Drop In Yield Strength Research Articles

Additive friction stir deposition of Al 6061-B4C composites: Process parameters, microstructure and property correlation

Al 6061-B4C metal matrix composites were fabricated using a novel solid state additive manufacturing technique of additive friction stir deposition (AFSD). Incorporation of B4C particles into the deposit was achieved by employing hollow Al 6061 feed stock filled with B4C powder and three different AFSD tool rotational speeds (200, 300 and 400 rpm). Fabricated deposits exhibited remarkable metallurgical bonding across the substrate/deposit interface and interlayer interfaces for all tool rotational speeds, indicating a wider fabrication window. A physics based pseudo-thermo mechanical model was employed to evaluate the thermo-kinetic conditions experienced by the depositing material to correlate with the microstructural observations. Deposited layers exhibited refinement of grains to ∼10 μm from the corresponding coarse grain structure of feedstock (136 μm). The regions near the B4C particles were observed to have finer grains (<2 μm) signifying particle stimulated nucleation. Although considerable loss in the tensile properties (60 % drop in yield strength) was observed in the AFSD deposits due to the associated dissolution of β’’ precipitates, subsequent heat treatment restored the tensile properties to the level of as received material (∼290 MPa). The thermo-kinetic conditions (temperature in excess of 500 °C and strain rates in excess of 250 s−1) experienced by the builds under higher AFSD tool rotational speeds (300 and 400 rpm) appeared to result in refinement of Al–Fe–Si based secondary intermetallic precipitates/dispersoids leading to the observed higher stability of their microstructure during the subsequent post AFSD heat treatment. Samples AFSD fabricated with a lower tool rotational speed of 200 rpm, however, exhibited significant grain growth on account of relatively larger secondary precipitates/dispersoids.

Thermal stability of dislocation structure and its effect on creep property in austenitic 316L stainless steel manufactured by directed energy deposition

The objective of this study is to investigate the thermal stability of dislocation structure and its effect on the creep behaviour of laser-directed energy deposited 316L stainless steel (L-DED-316L SS). Post-processing heat treatments at temperatures ranging from 300 to 1200 °C were performed on the as-deposited DED samples. The microstructural changes induced by the heat treatment were correlated to the corresponding variations of the room temperature tensile properties and creep behaviour at 650 °C/225 MPa. Results show that dislocations produced during DED process tend to distribute uniformly, with only a few localized fine dislocation cells (average cell size of ∼0.4 μm) being detected. At 600 °C, the remaining dislocations rearrange and organize into a coarse dislocation cell structure with an average cell size of ∼1.6 μm, leading to a slight decrease in yield strength, while the creep performance is not obviously affected. At 800 °C, the annihilation of dislocations and destruction of dislocation cell structure, as well as elemental diffusion contribute to a significant drop in yield strength and creep rupture time with a noticeable increase in steady creep rate. Further increasing heat treatment temperature above 1000 °C removes the dislocation cell structure and elemental segregation on cell walls, which results in a continuous increase in steady creep rate. The present work demonstrates that the presence of chemical micro-segregation is crucial for the stabilization of dislocation cells structure and the resulted creep performance of the heat-treated L-DED samples.

