Bulk nanocrystalline aluminum 5083 alloy fabricated by a novel technique: Cryomilling and spark plasma sintering

Dense, bulk nanocrystalline aluminum 5083 alloy was fabricatedvia a combined technique: cryomilling (mechanical milling at cryogenic temperature) to achieve the nanocrystalline Al 5083 powder and spark plasma sintering (SPS) to consolidate the cryomilled powder. The results of X-ray diffraction analysis indicate that the average grain size in the SPS consolidated material is 51 nm, one of the smallest grain sizes ever reported in bulk Al alloys produced by powder metallurgy derived methods. In contrast, transmission electron microscopy (TEM) analysis revealed a bimodal grain size distribution, with an average grain size of 47 nm in the fine-grained regions and approximately 300 nm in the coarse-grained regions. Nanoindentation was used to evaluate the mechanical properties and the uniformity of the consolidated nanocrystalline Al 5083. The hardness of the material is greatly improved over that of the conventional equivalent, due to the fine grain size. The mechanisms for spark plasma sintering and the microstructural evolution are discussed on the basis of the experimental findings.     

A numerical study of oxidation behavior during reactive atomization and deposition

The oxidation behavior of droplets during reactive atomization and deposition (RAD) is analyzed on the basis of a numerical framework proposed here. Commercial 5083 Al is chosen as a model material; moreover, in the numerical model, nonspherical droplets are approximated as cylinders with a length/diameter ratio of 3. An equation that represents the growth rate of the oxide phases, together with models that describe the dynamic and thermal behavior of droplets, is implemented in an effort to elucidate the oxidation behavior of individual droplets. The numerical results reveal that the oxidation rate of a droplet is extremely high and that the oxide phase grows very rapidly initially, eventually attaining a steady state of limited oxide growth. The overall volume fraction of oxide phases in the RAD material increases with increasing atomization pressure, superheat temperature, and O₂ concentration, whereas it decreases with increasing melt flow rate. The oxygen concentrations in the RAD powders and deposited materials predicted on the basis of numerical analysis are in good agreement with the results from chemical analysis when O₂ concentration is lower than 16 vol pct.     

Influence of Solute Content and Solidification Parameters on Grain Refinement of Aluminum Weld Metal

Grain refinement provides an important possibility to enhance the mechanical properties (e.g., strength and ductility) and the weldability (susceptibility to solidification cracking) of aluminum weld metal. In the current study, a filler metal consisting of aluminum base metal and different amounts of commercial grain refiner Al Ti5B1 was produced. The filler metal was then deposited in the base metal and fused in a GTA welding process. Additions of titanium and boron reduced the weld metal mean grain size considerably and resulted in a transition from columnar to equiaxed grain shape (CET). In commercial pure aluminum (Alloy 1050A), the grain-refining efficiency was higher than that in the Al alloys 6082 and 5083. Different welding and solidification parameters influenced the grain size response only slightly. Furthermore, the observed grain-size reduction was analyzed by means of the undercooling parameter P and the growth restriction parameter Q, which revealed the influence of solute elements and nucleant particles on grain size.     

Room-temperature mechanical behavior of cryomilled al alloys

Room-temperature mechanical properties of cryomilled Al-7.5 pct Mg and Al 5083 alloys are discussed in the context of a duplex microstructure, which arises during processing. After consolidation via hot isostatic pressing (“hipping”), coarse-grained regions are formed in former interparticle void volumes, and these regions become elongated during extrusion. Comparison of tensile and compression testing results on both “as-hipped” and extruded materials shows that tension-compression asymmetry is the result of these coarse-grained regions and not necessarily a fundamental property of ultrafine grained Al. The strength of the extruded materials is consistent with the Hall-Petch model of strengthening by grain size refinement, but the hipped material deviates from this trend, with a lower strength despite finer average grain size. This can also be attributed to the presence of coarse-grained regions, which substract from the strength in a predictable manner and also enhance the ability of the cryomilled material to work harden.     

