Consolidation of Carbon Nanotube Reinforced Aluminum Matrix Composites by High-Pressure Torsion

Al-3 vol pct carbon nanotube (CNT) composites are fabricated by consolidation through high-pressure torsion (HPT) at room temperature. The densification behavior, microstructural evolution, and mechanical properties of Al/CNT composites are studied. The results show that density and microstructural homogeneity increase with increasing number of revolutions under a high pressure of 6 GPa. Substantial grain refinement is achieved after 10 turns of HPT with an average grain thickness of ~38 nm perpendicular to the compression axis of HPT. The Al/CNT composite shows a considerable increase in hardness and strength compared to the Al matrix. The strengthening mechanisms of the Al/CNT composite are found to be (i) grain refinement of Al matrix and (ii) Orowan looping. Raman spectroscopy and high-resolution transmission electron microscopy reveal that the structure of most of CNTs is changed during processing through mechanical milling and HPT.     

Microstructure Evolution and Mechanical Properties of Al-1080 Processed by a Combination of Equal Channel Angular Pressing and High Pressure Torsion

Equal channel angular pressing (ECAP) and high pressure torsion (HPT) are the most promising severe plastic deformation (SPD) methods. Both methods impose very high strains, leading to extreme work hardening and microstructural refinement. In this paper, billets of Al-1080 were successfully processed by ECAP conducted for up to 10 passes, HPT at an applied pressure of 8 GPa for 5 revolutions, and a combination of ECAP and HPT (ECAP + HPT) at room temperature. The effects of the different SPD processes (ECAP, HPT, and ECAP + HPT) on the evolution of the microstructure and mechanical properties of Al-1080 were investigated. The HPT and ECAP + HPT processes were observed to produce finer grain sizes with greater fractions of high angle grain boundaries (HAGBs) than the ECAP alone. Although the grain sizes after HPT and ECAP + HPT were similar, the ECAP + HPT sample had more dislocations than the HPT sample. HPT after ECAP enhanced the mechanical properties (hardness, tensile strength, and ductility) of the ECAP-processed Al-1080, showing larger dimple size in the tensile fracture surfaces.     

Nitrided 08Kh17T steel after high-pressure torsion

The effect of severe plastic deformation by high-pressure torsion (HPT) on the microstructure, the phase composition, the microhardness, and the thermal stability of 08Kh17T steel preliminarily subjected to high-temperature bulk nitriding followed by annealing is studied. Nitriding is performed in a pure nitrogen atmosphere at 1075°C. HPT is found to cause the formation of a homogeneous nanostructure with a grain size of 55–85 nm. The microhardness of the steel after HPT increases by a factor of 2.2–2.7, to HV 780–860 depending on nitriding conditions. Hardening is retained when the material is heated to 450°C.     

Enhancement of Strength and Ductility of Al-Ag Alloys Processed by High-Pressure Torsion and Aging

This research was conducted by the application of high-pressure torsion (HPT) to Al-X wt pct Ag alloys (X = 5, 11, 20). Grain refinement was achieved to the size of ~300 nm after HPT processing at room temperature. The aging behavior of the alloys after HPT processing was investigated using Vickers microhardness measurement, tensile testing, scanning electron microscopy, and transmission electron microscopy. This study confirms the dual effect of grain refinement and fine precipitation on the enhancement of the strength. It is also shown that at peak-aged condition, the tensile strength is enhanced while maintaining considerable ductility.     

Mechanical Properties and Microstructures of Al-Fe Alloys Processed by High-Pressure Torsion

Al-Fe alloys in the form of thin disks 10?mm in diameter, with Fe nominal weight fractions of 0.5, 1, 2, and 5?pct, were extracted from bulk-extruded rods and processed by high-pressure torsion (HPT). A group of these bulk samples was processed in an as-received state, whereas another group was annealed at 773?K (500?°C) for a period of 1?hour, prior to the application of HPT. An additional set of samples was prepared by mixing high-purity powders with similar Fe contents and consolidated directly in the HPT facility. The samples were processed up to 10?revolutions. Vickers microhardness, tensile strength, and elongation to failure were evaluated for all cases along with observations of the Al matrix by transmission electron microscopy (TEM). Basic characterization of the microstructures was carried out by X-ray diffraction (XRD) and optical microscopy (OM). Significant strengthening with ductility retained was achieved in the bulk samples as a consequence of grain refinement and dispersion of intermetallic phases. Powder samples had a more gradual increase in strength but increased ductility as a result of the imposed strain.     