DEVELOPMENT OF ENERGY AND RESOURCE SAVING TECHNOLOGIES OF PROCUREMENT PRODUCTION

The article presents the results of development and research of energy- and resource-saving low-waste technologies of pressure metal processing (OMT). When developing OMT processes, it is important to expand the technological capabilities of low-waste production of parts of complex shape. This is facilitated by the development of local deformation processes. The purpose of this study is to improve the quality of parts, save material, energy and labor resources based on the development of the calculation and design of rolling stamping and rolling processes with an assessment of the deformability of the material of the workpieces. For this purpose, a method was developed for calculating rational routes in the design of stamping and rolling processes, which provides: a reduction in the cost of production due to a reduction in the power of the equipment, the number of transitions and the elimination of scrap from destruction; improving the quality of products due to the formation of a favorable microstructure of the material, reducing the anisotropy of mechanical properties and limiting the resource of plasticity. It is shown that the accuracy of the dimensions of the workpiece after local deformation depends on the technological scheme, the accuracy of the dimensions of the tool and its setting, and the roughness of the surface of the product depends on the technological scheme, the roughness of the tool and the quality of lubrication. When stamping by rolling and rolling, the accuracy of the dimensions of the product is ensured according to quality 7-11, and the roughness of the surfaces, if the technological requirements are met, is μm. On the example of reshaping by stamping and rolling of square sheet blanks into round ones, it was proven that the degree of planar anisotropy decreased by 70-80% and the relative drop in yield strength in the plane of the sheet from 0.10-0.15 to 0.03-0.05, as well as an increase in ultimate deformation for tension by 8-10% and ultimate uniform deformation by 5-8%. When drawing cylindrical products from reshaped blanks, the size of the festoons decreases by 2-2.5 times, and the size of the ultimate degree of drawing increases by 10-15%. At the same time, the relative difference in wall thickness along the perimeter of the elongated workpiece decreases, and the decrease in wall thickness along its height becomes linear.

Microstructure evolution and mechanical properties of an annealed dual-phase CrFeCoNiAl0.7 high-entropy alloy

The present work investigated microstructures and mechanical properties of an annealed dual-phase high-entropy alloy (HEA) CrFeCoNiAl0.7 between 700 and 1100 °C. It revealed that with the increased annealing temperature, there were rod-like B2 phase precipitating from the FCC-solution matrix, and the orientation between the precipitated B2 phase and the FCC matrix obeyed Kurdjumov-Sachs (K–S) relationship {111}FCC//{011}B2 and ⟨011⟩FCC//⟨111⟩B2. The higher annealing temperature resulted in a drastic drop in yield strength, but a significant increase in fracture strain and work-hardening capacity. The positive value of the strain rate sensitivity (SRS) decreased with strain and the SRS of the HEA sample annealed at a lower temperature decreased greatly. Due to the formation of co-clusters, the activation volume of the annealed HEA presented an abnormal trend, that was, it increased with strain.

Numerical analysis of the effects of ultrasonic vibrations and elevated temperature in incremental sheet forming

Incremental sheet forming (ISF) is one of the novel processes practised for more than the last decade in the field of sheet metal forming. ISF, although recognised as a promising manufacturing process over conventional forming, does not give good results in terms of geometrical accuracy, sheet thinning, surface finish and the forming forces. To make the ISF process more efficient, strategies to enhance the forming limits and capabilities are constantly being developed. Two such adaptations to the ISF process are ultrasonic vibration-assisted incremental sheet forming (UVaISF) and elevated temperature incremental sheet forming (ET-ISF). In the present work, investigations have been carried out to compare ISF, UVaISF and ET-ISF on 1 mm thick AA1050 and Titanium Grade 2 sheets with the help of numerical simulations using Abaqus/CAE. Response parameters like stress, strain, thickness distribution, the final shape of the geometry, strain energy, energy dissipated during plastic deformation and the forming forces are evaluated to measure the effectiveness of the processes. It is observed that there is a considerable reduction in the forming forces and spring-back with the UVaISF and ET-ISF processes, in comparison to the ISF at room temperature. This considerable reduction in the forming forces and spring-back is due to the softening behaviour of sheet material and the drop in yield strength during UVaISF and ET-ISF processes. The work overcomes some of the limitations of the ISF process and enlarges the scope of application of the process.

