutectic Si, the diffusion of Zn in the SMATed Al-Si alloy is much faster than that in the SMATed pure Al. It is believed that the high diffusivity of Zn in the nanocrystalline Al grains is attributed to the large fraction of grain boundaries that act as fast diffusion channel. The effect of thermal stability of the nanocrystalline grains on Zn diffusion in the SMATed Al alloys is also discussed.     

Microstructure,Mechanical and Abrasive Wear Behavior of 8.0 wt pct Cr White Iron Subjected to Continuous and Cyclic Annealing Treatment

Continuous annealing treatment (austenitization for 4 hours followed by furnace cooling) and cyclic annealing treatment (four cycles of austenitization, each of 0.66 hours duration followed by forced air cooling) of 8.0 wt pct Cr white iron samples are undertaken at 1173 K, 1223 K, 1273 K, 1323 K, and 1373 K (900 °C, 950 °C, 1000 °C, 1050 °C, and 1100 °C) as steps of destabilizing the as-cast structure. Continuous annealing results in precipitation of secondary carbides on a matrix containing mainly pearlite, while cyclic annealing treatment causes similar precipitation of secondary carbides on a matrix containing martensite plus retained austenite. On continuous annealing, the hardness falls below the as-cast value (HV 556), while after cyclic annealing treatment there is about 70 pct increase in hardness, i.e., up to HV 960. Decrease in hardness with increasing annealing temperature is quite common after both heat treatments. The as-cast notched impact toughness (4.0 J) is nearly doubled by increasing to 7.0 J after both continuous and cyclic annealing treatment at 1173 K and 1223 K (900 °C and 950 °C). Cyclic annealing treatment gives rise to a maximum notched impact toughness of 10.0 J at 1373 K (1100 °C). Abrasive wear resistance after continuous annealing treatment degrades exhibiting wear loss greater than that of the as-cast alloy. In contrast, samples with cyclic annealing treatment show reasonably good wear resistance, thereby superseding the wear performance of Ni-Hard IV.

     

Three-Dimensional Network Structure and Mechanical Properties of Al-Cu-Ni-Fe Cast Alloys with Gd Micro-addition

High-temperature tensile testing and X-ray microscope (XRM) characterization were performed to assess the effects of the micro-addition of Gd and the three-dimensional (3-D) network structure on the improvement of Al-6Cu-3.5Ni-0.8Fe alloy under as-cast conditions. Gd addition contributed to the modification of the microstructure, where a new thermally stable micro-sized Al₃CuGd phase was formed, and the refinement occurred in Al₃CuNi and Al₉FeNi. The tensile test results revealed that the alloy modified with 0.4 pct Gd exhibited optimal properties at 623 K (350 °C), with an ultimate tensile strength, yield strength, and elongation of 74.1 MPa, 61.2 MPa, and 15.5 pct, respectively. Fractographic analysis after the tensile tests indicated that at ambient temperature, brittle cleavage-type fracture of the precipitates and ductile fracture of the matrix were dominant, whereas the transformation from mixed fracture to fully ductile trans-crystalline fracture was detected at elevated temperatures. According to the CT characterization, there was no significant difference in the curvature or interconnectivity of the 3-D network structure formed by the aluminides between before and after the tensile test at 623 K (350 °C). It is believed that the 3-D continuous network structure of aluminides, equipped with excellent heat resistance, plays a pivotal role in the high-temperature performance of the studied alloys. This work provides a new and promising idea for solving the current heat resistance problems of cast Al alloys.

     

Effects of boron doping on the grain-growth kinetics and mechanical properties of γ/γ′ nickel-aluminum alloys

The effects of 0.5 at. pct of boron doping on the microstructures and mechanical properties of γ/γ′ nickel-aluminum alloys have been investigated in the present study. A nickel-rich grain-boundary zone was observed in the boron-doped alloy after homogenization at 1100 °C and prolonged annealing at 1200 °C. Boron doping also caused remarkable improvements in toughness and tensile elongation and caused the fracture mode to change from completely intergranular to completely transgranular. The grain growth following recrystallization at 1200 °C was found to be retarded upon boron doping. A sudden increase in tensile elongation and a sudden drop in hardness were also observed upon prolonged heating during isothermal annealing at 1200 °C. The results are interpreted with reference to boron-nickel cosegregation at the grain boundaries.     

Effect of Al and Gd Solutes on the Strain Rate Sensitivity of Magnesium Alloys

Pure magnesium and two binary alloys, Mg-1 wt pct Al and Mg-1.4 wt pct Gd, have been prepared with comparable grain sizes and textures. The alloys have been tensile tested at various strain rates and temperatures to examine the strain rate sensitivity (SRS). It has been found that Mg and Mg-Al show increasing SRS with increasing deformation temperatures. The Mg-Gd alloy showed decreasing SRS with increasing deformation temperatures and exhibited a negative SRS at 200 °C and 250 °C. Above these temperatures, the SRS returned to a positive value. The elongation to fracture was not effected by the SRS, and it has been concluded that for the alloys and conditions examined, the influences of mechanical twinning and dynamic recrystallization dominate the elongation behavior, rather than the SRS.