Microstructures and Properties of Nanocrystalline NiFe Alloy With and Without Particulate TiC Reinforcement by Mechanical Alloying

In the current study, Ni₅₀Fe₅₀ alloy powders were prepared using a high-energy planetary ball mill. The effects of TiC addition (0, 5, 10, 20, and 30 wt pct) and milling time on the sequence of alloy formation, the microstructure, and microhardness of the product were studied. The structure of solid solution phase, the lattice parameter, lattice strain, and grain size were identified by X-ray diffraction analysis. The correlation between the apparent densities and the milling time is explained by the morphologic evolution of the powder particles occurring during the high-energy milling process. The powder morphology was examined using scanning electron microscopy. It was found that FCC γ (Fe–Ni) solid solution was formed after 10 hours of milling, and this time was reduced to 7 hours when TiC was added. Therefore, brittle particles (TiC) accelerate the milling process by increasing crystal defects leading to a shorter diffusion path. Observations of polished cross section showed uniform distribution of the reinforcement particles. The apparent density increases with the increasing TiC content. It was also found that the higher TiC amount leads to larger lattice parameter, higher internal strain, and lower grain size of the alloy.     

Deformation Heterogeneity on the Cross-Sectional Planes of a Magnesium Alloy Processed by High-Pressure Torsion

Disks of an extruded AZ31 magnesium alloy were processed by high-pressure torsion (HPT) at a pressure of 6.0 GPa through 1/4, 1, and 5 turns either at room temperature (296 K (23 °C)) or at an elevated temperature of 463 K (190 °C). The cross-sectional planes of the disks were examined after processing, and it is shown that there is a high level of heterogeneity throughout all of the samples. This heterogeneity is revealed through the nature of the flow patterns, through the distributions of grain sizes, and by comprehensive microhardness measurements. The results demonstrate that it is possible to double the strength of the alloy in some areas of the disk after processing through 5 turns at room temperature.     

Effect of Partial Substitution of Mn for Ni on Mechanical Properties of Friction Stir Processed Hypoeutectic Al-Ni Alloys

In this study, the mechanical properties of as-cast and FSPed Al-2Ni-xMn alloys (x?=?1, 2, and 4 wt pct) were investigated and compared with those of the as-cast and FSPed Al-4Ni alloy. According to the results, the substitution of 2 wt pct Mn for 2 wt pct Ni leads to the formation of fine Mn-rich intermetallics in the microstructure increasing the tensile strength, microhardness, fracture toughness, and specific strength of alloy by 22, 56, 45, and 35 pct, respectively. At higher Mn concentrations, the formation of large Mn-rich platelets in the microstructure reduces the tensile properties. Friction stir processing at 12 mm/min and 1600 rpm significantly enhances both the strength and ductility of the alloy. The tensile strength, yield strength, fracture strain, fracture toughness, microhardness, and specific strength of FSPed Al-2Ni-4Mn alloy improved by 97, 83, 30, 380, 152, and 110  pct, respectively, as compared to those of the as-cast Al-4Ni alloy. This can be attributed to dispersion strengthening of Ni- and Mn-rich dispersoids, formation of ultrafine grains, and elimination of casting defects. The fractography results also show that the brittle fracture mode of the as-cast Mn-rich alloys turns to a more ductile mode, comprising fine and equiaxed dimples in FSPed samples.     

High Strain Rate Compression of Martensitic NiTi Shape Memory Alloy at Different Temperatures

The compressive response of martensitic NiTi shape memory alloy (SMA) rods has been investigated using a modified Kolsky compression bar at various strain rates (400, 800, and 1200 s^?1) and temperatures [room temperature and 373 K (100 °C)], i.e., in the martensitic state and in the austenitic state. SEM, DSC, and XRD were performed on NiTi SMA rod samples after high strain rate compression in order to reveal the influence of strain rate and temperature on the microstructural evolution, phase transformation, and crystal structure. It is found that at room temperature, the critical stress increases slightly as strain rate increases, whereas the strain-hardening rate decreases. However, the critical stress under high strain rate compression at 373 K (100 °C) increase first and then decrease due to competing strain hardening and thermal softening effects. After high rate compression, the microstructure of both martensitic and austenitic NiTi SMAs changes as a function of increasing strain rate, while the phase transformation after deformation is independent of the strain rate at room temperature and 373 K (100 °C). The preferred crystal plane of the martensitic NiTi SMA changes from (\( 1\bar{1}1 \))_M before compression to (111)_M after compression, while the preferred plane remains the same for austenitic NiTi SMA before and after compression. Additionally, dynamic recovery and recrystallization are also observed to occur after deformation of the austenitic NiTi SMA at 373 K (100 °C). The findings presented here extend the basic understanding of the deformation behavior of NiTi SMAs and its relation to microstructure, phase transformation, and crystal structure, especially at high strain rates.     