Performance of additively manufactured polylactic acid (PLA) in prolonged marine environments

Additive manufacturing has seen rapid growth in a variety of sectors due to its advantages in cost and lead-time reductions. However, there is still skepticism regarding the ability for additively manufactured materials to withstand extreme conditions. In this report, AM polylactic acid (PLA) was submerged in seawater at several temperatures to deduce long-term effects of marine environments for these materials. An accelerated aging model was developed to predict life of PLA over years with only weeks or days of exposure. Samples were submerged in artificial seawater between one and ten weeks with temperatures ranging from 22 °C to 60 °C. Several mechanical tests were then conducted on the submerged samples, such as quasi-static tensile, nanoindentation, and direct impact Hopkinson pressure bar tests to deduce different mechanical properties after aging. For the seawater samples, diffusion data was collected to measure water absorbed by the specimens. An Arrhenius relationship was then studied, and a model for accelerated aging was developed with resulting acceleration factors. Results indicated clear aging in PLA over the timeframe studied, and it was deduced that the acceleration factors of aging between 22 °C and 40 °C, 22 °C and 50 °C, and 22 °C and 60 °C, were 1.91, 3.56, and 5.57, respectively. Results demonstrated a clear aging in PLA; it can be predicted that PLA will lose 0.5% ductility at around 5166 h (30.75 weeks) and will suffer a 10 MPa drop in yield strength at around 2084 h (12.4 weeks) while submerged in 17 °C seawater.

Deformation behaviour of Cu and Cu–Al in the dislocation starved regime: A molecular dynamics study

In general, with the addition of solutes, the yield strength of alloys is expected to increase; this phenomenon is known as solid solution strengthening. However, this model of strengthening implicitly assumes that dislocations are available and it is their motion that causes plastic deformation. On the other hand, in dislocation starved regimes, it is the nucleation of dislocations that controls the deformation behaviour. In this regime, the solute atoms can act as sites of heterogeneous nucleation for dislocations and “anomalous” softening with alloying additions are not uncommon. In this study, using Molecular Dynamics (MD) simulations, we show anomalous softening in Cu–Al alloys deformed at 300 K. We show that there are two components to this softening; in addition to the heterogeneous nucleation at solute sites, the reduction in stacking fault energy with alloying addition promotes the (homogeneous) nucleation of partial dislocation loops. In order to be consistent, we interpret the MD simulation results of deformation behaviour using parameters and energies derived from the same potentials used in the simulations. Specifically, we carry out the thermodynamic integration to evaluate the free energies in crystals with and without the stacking faults and hence calculate the SFE as a function of Al content at 300 K. Using the SFE values thus obtained, and using a continuum model of homogeneous nucleation of partial dislocation loops, we rationalise the deformation behaviour seen in pure copper. On the other hand, in Cu–Al alloys, the drop in yield strength can only be explained using a combination of the continuum model (which assumes homogeneous nucleation), and the reduction in heterogeneous nucleation barrier (which is a function of the ratio of the unstable and stable stacking fault energies). Thus, our results indicate that deformation experiments of pure copper and copper–aluminium alloys in the dislocation starved regime could be interesting and might show qualitatively and quantitatively different behaviour.

Microstructural and Mechanical Properties of a Solid-State Additive Manufactured Magnesium Alloy

Abstract In recent years, additive manufacturing (AM) has gained prominence in rapid prototyping and production of structural components with complex geometries. Magnesium alloys, which have a strength-to-weight ratio that is superior compared with steel and aluminum alloys, have shown potential in lightweighting applications. However, commercial beam-based AM technologies have limited success with magnesium alloys due to vaporization and hot cracking. Therefore, as an alternative approach, we propose the use of a near net-shape solid-state additive manufacturing process, additive friction stir deposition (AFSD), to fabricate magnesium alloys in bulk. In this study, a parametric investigation was performed to quantify the effect of process parameters on AFSD build quality including volumetric defects and surface quality in magnesium alloy AZ31B. In order to understand the effect of the AFSD process on structural integrity in the magnesium alloy AZ31B, in-depth microstructure and mechanical property characterization was conducted on a bulk AFSD build fabricated with a set of acceptable process parameters. Results of the microstructure analysis of the as-deposited AFSD build revealed bulk microstructure similar to wrought magnesium alloy AZ31 plate. Additionally, similar hardness measurements were found in AFSD build compared with control wrought specimens. While tensile test results of the as-deposited AFSD build exhibited a 20% drop in yield strength (YS), nearly identical ultimate strength was observed compared with the wrought control. The experimental results of this study illustrate the potential of using the AFSD process to additively manufacture Mg alloys for load bearing structural components with achieving wrought-like microstructure and mechanical properties.

