Novel microstructural characteristics and properties of spray formed Al-RE-TM based alloys

Recent studies on the synthesis of bulk Al-RE (Rare Earth)-TM (Transition Metal) based alloys, from melt spun ribbons and gas atomized powders, have shown that a partially amorphous or nano-crystalline structures lead to a high specific strength. In the present study, therefore, spray atomization and deposition process has been used to produce plates of Al85Y8Ni5Co2 (deposit D1) and Al83Y5La5Ni5Co2 (deposit D2) based alloys so as to synthesize bulk deposit of nano-crystalline and/or partial amorphous matrix composite in a single step. The rapid solidification and high undercooling of droplets during atomization and a chilling effect on undercooled liquid upon deposition are expected to give rise to the above microstructural features. The microstructural features of deposits as well as overspray powders were studied using optical, scanning and transmission electron microscope. The alloys invariably showed a large fraction of nano-crystalline and amorphous structures, characterized by featureless regions at optical resolution, along with distribution of primary equilibrium phases. The differential scanning calorimetric (DSC) analysis of the deposits showed all the crystallization peaks as is observed during crystallization of fully amorphous melt spun ribbons of respective compositions. A glass transition phenomenon is observed in Al-Y-Ni-Co based deposit. The transmission electron microscopy of deposit D1 showed the presence of 50-100 nm size fcc-Al precipitates in an amorphous matrix decorated with 5-20 nm fcc-Al crystallites. The annealing treatment of deposits at different temperatures, determined from the crystallization peaks of the deposit, showed precipitation of nanoscale fcc-Al and intermetallic phases giving rise to a remarkable increase in hardness. The bulk hardness of the deposits D1 and D2 was 391 and 427 HV, respectively. Whereas, the heat treated deposits showed a bulk hardness value of 476 HV for deposit D1 at 298 oC and 582 HV for deposit D2 at 380 oC. An attempt has been made to bring out the possible mechanism of microstructural evolution during spray deposition of these alloys, and the effect of microstructural features on the mechanical properties has been discussed

Pressure-Assisted Sintering Of Al–Gd–Ni–Co Amorphous Alloy Powders

The influence of pressure (0–764 MPa), on the densification and crystallization behavior of Al84Ni7Gd6Co3 glassy powders during sintering has been studied by hot pressing the amorphous alloy powders within the supercooled liquid region. The applied pressure significantly influenced the densification process: decreased the porosity level, changed the area of the inter-particle region and varied the volume fraction of α-Al in the inter-particle regions. The main densification mechanisms operating were particle (re)arrangement, plastic flow, viscous flow, and Al diffusion from the amorphous matrix to the inter-particle region at the surface. Furthermore, pressure affected the thermal stability of the amorphous powders during consolidation, influenced the amount of crystalline phases in the amorphous matrix, and the subsequent crystallization of the retained glassy phase. With increasing pressure, the primary crystallization of α-Al was hindered whereas crystallization of the intermetallic phases was promoted. This indicated that amorphous powders sintered at lower pressures allowed one to obtain higher volume fraction of α-Al and suppress the formation of brittle intermetallic phases. This study provides new and deep insights into pressure-assisted sintering of amorphous alloy powders and allows an innovative way of designing novel and advanced alloys with superior properties

Mechanical properties of Al-based metal matrix composites reinforced with Zr-based glassy particles produced by powder metallurgy

Al-based metal matrix composites consisting of pure Al reinforced with different amounts of mechanically alloyed Zr57Ti8Nb2.5Cu13.9Ni11.1 Al7.5 glassy powder were produced by powder metallurgy, and their mechanical properties were investigated by room temperature compression tests. The samples were consolidated into highly dense bulk specimens at temperatures within the supercooled liquid region in order to take advantage of the viscous flow behavior of the glassy powder. Compression tests show that the addition of the glass reinforcement increases the strength of pure Al from 155 to 250 MPa, while retaining appreciable plastic deformation with a fracture strain ranging between 70% and 40%. The yield strength and the elastoplastic deformation of such composites containing a high volume fraction of glassy particles were accurately modeled using a shear lag model and a self-consistent effective medium approach. Finally, the fracture characteristics of the reinforcing particles were rationalized using a proposed fracture criterion

Formation of Nanocrystalline Matrix Composite during Spray Forming of Al83La5Y5Ni5Co2

In the present investigation, a multicomponent glass-forming Al83Y5La5Ni5Co2 (at. pct) alloy was spray deposited on a copper substrate to produce an 8-mm-thick plate. The substrate was 30-mm thick and heated to a temperature of 160 C prior to spray deposition. The temperature of the substrate and the deposit was measured during and after deposition. The deposits as well as oversprayed powders were characterized in terms of the microstructural features by optical microscopy and scanning electron microscopy (SEM). The phase constitution and transformation were studied by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The oversprayed powder revealed different microstructural characteristics showing crystalline, partially crystalline, and fully featureless particles. The spray deposit showed large fraction of featureless regions with embedded dendrites of 1- to 10-lm size intermetallic phases. These regions were observed to have a nanocrystalline structure with an average grain size of approximately 100 nm. The XRD analysis also revealed the nanocrystallinity in terms of a halo and peak broadening. These microstructural features have been attributed to the deposition of undercooled liquid on a highly conductive copper substrate and rapid heat extraction from the droplets due to proper metallic contact between the deposit and the substrate. These results have been discussed in light of processing conditions and the microstructural evolution of droplets in flight and during deposition

Production of high-strength Al85Y8Ni5Co2 bulk alloy by spark plasma sintering

Highly dense bulk samples were produced by spark plasma sintering (SPS) through combined devitrification and consolidation of partially amorphous Al85Y8Ni5Co2 gas atomized powders. The microstructure of the consolidated samples shows a mixed structure containing crystalline, ultrafine-grained and amorphous/nanocrystalline particles. The sintered sample exhibits a remarkable high strength of about 1050 MPa combined with 3.7 % fracture strain