Effects of the Process Parameters on the Microstructure and Properties of Nitrided 17-4PH Stainless Steel

The effects of process parameters on the microstructure, microhardness, and dry-sliding wear behavior of plasma nitrided 17-4PH stainless steel were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and wear testing. The results show that a wear-resistant nitrided layer was formed on the surface of direct current plasma nitrided 17-4PH martensitic stainless steel. The microstructure and thickness of the nitrided layer is dependent on the treatment temperature rather than process pressure. XRD indicated that a single α _N phase was formed during nitriding at 623 K (350 °C). When the temperature increased, the α _N phase disappeared and CrN transformed in the nitrided layer. The hardness measurement demonstrated that the hardness of the stainless substrate steel increased from 320 HV_0.1 in the untreated condition increasing to about 1275HV_0.1 after nitriding 623 K (350 °C)/600 pa/4 hours. The extremely high values of the microhardness achieved by the great misfit-induced stress fields associated with the plenty of dislocation group and stacking fault. Dry-sliding wear resistance was improved by DC plasma nitriding. The best wear-resistance performance of a nitrided sample was obtained after nitriding at 673 K (350 °C), when the single α _N-phase was produced and there were no CrN precipitates in the nitrided layer.     

Effects of High Strain Rate on Properties and Microstructure Evolution of TWIP Steel Subjected to Impact Loading

 The mechanical properties of the TWIP steel subjected to impact loading at various strain rates were analyzed by the Split Pressure Hopkinson Bar. Meanwhile the microstructure of the TWIP steel fore-and-after the dynamic deformation were characterized and analyzed by optical microscopy (OM), X-ray diffraction (XRD), and transmission electron microscope (TEM). The result shows that when the TWIP steel was deformed under dynamic station, the stress, microshardness and work hardening rate increase with the increment of strain and strain rate; there exist stress fluctuation and decline of work hardening rate for adiabatic temperature rising softening. There exist many pin-like deformation twins in the microstructure of the TWIP steel subjected to impact loading, the grain size after deformation is bigger than that before; the interaction of twins with dislocation and twins with twins, especial emergence of high order deformation twins are the main strengthening mechanisms of the TWIP steel. The nucleation mechanism of deformation twins will be “rebound mechanism”; the incomplete deformation twins can be observed when the strain rate is low; when strain rate raises, deformation twins unite together; furthermore, deformation twins become denser because the nucleation rating enhancing with strain rate increasing.     

Strengthening via the Formation of Strain-Induced Martensite and the Effects of Laser Marking on the Microstructure of Austenitic Stainless Steel

The deformation behavior and microstructure characteristics of 304L stainless steel during strip rolling and bar extrusion at different strains and temperatures, from room to liquid-nitrogen temperature, were investigated with Vickers hardness, light microscopy, and electron-backscatter-diffraction. The relative volume fractions of transformed martensite at different stages of the deformation process were assessed using Ferritescope MP-30. It was found that during rolling and extrusion the relative volume fraction of martensite increases with increasing strain and decreasing temperature. According to the enhancement of the mechanical and magnetic properties after isothermal treatment at 673 K (400 °C), it is assumed that both, ε-martensite and α′-martensite, are present in the deformation microstructure, indicating the simultaneous stress-induced transformation and strain-induced transformation of austenite. The effects of the laser surface treatment and the local appearance of a non-magnetic phase due to the α′ → γ transformation after the laser surface treatment were also investigated.     