Microstructural and Mechanical Characterization of Additive Friction Stir-Deposition of Aluminum Alloy 5083 Effect of Lubrication on Material Anisotropy.

Additive Friction Stir-Deposition (AFS-D) is a transformative, metallic additive manufacturing (AM) process capable of producing near-net shape components with a wide variety of material systems. The solid-state nature of the process permits many of these materials to be successfully deposited without the deleterious phase and thermally activated defects commonly observed in other metallic AM technologies. This work is the first to investigate the as-deposited microstructure and mechanical performance of a free-standing AA5083 deposition. An initial process parameterization was conducted to down-select optimal parameters for a large deposition to examine build direction properties. Microscopy revealed that constitutive particles were dispersed evenly throughout the matrix when compared to the rolled feedstock. Electron backscatter diffraction revealed a significant grain refinement from the inherent dynamic recrystallization from the AFS-D process. Tensile experiments determined a drop in yield strength, but an improvement in tensile strength in the longitudinal direction. However, a substantial reduction in tensile strength was observed in the build direction of the structure. Subsequent fractographic analysis revealed that the recommended lubrication applied to the feedstock rods, necessary for successful depositions via AFS-D, was ineffectively dispersed into the structure. As a result, lubrication contamination became entrapped at layer boundaries, preventing adequate bonding between layers.

New insights on cellular structures strengthening mechanisms and thermal stability of an austenitic stainless steel fabricated by laser powder-bed-fusion

Rapid solidification cellular structures are known to play a crucial role in helping achieve high strength and high ductility in 316L austenitic stainless steels fabricated by laser powder-bed-fusion (L-PBF). Despite this, the understanding of their intrinsic characteristics (e.g., crystallographic orientations, dislocations, precipitates, elemental segregations) and the respective impacts on the material's strength and thermal stability remains nebulous. We conduct several dedicated transmission electron microscopy (TEM) studies to investigate these strengthening mechanisms and identified that cell walls follow specific crystallographic orientations. The high density of tangled dislocations inside cell walls are found to have a higher tendency to dissociate, forming wider stacking faults while oxide precipitates are confined inside cell walls. These features act as barriers to moving dislocations upon plastic deformation and contribute to the high strength. Our dislocation dynamic simulations indicate that segregated particles are effective in blocking dislocations locally, helping the formation of dislocation cells and participating to the material strengthening. To study the thermal stability of L-PBF 316L SS, we perform systematic post-processing heat treatments from 400-1200°C. Microstructure characterizations using electron backscatter diffraction, TEM, and synchrotron X-ray diffraction coupled with dislocation dynamics and CALPHAD simulations and tensile testing reveal three heat treatment zones where the structure-property relationship can be tuned. After annealing up to 600°C, the microstructure remains stable; but the work hardening behavior is altered with a material that retains high strength and high ductility. Annealing between 600-1000°C activates elemental diffusion and gradual disappearance of cell walls, leading to a sharp drop in yield strength and a tradeoff between strength and ductility. Low-angle grain boundaries remain stable up to 1000°C while the average grain size defined by high angle grain boundaries is near constant at annealing temperatures up to 800°C. Annealing above 1100°C removes all L-PBF microstructure footprints and renders a conventional-like microstructure. Compared to conventional materials, L-PBF 316LSS displays substantially higher thermal stability and superior performance at elevated temperatures.