Superplastic behavior of spray-deposited 5083 Al

A 5083 Al alloy was synthesized using spray deposition processing with N₂ as the atomization gas. It was noted that the grains that were present in as-spray-deposited 5083 Al were equiaxed with an average size of 15.2 μm. The matrix of the material was supersaturated with Mg and Mn. The asspray-deposited microstructure contained irregular pores with porosity in the range of 0.1 to 5.4 vol pct, depending on spatial location in the preform. The spray-deposited alloy was thermomechanically processed using extrusion and multiple-pass warm rolling to reduce grain size and close porosity. It was observed that spray-deposited 5083 Al exhibited superplasticity following thermomechanical processing by extrusion followed by rolling. Superplasticity was observed in the 500 °C to 550 °C temperature range and 3 × 10⁻⁵ to 3 × 10⁻³ s⁻¹ strain rate range. The corresponding strain-rate-sensitivity factors were in the 0.25 to 0.5 range and increased with decreasing strain rate. A maximum elongation of 465 pct was noted at 550 °C and 3 × 10⁻⁵ s⁻¹. The spray-deposited 5083 Al, thermomechanically processed by direct rolling, exhibited superplasticity in the same temperature and strain rate ranges as those for the extruded and rolled materials. The superplastic elongation of the spray-formed and rolled material was relatively low, being in the range of 250 to 300 pct. The deformation behavior is discussed in light of the presence of porosity in the microstructure.     

Thermal stability in bulk cryomilled ultrafine-grained 5083 Al alloy

Thermal stability in bulk ultrafine-grained (UFG) 5083 Al that was processed by gas atomization followed by cryomilling, consolidation, and extrusion, and that exhibited an average grain size of 305 nm, was investigated in the temperature range of 473 to 673 K (0.55 to 0.79 T _m , where T _m is the melting temperature of the material) for different annealing times. Appreciable grain growth was observed at temperatures > 573 K, whereas there was limited grain growth at temperatures < 573 K even after long annealing times. The values of the grain growth exponent, n, deduced from the grain growth data were higher than the value of 2 predicted from elementary grain growth theories. The discrepancy was attributed to the operation of strong pinning forces on boundaries during the annealing treatment. An examination of the microstructure of the alloy suggests that the origin of the pinning forces is most likely related to the presence of dispersion particles, which are mostly introduced during cryomilling. Two-grain growth regimes were identified: the low-temperature region (<573 K) and the high-temperature region (>573 K). For temperatures lower than 573 K, the activation energy of 25 ± 5 kJ/mol was determined. It is suggested that this low activation energy represents the energy for the reordering of grain boundaries in the UFG material. For temperatures higher than 573 K, an activation energy of 124 ± 5 kJ/mol was measured. This value of activation energy, 124 ± 5 kJ/mol, lies between that for grain boundary diffusion and lattice diffusion in analogous aluminum polycrystalline systems. The results show that the strength and ductility of bulk UFG 5083 Al, as obtained from tensile tests, correlate well with substructural changes introduced in the alloy by the annealing treatment.     

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.

     

Microstructural characterization of Ni3Al processed by reactive atomization and deposition

Reactive atomization and deposition (RAD) is a new processing technique that has been developed to synthesize dispersion-strengthened materials. In this process, atomization,in situ reaction, and consolidation are combined into a single step by spray atomization and deposition with a reactive gas. The matrix material selected for this study is an Ni₃Al + Y + B alloy in combination with N₂-O₂ atomization gas. The as-deposited microstructures reveal a spheroidal grain morphology, a banded structure, and a γ + γ′ mosaiclike structure. The formation of the γ + γ′ mosaiclike structure is attributed to an annealing effect during deposition. Matrix-lattice mismatches of 0.5 to 1 pct at the γ/γ′ interface and {100} growth orientations of γ′ phase are deduced from microscopic observations. The formation of the banded structure is attributed to the high cooling rate that is inherent to RAD processing. Anticipated dispersoids, such as Y₂O₃, Al₂O₃, and Y₃Al₅O₁₂ are identified using transmission electron microscopy (TEM). Dislocation pileups and grain boundary pinning are observed in the vicinity of oxide dispersoids. The origin and movement of dislocations in the as-deposited materials may be attributed to the residual stresses that originate from thermal gradients and the large amount of deformation experienced by the solid and semisolid droplets during impact. The preliminary results and analyses reported here suggest that the high thermal stability of the RAD processed Ni₃Al using N₂-15 pct O₂ may be attributed not only to the hindering effect of oxide dispersoids on grain boundary mi-gration, but also to the high cooling rate experienced by the droplets during atomization and the short annealing effect experienced by the material during deposition.     

