Structure stability of AA3003 alloy with ultra-fine grain size

The study on the structure stability of AA3003 alloy produced by an intense plastic straining process named accumulative roll bonding (ARB) has been conducted. The results show that continuous recrystallization took place in the ARBed 3003 alloy with increasing the annealing time at 250°C and increasing the annealing temperature to 275°C. While, discontinuous recrystallization began in some regions after 300°C annealing, and nearly finished after 400°C annealing. Furthermore, an unusual tensile behavior was observed in this alloy after annealing at 250–275°C. The Hall-Petch dependence was observed in the plot of microhardness versus d ^–1/2 of the ARBed 3003 alloy, but its dependence slope was changed. The ultra-fine grains (<1 m) formed in the ARBed 3003 alloy can be stable until annealing at 250°C for 1 h, and the fine grains (<2 m) can be stable until annealing at 275°C for 1 h. Therefore, grain structure formed in the ARBed 3003 alloys after intense plastic strain is reasonably stable. In addition, the mechanism of structure stability and mechanical behavior were also discussed.     

Synthesis and properties of carbons dispersed with Fe-Co alloy by pressure pyrolysis of organoiron-organocobalt copolymer

Carbons dispersed with Fe-Co alloy were synthesized by the pressure pyrolysis of vinylferro cene-phenylethynylcobaltocene-divinylbenzene copolymer at temperatures below 700° C and at 125 M Pa. As-prepared carbon synthesized at 550° C contained finely dispersed metallic particles of less than 10nm diameter with low crystallinity, which crystallized to form Fe-Co alloy particles with a higher crystallinity by subsequent heat treatment at 800° C. Larger particles of the alloy of more than 50nm diameter were dispersed in the carbon matrix synthesized at 700° C. Thermomagnetization measurement of the as-prepared carbon synthesized from divinylbenzene-2.1 mol% vinylferrocene-4.8mol% phenylethynylcobaltocene copolymer at 550° C and 125 M Pa confirmed that iron formed an alloy with cobalt in the carbon matrix. Fine, superparamagnetic metallic particles in the as-prepared carbon aggregated and crystallized by the heat treatment during the thermomagnetic measurement to increase the magnetization of the alloy-dispersed carbon. The saturation magnetization and the coercive force of alloy-dispersed carbon increased from 128 to 187e.m.u.g^–1 and from a few to 50 Oe, respectively, on increasing the pyrolysis temperature of the starting copolymer from 550 to 700° C. The saturation magnetization of alloy-dispersed carbon from divinylbenzene containing iron and cobalt with a ratio of 52 was higher than that from divinylbenzene including those with a ratio of 25. The carbon with finely dispersed Fe-Co alloy showed a high saturation magnetization of 213 e.m.u.g^–1 and a coercive force of 230 Oe, and the magnetization persisted above 800° C.     

Effect of thermomechanical treatments on microstructure and properties of Cu-base leadframe alloy

The effect of thermomechanical treatments (TMT) on the microstructuresand properties of Cu-1.5Ni-0.3Si-0.03P-0.05Mg leadframe alloy wasinvestigated. The Cu-base leadframe alloy was received as hot rolledplates with 8 mm thickness. The hot rolled plates were solutiontreated at 700°C or 800°C for 1 hour, and coldrolled with 40–85% reduction, then followed by aging treatment at450°C. The leadframe alloy solution treated at 800°Cshowed larger grain size of 15 m comparing with the grain size of10 m in leadframe alloy solution treated at 700°C. Theleadframe alloy with smaller grain size of 10 m showed highertensile strength and lower electrical resistivity than that withlarger grain size of 15 m. The dislocation density increased withincreasing reduction ratio of cold rolling from 40% to 85% andresulted in finer Ni₂Si precipitates. Tensile strength increasedand electrical resistivity decreased with increasing reduction ratioof cold rolling due to the formation of finer Ni₂Si precipitates.Two types of thermomechanical treatments were performed to enhance theproperties of leadframe alloy. One type of thermomechanical treatmentis to refine the grain size through the overaging, cold rollingfollowed by recrystallization. The recrystallization process improvedthe tensile strength to 540 MPa and elongation to 15% by reducing thegrain size to 5 m. The other type of thermomechanical treatmentis to refine the precipitate size by two-step aging process. Thetwo-step aging process increased the tensile strength to 640 MPa andreduced the electrical resistivity to1.475 × 10^–8 m by reducing the size of Ni₂Si precipitates to 4 nm.     

Role of second phase particles on microstructure and texture evolution of ARB processed aluminium sheets

Abstract

The accumulative roll bonding (ARB) process was carried out on a high purity alloy (AA1100) and a particle containing aluminium alloy (AA3003) for up to eight cycles. The electron backscattered diffraction (EBSD) method was utilised to investigate the microstructural and microtextural evolution in ARB processed sheets. The results indicate that the lack of second phase particles in pure aluminium hinders grain refinement and leads to the formation of unrefined bands, which results in the increase of the overall texture intensity and the development of a strong texture. A submicrometre grain structure in this alloy develops at the final stages of the process. It was also found that the presence of second phase particles in AA3003 alloy prevents the development of such unrefined bands and improves grain refinement during the ARB process, which results in a more homogenous microstructure of ultrafine grains.     