Novel Approach to Thermal Processing Development for Precision Sand Casting Process (PSCP) of Aluminum Engine Blocks

The latest generation of high-performance engine blocks are produced with the precision sand casting process (PSCP). Their enhanced mechanical properties are provided by an integrated cast iron chill in the bulkhead section and thus result in large variations in secondary dendrite arm spacing (λ2) throughout the whole casting, providing unique challenges in both heat treat (T7 temper) development and compliance to the CQI-9 protocol. This research examines the influence of chemistry, controlled solution/quench, residence time between quench and the start of artificial age and artificial age temperature, on the mechanical properties and cryogenic testing observations on PSCP engine blocks. The results are obtained from large population engine block castings (n = 204), heat treated within an industrial plant environment with strict control over furnace temperature gradients. The results indicated that age temperature differences of as little as 4 °C (within CQI-9 for furnace temperature specification of ± 5 °C) can alter yield strength and elongation significantly, which in turn changes crack susceptibility (warranty issues). In addition to the above analysis, it was found that extending the time between the solution/quench operation and the start of artificial age results in a continual drop in yield strength due to the influence of natural aging.

Microband induced plasticity and the temperature dependence of the mechanical properties of a carbon-doped FeNiMnAlCr high entropy alloy

The uniaxial mechanical properties of the high entropy alloy (HEA) Fe40.4Ni11.3Mn34.8Al7.5Cr6 doped with 1.1 at.% C have been determined for both the as-cast and recrystallized states at temperatures from 77 to 973 K. Very high yield strengths were obtained at 77 K of 928 MPa for the as-cast HEA and 1037 MPa for the recrystallized HEA. The yield strength decreased sharply to 293 K and more slowly to 973 K, where it was ~200 MPa [11] in both conditions. The lattice friction stress, which results from the increase of the Peierls barrier, was the major contributor to the large yield strength at 77 K. On the other hand, the grain boundaries lose their strengthening ability at a high temperature, which largely accounts for the sharp drop in yield strength of the fine-grained (5 μm) recrystallized HEA at 973 K. The elongation to failure was highest at 293 K and 673 K with the lowest values at 77 K. Transmission electron microscopy was used to study the effects of temperature on the deformation mechanisms of the HEAs, especially the temperature dependence of the microband-induced plasticity effect. At temperatures ranging from 77 K to 673 K microbands form in the as-cast HEA, while dislocation clusters were observed in the recrystallized HEA over this temperature range. Only tangled dislocations were found at 973 K for both the as-cast and recrystallized HEA due to thermally-activated deformation.

The Effect of Aging on the Microstructure of Selective Laser Melted Cu-Ni-Si

Precipitation hardening copper alloy C70250 was selectively laser melted to successfully produce components around 98 pct dense with high mechanical strength and electrical conductivity. Aging heat treatments were carried out at 723 K (450 °C) directly on as-printed samples up to 128 hours. Mechanical testing found that peak yield strength of around 590 MPa could be attained with an electrical conductivity of 34.2 pct IACS after 8 hours of aging. Conductivity continues to increase with further aging while the peak strength appears to be less sensitive to aging time exhibiting a broad range of time where near-peak properties exist. After aging for 128 hours, there is a drop in yield strength to 546 MPa with an increase in conductivity to 43.2 pct IACS. Electron microscopy analysis revealed nanometer-scale silicon-rich oxide particles throughout the material that persist during aging. Deformation twinning is observed in the peak-age condition after tensile testing and several strengthening mechanisms appear to be active to varying degrees throughout aging which account for the broad range of aging time where nearly the peak mechanical properties exist.

High-temperature tensile deformation behavior of hot rolled CrMnFeCoNi high-entropy alloy

In the present study, high temperature (700–1100 °C) tensile deformation behavior of hot-rolled CrMnFeCoNi high-entropy alloy was investigated. A single face-centered cubic phase was retained in the hot deformed specimens under different deformation conditions. The flow behavior was significantly influenced by temperature and microstructure evolution. A strong temperature dependence of yield stress was observed with a drastic drop in yield strength from 413 MPa at 700 °C to 94 MPa at 900 °C. A profound increase in ductility was observed above 700 °C and a perceptive decrease in ductility was observed above 900 °C. Electron backscatter diffraction analysis indicates incomplete dynamic recrystallization at 700 °C leading to a significant difference in the flow behavior.