Cold deformation behavior of graphitized carbon steel

The cold compression test of a strain rate 1s-1 for graphitized carbon steel containing C of 0. 43 mass% with ferrite and graphite was carried out using Gleeble- 3500 thermal simulation machine, the characteristics of the stress- strain relationship was analyzed, and metallographic analysis in the large deformation zone of the compressed samples with different reduction was investigated by optical microscope, field emission scanning electron microscopy and micro hardness tester. The results show that there exists peak stress on the stress- strain curve, namely, 645MPa, the corresponding peak strain is 0. 43; in process of the compressive deformation, morphologies of ferrite and graphite in the large deformation zone on the longitudinal section of the compressed samples gradually become fibrous with increasing the reduction; thereinto, the deformation of graphite particles is realized by means of sliding deformation between basic planes of graphite, shear deformation between substructures of graphite particles, as well as elongation of compacted section near the base of the ferrite of the deformed graphite particles; microhardness of ferrite increases with increasing the reduction, it indicate that ferrite is in a state of hardening, but, increase amplitude of microhardness in the process of compressive deformation after peak stress is decreased. Therefore, based on increase amplitude of microhardness of ferrite and microstructure of the deformed graphite particles, it can be concluded that in process of the compressive deformation after peak stress, deformation of graphite particles in the large deformation zone plays a major role, a deformation of ferrite plays a secondary role (it is mainly to coordinate the deformation of graphite particles), this is one of the main reason that stress decreases in the compressive deformation after peak stress.     

Microstructure and Mechanical Properties of Al Tube Processed by Friction Stir Tube Back Extrusion (FSTBE)

Friction stir tube back extrusion is presented as a method to produce fine-grained tubes based on friction stir back extrusion process. In this method, a tubular aluminium alloy specimen whose inner diameter is less than the diameter of the rotating tool, is placed on a cylindrical die and penetrated by a rotating tool. Mechanical properties and microstructural evaluation of the processed tube have been investigated. The optical micrographs of the microstructure demonstrate strain gradient along the tube wall. Reduction in grain size and grain refinement occurs across the inner wall of the formed tube as well. Hence, the grain size with an initial value of ~100 µm is decreased to ~39 µm in the inner side of the formed tube wall. The obtained results from tensile tests illustrate that the ultimate strength and elongation at failure increases from the initial value of 183–225 MPa and 19–22 %, respectively. Microhardness assessments represent an increase in initial value from 74 to 86 Hv. Moreover, the microhardness values of the inner surface of the formed tube are more than the outer surface. The results show that this method is suitable for fabricating a kind of ultrafine-grained tube which possess an acceptable combination of strength and ductility.     

Microstructure Evolution During Hot Deformation of a Micro-Alloyed Steel

In the present investigation, hot deformation by uniaxial compression of a microalloyed steel has been carried out, using a deformation dilatometer, after homogenization at 1200 °C for 20 min up to strains of 0.4, 0.8 and 1.2 at different temperatures of 900, 1000 and 1100 °C, at a constant strain rate of 2 s^?1 followed by water quenching. In all the deformation conditions, initiation of dynamic recrystallization (DRX) is observed, however, stress peaks are not observed in the specimens deformed at 900 and 1000 °C. The specimens deformed at 900 °C showed a combination of acicular ferrite (AF) and bainite (B) microstructure. There is an increase in the acicular ferrite fraction with increase in strain at all these deformation temperatures. At high deformation temperature of 1100 °C, coarsening of DRXed grains is observed. This is attributed to the common limitations involved in fast quenching of the DRXed microstructure, which leads to increase in grain size by metadynamic recrystallization (MDRX). The strain free prior austenite grains promote the formation of large fraction of both bainite and martensite in the transformed microstructures during cooling. The length and width of bainitic ferrite laths also increases with increase in deformation temperature from 900 to 1100 °C and decrease in deformation strain.     

Study of Dissimilar Header Welding Between 2.25Cr–1Mo Steel and 9Cr–1Mo Steel with 9018 B9 Electrode Under Various Conditions of Post Weld Heat Treatment

Headers are an integral part of the power plant equipment which serves as junction for receiving and distribution of fluid. Headers are routinely used in high temperature applications in which various combinations of steels are used to achieve weight and cost savings thus optimising the use of steels. This paper intends in studying the evolution of microhardness and microstructure in a dissimilar header fillet welding between the base materials 2.25Cr–1Mo steel and 9Cr–1Mo steel when welded using 9018 B9 electrode and a constant preheat of 220 °C. The post weld heat treatment (PWHT) is varied at temperatures from room temperature to 770 °C and the soaking duration is kept at 1 h. The changes in microstructure and microhardness are examined with the help of micrographs, electron dispersive spectrum and scanning electron microscopy analysis. As the PWHT temperatures changes, the variation in microstructure and microhardness becomes very much evident which is detailed out in this paper. Also, carbon migration phenomena and its relation with the PWHT temperatures has been studied in this paper.     