Nanostructured thin palladium-silver membranes: Effects of grain size on gas permeation properties

Submicron-thick Pd-Ag alloy membranes, prepared on 4 nm pore -alumina support by magnetron sputter deposition, are nanocrystalline with a grain (crystallite) size of about 20 nm. The membranes show good selectivity for hydrogen over helium (about 4000 at 300°C). Hydrogen permeation is dominated by the surface reaction steps in 100–200°C with an activation energy of about 30 kJ/mol. Bulk diffusion resistance becomes important at higher temperatures (>200 °C). Grain size is the most critical parameter affecting the hydrogen permeance of the thin nanostructued Pd-Ag membranes. Increase in Pd-Ag grain size from about 20 to 60 nm results in a substantial improvement in hydrogen permeance with a higher apparent activation energy in 100–300°C. Grain growth appears to increase the hydrogen permeability in the bulk phase of the Pd-Ag membranes. Helium permeance through the grain boundary decreases with increasing temperature or hydrogen partial pressure due to grain expansion. Carbonation and the accompanied grain expansion have detrimental effects on the perm-selectivity of the Pd-Ag membranes.     

Fabrication of nanostructured deoxidized low-phosphorous copper processed by three-layer stack accumulative roll-bonding

A nanostructured deoxidized low-phosphorous copper (DLPC) was fabricated by three-layer stack accumulative roll-bonding (ARB) process. The microstructural evolution and the variation of mechanical properties with three-layer stack ARB were investigated in detail. It was found that the microstructure has been evolved from a dislocation cell structure to ultrafine grained structure as the number of ARB cycles increases. In addition, the mean spacing of grain boundaries, which was 45 microm in initial material, reduced to 2.1 microm after 1 cycle, 360 nm after 3 cycles, 250 nm after 5 cycles, then 170 nm after 7 cycles, progressively. The fraction of high-angle grain boundaries after 1-cycle ARB was no more than 0.27, but it increased with the number of ARB cycles, and became surprisingly more than 0.7 after 7-cycle ARB. The tensile strength increased with the number of ARB cycles, and then after 7 cycles it reached about 600 MPa, which is about 2.5 times higher than that of the initial material. Therefore, the three-layer stack ARB is very effective for development of ultrafine grains and high strengthening of DLPC alloy.     

Effect of cooling rate in overageing and other thermomechanical process parameters on grain refinement in an AA7475 aluminium alloy

Fine-grained AA7475 aluminium alloy sheets were produced in this study by a thermomechanical treatment involving solution anneal, overageing, rolling and recrystallization steps. It has been found that the cooling rate after the intermediate overageing treatment should be fast to obtain the finest grain size. The fast cooling rate ensured the presence of relatively large particles of MgZn₂ and some supersaturation prior to cold rolling. Generally, the final grain structure was heterogeneous, with bands of fine grains lying parallel to the rolling direction. In material rapidly cooled after overageing, bands of fine grains were also observed in the transverse direction and these bands were associated with shear bands formed during rolling. The fine-grained AA7475 alloy sheets with an average grain size of about 9 m showed large tensile elongations of about 800% when deformed at 516 °C and with an initial strain rate of 5×10^–4s^–1.     

Preparation of nanocomposite materials containing WS2, δ-WN,Fe3O4, or Fe9S10 in a silica aerogel host

Nanocomposite materials based on silica aerogel hosts have been produced using chemical vapour infiltration/decomposition methods and characterized by X-ray diffraction and electron microscopy. Amorphous tungsten in SiO₂ aerogel was formed by the decomposition of W(CO)₆ at 250 °C. Alternatively, reaction of this material with sulphur at 700 °C produced needle-shaped WS₂ crystals with lengths ranging from 25–230 nm. Reaction of the W/SiO₂ composite with anhydrous NH₃ formed crystals of -WN with diameters of 1–5 nm. Fe(CO)₅ is readily absorbed into the silica aerogel, forming an amorphous iron oxide/SiO₂ composite after slow oxidation in air. Treatment of this material with additional Fe(CO)₅ produced an Fe₃O₄/SiO₂ aerogel composite. Fe₃O₄ particle sizes were 20–55 nm. After additional heat treatment, this composite exhibited soft ferromagnetic behaviour with a coercivity of 170 Oe. Fe₉S₁₀ crystals with diameters of 30–90 nm were formed by the reaction of the amorphous iron oxide/SiO₂ composite with H₂S at 900 °C.     

Evaluation of the structure and properties of steel 35KhN2MFA under static and dynamic loading

We investigated the effect of the structure of heat-treated steel 35KhN2MFA on the mechanical properties under static and dynamic loading; the conditions of heat treatment were: hardening from 740–1100°C, isothermal quenching from 360°C, tempering at 300–550°C, heat cycling. It was established that a higher hardening temperature entails greater endurance: from 5700 cycles (T_h=740°C) to 10,100 cycles (T_h=1100°C), yet the required strength (_u=1740 MPa) is retained, and ductility and impact toughness are somewhat reduced. Isothermally quenched and high-tempered specimens have high endurance (N=12,000–13,500 cycles) but a low level of strength (_u=1330 MPa). Specimens subjected to heat cycling under the conditions hardening from 860°C and tempering at 650°C have the best complex of mechanical properties under static and cyclic loading.Translated from Problemy Prochnosti, No. 1, pp. 17–20, January, 1992.     