Influence of hot isostatic pressing on microstructure and mechanical behaviour of nanostructured Al alloy

Abstract

Aluminium alloy AA 5083 [Al–4·4Mg–0·7Mn–0·15Cr (wt-%)], powder was ball milled in liquid nitrogen via the cryomilling method to obtain a nanocrystalline (NC) structure. Samples of the powder were hot vacuum degassed to remove interstitial contaminants, then consolidated by hot isostatic pressing (HIPing) at six temperatures (from 0·46T_m to 0·89T_m), before being high strain rate forged (HSRF) to produce plate material. The microstructure was analysed at the different processing stages. The compressive properties of the as HIPed material, plus tensile properties of the final product were studied. Despite grain growth during HIPing, an ultrafine grain (UFG) structure was retained in the consolidated material, which consequently had increased strength over conventionally processed AA 5083. As the HIP temperature was increased, the density increased. Strength changes were minimal in compression and tension with varying HIP temperature, once near full density was attained at 275°C (～0·64T_M). Yield strength data indicate negligible variation in the grain size of the materials.     

Mechanical behavior and microstructure of a thermally stable bulk nanostructured Al alloy

A commercial aluminum alloy, 5083, was processed using a cryomilling synthesis approach to produce powders with a nanostructured grain size. The powders were subsequently degassed, hot isostatically pressed, and extruded. The grain size at each processing step was measured utilizing both X-ray diffraction and transmission electron microscopy (TEM). The mechanical properties of the n-5083 extruded material were determined utilizing ASTM E8-93, Standard Test Methods for Tension Testing of Metallic Materials. This processing technique was found to produce a thermally stable nanostructured aluminum alloy which maintained an average grain size of 30 to 35 nm through several processing steps up to 0.61 T _mp . The thermal stability was attributed to Zener pinning of the grain boundaries by AIN and Al₂O₃ particles and solute drag of numerous atomic species. The nanostructured 5083 was found to have a 30 pct increase in yield strength and ultimate strength over the strongest commercially available form of 5083, with no corresponding decrease in elongation. The enhanced ductility is attributed to the presence of a few large, single-crystal aluminum grains acting as crack-blunting objects.     

On the mechanism of grain formation during spray atomization and deposition

The mechanisms governing the morphological changes in the microstructure of spray atomized and deposited Ni₃Al were studied, with particular emphasis on the formation of a spheroidal grain morphology. Accordingly, the various microstructural features present in the spray deposited material were rationalized on the basis of thermal energy considerations. The formation of spheroidal grains was proposed to evolve from: (a) the homogenization of dendrites that did not deform extensively during deposition; and (b) the growth and coalescence of deformed or fractured dendrite fragments. Support for this suggestion was provided by experimental results and numerical analyses which show that the microstructure of Ni₃Al is exposed to a high temperature anneal during deposition. Moreover, the results show that during high temperature annealing, the deformed or fractured dendrite fragments that were initially present in the spray deposited materials grew and coalesced leading to the development of a spheroidal grain morphology. On the basis of a coarsening mechanism, the relative annealing time under a particular cooling rate may be calculated and converted into a spheroidal grain size, d_sph, from the following equation, d_sph = 15.2· Ṫ^−0.35. The experimental results were observed to concur with this relationship.     