Mechanical and ballistic properties of polycarbonate apposite to riot shield applications

Polycarbonate is commonly used as the material of riot shields. Firings of 8˙3 g steel ball bearings at velocities of 23–98 ms–1 produced damage in the form of dents. Subsequent tensile testing showed a small drop in yield strength and ductility for impact velocities greater than 58 ms–1. This drop in yield strength and ductility was more marked for specimens that had been soaked in acetone for 10 min after ballistic impact. Soaking in n-heptane produced no similar effect to that seen with acetone. Measurements were taken of the velocities of projectiles likely to be thrown by rioters. These showed that the measured velocities were lower than those that caused macroscopic damage to polycarbonate when hit by a ball bearing. Thus susceptibility to brittle failure is likely to be associated with shields that have been hit by sharp objects and exposed to solvents after impact. Mechanical stressing of polycarbonate at the point of impact by the ball bearing had no effect on the subsequent mechanical properties. This is in contrast to the embrittlement seen in polycarbonate when internal strains are present produced on fast cooling from the melt.

Plastic deformation inhomogeneity in a single crystal nickel-base superalloy

Extension and compression tests at different temperatures with various strain rates have been carried out on DD8 single crystal superalloys with [001] [110] and [111] orientations. The results show that plastic deformation heterogeneity (P–L effect) occurs in the temperature range from 300 to 700°C at the strain rate of 3.3×10−4 S−1 and also in the strain rate range of 1.3×10−4–5.3×10−4 S−1 at 700°C for the [001]-oriented single crystal nickel-base superalloy during extension. The dependence of the plastic deformation inhomogeneity on the crystallographic orientations during compression under the strain rate of 1.0×10−4 S−1 at 550°C were investigated, and the results indicate that the tendency to the plastic deformation inhomogeneity decreases in the order of [011], [001] and [111]. The mechanical tests display that this plastic instability may lead to the catastrophic drop in yield strength of the alloys, but can be removed by a new heat treatment, with which the yield strength can be improved.

The Effects of Artificial Aging on the Microstructure and Fracture Toughness of Al-Cu-Li Alloy 2195

Aluminum-lithium alloys have shown promise for aerospace applications, and National Aeronautics and Space Administration (NASA) has selected the aluminum-lithium Alloy 2195 for the main structural alloy of the super light weight tank (SLWT) for the space shuttle. This alloy has significantly higher strength than conventional2xxx alloys (such as 2219) at both ambient and cryogenic temperatures. If properly processed and heat treated, this alloy can display higher fracture toughness at cryogenic temperature than at ambient temperature. However, the properties of production materials have shown greater variation than those of other established alloys, as is the case with any new alloy that is being transitioned to a demanding application. Recently, some commercial 2195 plates for the SLWT program were rejected, mostly due to low CFT or FTR at ambient and cryogenic temperatures. Investigation of the microstructure property relationships of Al-Cu-Li based alloys indicates that the poor fracture toughness properties can be attributed to excessive T1 precipitation at subgrain boundaries. Lowering the aging temperature is one way to avoid excessive T1 precipitation at subgrain boundaries. However, this approach results in a significant drop in yield strength. In addition, low-temperature aging is associated with sluggish aging kinetics, which are not desirable for industrial mass production. Therefore, the present study was undertaken to develop an aging process that can improve fracture toughness without sacrificing yield and tensile strength. A multistep heating-rate controlled (MSRC) aging treatment has been developed that can improve the cryogenic fracture toughness of aluminum-lithium Alloy 2195. At the same levels of yield strength (YS), this treatment results in considerably higher fracture toughness than that found in Alloy 2195, which has received conventional (isothermal) aging. Transmission electron microscopy revealed that the new treatment greatly reduces the size and density of subgrain-boundary T1 precipitates. In addition, it promotes T1 and θ" nucleation, resulting in a fine and dense distribution of precipitate particles in the matrix. The MSRC aging treatment consists of (a) aging at 127‡C (260‡F) for 5 h, (b) heating continuously from 127‡C (260‡F) to 135‡C (275‡F) at a rate of 0.556‡C/h (1‡F/h), (c) holding at 135‡C (275‡F) for 5 h, (d) heating continuously from 135 to 143‡C (275 to 290‡F) at a rate of 0.556‡C/h (1‡F/h), and (e) holding at 143‡C (290‡F) for 25 h to obtain a near peak-aged condition.