An Investigation of Parameters Involved and Defects in the Fabrication of Al–SiC Nanocomposite Using Hot Extrusion Technique

In the present work, some parameters including temperature, pressure, boundary conditions (lubricant and friction), and also defects during hot extrusion technique (HET) of specimens were studied and their reasons were analyzed. The nanocomposite powders were prepared by a powder metallurgy route consisting of mechanical milling, cold pressing, and HET. Micron-sized Al with different amounts of SiC nanoparticles, 0, 1.5, and 3 vol%, were used to fabricate the specimens. The physical and mechanical properties of the extruded samples such as density, microhardness, tensile strength, and also the microstructure of the materials were evaluated. It was found that by increasing the nanoparticle contents, microhardness and tensile strength increased and ductility declined. The right and appropriate design of die, using foil or aluminum cans, proper lubricant, and extrusion rate were important parameters to be controlled to obtain minimum defects. The results showed that the temperature of 550 °C was more appropriate towards achieving superior tensile strength than at 500 °C and better surface finish than at 500 and/or 600 °C.     

Mechanical milling of gas-atomized Al-Ni-Mm (Mm = misch metal) alloy powders

Al-14Ni-14Mm (Mm = misch metal) alloy powders rapidly solidified by the gas atomization method were subjected to mechanical milling (MM). The microstructure, hardness, and thermal stability of the powders were investigated as a function of milling time using X-ray diffraction (XRD), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC) methods. In the early stages of milling, a cold-welded layer with a fine microstructure formed along the edge of the milled powder (zone A). The interior of the powder remained unworked (zone B), resulting in a two-zone microstructure, reminiscent of the microstructures in rapidly solidified ribbons containing zones A and B. With increasing milling time, the crystallite size decreased gradually reaching a size of about 10 to 15 nm and the lattice strain increased reaching a maximum value of about 0.7 pct for a milling time of 200 hours. The microhardness of the mechanically milled powder was 132 kg/mm² after milling for 72 hours and it increased to 290 kg/mm² after milling for 200 hours. This increase in microhardness is attributed to a significant refinement of microstructure, presence of lattice strain, and presence of a mixture of phases in the alloy. Details of the microstructural development as a function of milling time and its effect on the microhardness of the alloy are discussed.     

Age Hardening Behavior and Corresponding Microstructure of Dilute Al-Er-Zr Alloys

The age hardening and the microstructure of dilute Al-Er-Zr alloys were investigated by microhardness tests and TEM. The Al-0.04Er alloy shows a conventional age hardening behavior and obtains a maximum hardness of 410 MPa after aging for 2 h at 523 K (250 °C) due to precipitation of Al₃Er. The addition of Zr to Al-Er alloy can slow down the growth of the precipitates and make the age hardening effect remain for a long time in Al-0.04Er-0.04Zr alloy. Addition of Zr retards the decomposition of Al-Er and the Al-0.04Er-0.08Zr alloy can reach higher peak hardness than that of Al-0.04Er after aging for long time at elevated temperature. The precipitation behavior of Al-Er-Zr system is likely to be a new commercial way to developing creep-resistant aluminum alloy.     

Mechanical milling of gas-atomized Al−Ni−Mm (Mm=misch metal) alloy powders

Al−14Ni−14Mm (Mm=misch metal) alloy powders rapidly solidified by the gas atomization method were subjected to mechanical milling (MM). The microstructure, hardness, and thermal stability of the powders were investigated as a function of milling time using X-ray diffraction (XRD), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC) methods. In the early stages of milling, a cold-welded layer with a fine microstructure formed along the edge of the milled powder (zone A). The interior of the powder remained unworked (zone B), resulting in a two-zone microstructure, reminiscent of the microstructures in rapidly solidified ribbons containing zones A and B. With increasing milling time, the crystallite size decreased gradually reaching a size of about 10 to 15 nm and the lattice strain increased reaching a maximum value of about 0.7 pct for a milling time of 200 hours. The microhardness of the mechanically milled powder was 132 kg/mm² after milling for 72 hours and it increased to 290 kg/mm² after milling for 200 hours. This increase in microhardness is attributed to a significant refinement of mcirostructure, presence of lattice strain, and presence of a mixture of phases in the alloy. Details of the microstructural development as a function of milling time and its effect on the microhardness of the alloy are discussed.