Hot Extrusion of Nanostructured Al-Powder Alloys: Grain Growth Control and the Effect of Process Parameters on Their Microstructure and Mechanical Properties

Two nanostructured aluminum powder alloys (supersaturated Al4.5Cu prepared by mechanical alloying, and Al3.0Fe0.42Cu0.37Mn rich in precipitates and prepared by rapid solidification via gas atomization) were consolidated into bulk material under various processing conditions via hot extrusion. The microstructural modifications and mechanical properties of the consolidated alloys as a function of the extrusion conditions were investigated and are discussed here. The effect of pre-existing precipitates from nonsupersaturated alloy is shown to be more effective for controlling grain growth during consolidation. The increase in the extrusion load, with a concomitant increase in the extrusion rate and decrease in temperature, is shown to lead to microstructural modifications. The differences in mechanical properties measured by compressive tests are also discussed in association with the extrusion parameters. Furthermore, suggestions are given for rationalizing the extrusion rate and temperature for the consolidation of nanostructured aluminum powder alloys via hot extrusion.     

Influence of Nitrogen Content on Thermal Stability and Grain Growth Kinetics of Cryomilled Al Nanocomposites

Nanocomposite powders of Al 5083/B₄C were produced via cryogenic milling (cryomilling) of boron carbide (B₄C) particles in Al 5083 matrix. The effect of milling time (up to 24 hours), and consequential nitrogen content, on grain growth in the nanocrystalline Al 5083 matrix was investigated. Thermal stability was studied at temperatures as high as ~0.96 T _m and annealing times of up to 24 hours. Average grain sizes increased with time and temperature and tended to stabilize after longer annealing times, regardless of nitrogen content. Higher thermal stability was observed in samples with higher nitrogen content, with the average grain size remaining in the range of 30 nm, even after exposure to the most extreme annealing conditions. This behavior was attributed to the retarding effect that nitrides have on grain growth, as a result of pinning grain boundaries. Kinetic studies based on the Burke equation showed two thermally activated grain growth regimes—a low-temperature regime with an activation energy of 15 kJ/mol and a high-temperature regime with an activation energy of 58 kJ/mol.     

High-strain-rate superplasticity in bulk cryomilled ultra-fine-grained 5083 Al

The ductility and creep of bulk ultra-fine-grained (UFG) 5083 Al (grain size ∼440 nm) processed by gas atomization, cryomilling, and consolidation were studied in the temperature range 523 to 648 K. Also, the creep microstructure developed in the alloy was examined by means of transmission electron microscopy (TEM). The ductility as a function of strain rate exhibits a maximum that shifts to higher strain rates with increasing temperature. An analysis of the experimental data indicates that the true stress exponent is about 2, and the true activation energy is close to that anticipated for boundary diffusion in 5083 Al. These creep characteristics along with the ductility behavior of 5083 Al are a reflection of its creep behavior as a superplastic alloy and not as a solid-solution alloy. In addition, the observation of elongations of more than 300 pct at strain rates higher than 0.1 s⁻¹ is indicative of the occurrence of high-strain-rate (HSR) superplasticity. Microstructural evidence for the occurrence of HSR superplasticity includes the retention of equiaxed grains after deformation, the observation of features associated with the occurrence of grain boundary sliding, and the formation of cavity stringers. Grain size stability during the superplastic deformation of the alloy is attributed to the presence of dispersion particles that are introduced during gas spraying and cryomilling. These particles also serve as obstacles for dislocation motion, which may account for the threshold stress estimated from the creep data of the alloy.     

Effects of Solidification Conditions on Grain Refinement Capacity of TiC in Directionally Solidified Ti6Al4V Alloy

In this study, the effects of solidification conditions on the grain refinement capacity of heterogeneous nuclei TiC in directionally solidified Ti6Al4V alloy were investigated using experimental and numerical approaches. Ti6Al4V powder with and without TiC particles in a Ti6Al4V sheath was melted and directionally solidified at various solidification rates via the floating zone melting method. In addition, by using the phase field method, the microstructural evolution of directionally solidified Ti6Al4V was simulated by varying the temperature gradient G and solidification rate V. As the solidification rate increased, the increment of the prior β grain number by TiC addition also increased. There are two reasons for this: first, the amount of residual potent heterogeneous nuclei TiC is larger. Second, the amount of TiC particles that can nucleate becomes larger. This is because increasing the constitutional undercooling ΔT_c leads to the activation of a smaller radius of heterogeneous nuclei and a higher nucleation probability from each radius. At a cooling rate R higher than that in the floating zone melting experiment (R = 3 to 1000 K/s), the maximum degree of constitutional undercooling ΔT_c,Max has a peak value, which suggests that constitutional undercooling ΔT_c has a smaller contribution at higher cooling rates, such as those that occur during electron beam melting (EBM), including laser powder bed fusion (LPBF).