Effects of deviations from stoichiometry on the strength anomaly and fracture behavior of B-doped FeAl

Abstract Tensile tests were conducted at temperatures ranging from 145–1123 K on four different FeAl alloys, containing 40, 43, 45, and 48 at.% Al, each doped with 0.12 at.% B. The alloys were initially heat treated to obtain a relatively large grain size (~200 μm), after which they were given a long, low-temperature anneal (673 K for 5 d), to minimize, respectively, the effects of grain boundary strengthening and thermal vacancies on the measured yield strengths. Each alloy displayed bcc-type behavior at low temperatures (yield strength decreasing with increasing temperature), followed by a strength anomaly at intermediate temperatures (yield strength increasing with increasing temperature), and a sharp drop in yield strength at elevated temperatures (beyond the anomalous strength peak). Thermal vacancies that are generated during the hold time at the test temperature may contribute to the production of the strength anomaly. In specimens not given the vacancy-minimizing anneal, quenched-in vacancies were found to substantially increase low-temperature strength, thereby masking the yield strength anomaly. As the Al concentration of FeAl increased, the prominence of the yield strength anomaly decreased. Ductility also exhibited a peak at elevated temperatures, first increasing with temperature until it reached a maximum value and then decreasing with further increases in temperature. The peak in ductility occured at lower temperatures as the Al content increased. The fracture mode in all four alloys was mixed (intergranular + transgranular) at cryogenic temperatures, predominantly intergranular at around room temperature, dimpled rupture at peak ductility, and intergranular cavitation at elevated temperatures where the ductility dropped.

Tensile deformation of the forged l12compound al66ti25mn9

Abstract The L12 compound A166Ti25Mn9 was induction-melted, cast, homogenized and successfully isothermally forged. Cylindrical ‘buttonhead’ tensile specimens were machined from the plane of the forging and tested in the temperature range 298-1073K at a strain rate of ∼5 × 10−5s−1. The 0.2% offset yield in tension remained essentially constant up to 623 K and then underwent a discontinuous drop. Further increases in temperature caused a steady gradual decrease in strength. Serrated flow was observed in the load-displacement curves over the temperature range 623-923 K. A small (0.2%) but finite plastic elongation was measured at ambient which gradually increased to 1.3% at 623 K. Further increase in temperature caused a sharp decrease in ductility with a minimum at 773 K. Fracture mode transitioned gradually from transgranular cleavage to intergranular failure, although the onset of intergranular failure was coincident with neither the ductility minimum nor the drop in yield strength. The deformed specimen...

Aluminum-lithium powder metallurgy alloys with improved toughness

A technique has been developed for increasing the toughness of Al-Li products made by powder metallurgy. The technique which involves forming a duplex structure by the addition of unalloyed aluminum powder to Al-Li powder before compaction was evaluated with Al-Li-Cu-Mg-Zr alloys (A18090). The strength, ductility, toughness, and short transverse stress corrosion resistance of the alloys were determined. The addition of 15 pct aluminum to A18090 aged at 422 K for 40 hours produced an increase in impact toughness of 215 pct at the expense of a drop in yield strength of 11 pct. The powder metallurgy alloys with a duplex structure had superior longitudinal strengthtoughness combinations to ingot materials of similar composition processed identically.