Study of heavy ion beam induced damage in tungsten for high power target applications

The spallation material at ESS is pure tungsten, which is cooled by gaseous helium flow. To study the behaviour of tungsten under dynamic beam conditions at ESS, pure tungsten specimens have been irradiated at the M3-beamline of the UNILAC facility at GSI Helmholtz Centre for Heavy Ion Research. Tungsten specimens of two thicknesses, 26 μm and 3 mm, were exposed to pulsed uranium and gold ion beams for fluences up to 7.5 · 10 13 ions·cm −2 at 4.8 MeV/nucleon. Nanoindentation tests were performed on the cross section of the irradiated 3 mm sample, and microhardness was measured on the top surface. The measured data are compared with the calculated damage values, and a correlation between the radiation induced damage and the observed mechanical property is presented. Thermal diffusivities of foil samples irradiated up to four different fluences were measured with a Laser Flash Apparatus (LFA). The observed changes in the mechanical and thermal properties of irradiated tungsten were used to estimate the changes of operational temperature and mechanical stresses in the ESS target material with the progress of radiation damage, using coupled thermal and mechanical simulations. From the pulsed beam induced dynamic oscillations of thin tungsten specimens, information on fatigue properties of tungsten under irradiation was drawn. In addition to pure tungsten, oxidised tungsten samples were irradiated. This is to investigate the stability of the adhesive oxide layer under pulsed beam conditions, which would be formed due to oxygen impurities in the helium cooling loop. The irradiated oxide scale was examined using Auger Electron Spectroscopy (AES) and Scanning Electron Microscopy (SEM)

Spray forming of bulk Al85Y8Ni5Co2 with co-existing amorphous, nano- and micro-crystalline structures

A 12 mm thick Al85Y8Ni5Co2 plate was spray deposited on a 30 mm thick pre-heated copper substrate. The deposit was characterized using optical, high resolution scanning and transmission electron microscopy. Differential scanning calorimetry was performed to assess the crystallization behaviour of the deposit compared to melt spun ribbons of the same composition. The deposit shows an amorphous phase fraction of 83 and 56 vol.% (based on the total crystallization energy), in the bottom and top regions of the deposit. The deposit consists of amorphous and nanoscale structures along with microcrystalline intermetallic phases. The large amorphous phase fraction in the deposit is attributed to the chilling effect upon deposition of highly undercooled/partially crystallized droplets onto the pre-heated substrate and rapid heat extraction thereof due to bonding at the deposit/substrate interface

Microstructural characteristics of spray formed and heat treated Al–(Y, La)–Ni–Co system

Recent studies on the synthesis of bulk Al–RE (Rare Earth)-TM (Transition Metal) based materials, from melt spun ribbons and gas atomized powders, have shown that partially amorphous or nano-crystalline structures lead to a high specific strength. In the present study, therefore, spray atomization and depo- sition process has been used to produce plates of Al 85 Y 8 Ni 5 Co 2 (deposit D1) and Al 83 Y 5 La 5 Ni 5 Co 2 (deposit D2) systems so as to synthesize bulk deposit of nano-crystalline and/or partially amorphous materials in a single step. The rapid solidification and high undercooling of droplets during atomization and the chill- ing effect on undercooled liquid upon deposition give rise to the above microstructural features. The microstructural features of deposits as well as overspray powders were studied using optical, scanning and transmission electron microscope. The alloys invariably showed a large fraction of nano-crystalline structure and amorphous features, characterized by featureless regions at optical resolution, along with distribution of primary equilibrium phases. The differential scanning calorimetric (DSC) analysis of the deposits showed similar crystallization features as observed during crystallization of fully amorphous melt spun ribbons of respective compositions. The transmission electron microscopy of deposit D1 showed the presence of 50–100 nm size fcc-Al precipitates in an amorphous matrix decorated with 5– 20 nm fcc-Al crystallites. The annealing treatment of deposits at different temperatures, determined from the crystallization peak temperatures of the deposits, showed precipitation of nanoscale fcc-Al and inter- metallic phases giving rise to a remarkable increase in hardness values. The bulk hardness of the deposits D1 and D2 was 391 and 427 HV, respectively. Whereas, the annealed deposits showed a bulk hardness value of 476 HV for deposit D1 and 583 HV for deposit D2 at annealing temperature of 298 and 380 C, respectively. An attempt has been made to bring out the possible mechanism of microstructural evolution during spray deposition of the above alloy systems, and the effect of microstructural features on the hardness values has been discussed

Microstructure and mechanical properties of partially amorphous Al85Y8Ni5Co2 plate produced by spray forming

A 12mmthick Al85Y8Ni5Co2 plate was spray deposited on a 30mmthick copper substrate pre-heated to 383 K. The deposit microstructure consists of an amorphous phase, 50–150nm fcc-Al grains, 0.2–0.7m Al2Y and Al3Y intermetallic phases and some unidentified phases. The hardness of different microstructural features and the compressive strength of the deposit, after extrusion at 723 K, were evaluated. The small size overspray particles as well as the deposit show a glass transition phenomenon. The total crystallization energy of the deposit is 121 J/g compared to 83.9 J/g for the small sized overspray powder, indicating its high metastability. A compressive strength of 925MPa and a deformation strain of 9% were achieved for the deposit after extrusion. The novel microstructural features in the deposit are attributed to the “chilling effect” on highly undercooled or partially solidified large-size droplets during deposition onto the pre-heated substrate, and the rapid heat extraction thereof due to a close contact at the deposit/substrate interface