     

Effect of oxygen on microstructures of high-rate sputter-deposited Nb3Sn superconductors

The role of excess oxygen and deposition temperature on the microstructure and critical temperature (T _c ) has been studied in high-rate, sputter deposited Nb₃Sn. Excess oxygen does not significantly affect the grain size of the A15 phase whether it is deposited at low temperature followed by annealing or deposited directly at an elevated temperature. Oxygen does promote the formation of the amorphous phase during deposition at low temperature. During subsequent transformation of material sputter deposited at room temperature, gas bubbles form from the entrapped sputtering gas. Excess oxygen also promotes greater precipitate formation during deposition at elevated temperature. In no case was there a large change in the superconducting transition temperatureT _c ; however, the microstructural features may have significant effects on the critical current densityJ _c .     

Deformation behavior of bimodal nanostructured 5083 Al alloys

Cryomilled 5083 Al alloys blended with volume fractions of 15, 30, and 50 pct unmilled 5083 Al were produced by consolidation of a mixture of cryomilled 5083 Al and unmilled 5083 Al powders. A bimodal grain size was achieved in the as-extruded alloys in which nanostructured regions had a grain size of 200 nm and coarse-grained regions had a grain size of 1 μm. Compression loading in the longitudinal direction resulted in elastic-perfectly plastic deformation behavior. An enhanced tensile elongation associated with the occurrence of a Lüders band was observed in the bimodal alloys. As the volume fraction of coarse grains was increased, tensile ductility increased and strength decreased. Enhanced tensile ductility was attributed to the occurrence of crack bridging as well as delamination between nanostructured and coarse-grained regions during plastic deformation.     

Grain Selection in Spiral Selectors During Investment Casting of Single-Crystal Turbine Blades: Part I. Experimental Investigation

Spiral grain selectors are used to grow single-crystal (SX) turbine blades during investment casting. Competitive growth in the spiral selectors leads to the selection of a single grain that subsequently grows to form the blade. In this study, the effect of spiral design on grain selection during investment casting was investigated through a series of experiments. It is found that the spiral design can effectively reduce the grain number but is not able to optimize axial grain orientations during solidification, the effectiveness of grain selection is strongly dependent on the spiral “take-off” angle, and spirals with smaller take-off angles are most potent. It is proposed that grain selection in the spiral is controlled by the geometry of the spiral via a “geometrical blocking” mechanism.     

Microstructural factors governing hardness in friction-stir welds of solid-solution-hardened Al alloys

Microstructural factors governing hardness in friction-stir welds of the solid-solution-hardened Al alloys 1080 and 5083 were examined by optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The effect of grain boundary on the hardness was examined in an Al alloy 1080 which did not contain any second-phase particles. The weld of Al alloy 1080 had a slightly greater hardness in the stir zone than the base material. The maximum hardness was located in the thermomechanically affected zone (TMAZ). The stir zone consisted of recrystallized fine grains, while the TMAZ had a recovered grain structure. The increase in hardness in the stir zone can be explained by the Hall-Petch relationship. On the other hand, the hardness profiles in the weld of Al alloy 5083 were roughly homogeneous. Friction-stir welding created the fine recrystallized grains in the stir zone and recovered grains in the TMAZ in the weld of this alloy. The stir zone and the TMAZ had slightly higher dislocation densities than the base material. Many small Al₆(Mn,Fe) particles were detected in all the grains of the weld. The hardness profiles could not be explained by the Hall-Petch relationship, but rather by Orowan hardening. The results of the present study suggest that the hardness profile is mainly affected by the distribution of small particles in friction-stir welds of Al alloys containing many such particles.