Wetting behaviour and mullite formation at the interface of inviscid melt-spun CaO-Al2O3 fibre-reinforced Al-Si alloy (4032) composite

Inviscid melt-spun calcia-alumina fibre-reinforced aluminium-silicon alloy (4032) composites were produced using a melt-infiltration technique. Scanning electron microscopy, energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) were used to investigate interfacial wetting and interphase formation, and identify the crystalline phase of the interphase of these composites. The composites processed at 700°C showed a good interfacial wetting and silicon accumulation at the interface. The composites processed at 927°C showed formation of an interphase region of about 10–20 m thick, as well as excellent interfacial wetting. EDS analysis gave averaged compositions of this interphase region at 74 wt % Al and 26 wt % Si, which corresponds to the composition of mullite (3Al₂O₃ · 2SiO₂). The formation of mullite at the interface was confirmed by XRD analysis.     

Effect of thermomechanical treatment on the properties of Fe-11Al and Fe-14Al alloys

Fe-Al alloys have the potential to be relatively inexpensive soft magnetic materials if their formability could be improved. An investigation has been made on the effect of thermomechanical treatment on the properties of Fe-11 wt%Al and Fe-14 wt%Al alloys (designated Fe-11Al and Fe-14Al respectively). For the former the room temperature mechanical properties were found to be determined principally by the recrystallised grain size. A good combination of properties for Fe-11Al, i.e. high strength and ductility, was obtained when the grain size was less than about 100 m. The small grain size was produced by warm rolling at 600°C followed by 1 hour annealing at 600–700°C. On the other hand hot rolling followed by annealing resulted in large grain size, hence rendered the alloy brittle. The cold formability also exhibited a grain size dependence, with the Fe-11Al alloy with a fine recrystallised grain size having good cold rollability. In contrast Fe-14Al was brittle irrespective of the treatment given; ductility of less than 1% was observed in all cases and the cold rollability was limited. Ordering was not seen to be a factor affecting the observed mechanical properties and rollability of either alloy as all the thermomechanical treatments, other than an ordering treatment of 500 hours at 400°C, resulted in a disordered structure. The stress required to work these alloys at elevated temperatures were estimated from compression tests and it is apparent that for Fe-11Al the stress is greatly reduced (50%) from the room temperature value at 600°C and that at 750°C both alloys required a similar stress which was about 15% of the room temperature value. The magnetic properties of Fe-11Al compared favourably with Fe-14Al; the former has a higher saturation induction, a similar coercive force but a lower permeability than Fe-14Al.     

High-temperature flow behaviour and concurrent microstructural evolution in an Al-24 wt% Cu alloy

Tensile specimens of an Al-24 wt% Cu alloy of grain sizes in the range 7.6–20.6 m were deformed at 400–540 °C using constant initial strain rates ranging from 5×10^–6 to 2×10^–2 s^–1. Initially the stress-strain (-) curves show work hardening which is followed by strain softening at higher strain rates and lower temperatures. At lower strain rates and higher temperatures, on the other hand, continues to increase with strain or tends to be independent of strain. Grain growth and cavitation occur to varying extents depending on strain rate and test temperature. While the grain growth can account for the work hardening at higher temperatures as well as at lower strain rates, it fails to do so at higher strain rates. The strain softening is associated with cavitation. The presence of non-steady-state flow influences the parameters of the constitutive relation to varying extents.     

Characterization of pyrolysed silacyclobutasilazane

Silacyclobutasilazane (SCBZ) is a candidate organosilicon polymer suited to many hightemperature applications. Pyrolysis of SCBZ occurs over two distinct temperature ranges 400–800 and 1400–1800 °C. X-ray diffraction analysis showed that amorphous SCBZ transforms to crystalline SiC above 1400 °C. Thin foils of the pyrolysed (1800 °C) SCBZ were prepared and examined using the analytical electron microscope. The material was found to contain 200 nm-SiC particles scattered through a matrix consisting of 50 nm radiating clusters of pyrolytic graphite crystals and a highly carbonaceous amorphous background. It is suggested that during 1400–1800 °C pyrolysis two major reactions occurred. Initially, SCBZ through nitrogen loss, decomposed to crystalline SiC and amorphous carbon. Subsequent graphitization then produced the radiating crystal clusters.     

Superplastic flow in a nanostructured aluminum alloy produced using high-pressure torsion

Samples of a spray-cast Al-7034 alloy were processed by high-pressure torsion (HPT) at temperatures of 293 or 473 K using an imposed pressure of 4 GPa and torsional straining through five revolutions. Processing by HPT produced significant grain refinement with grain sizes of 60 and 85 nm at the edges of the disks for the two processing temperatures. In tensile testing at room temperature, the alloy processed by HPT exhibited higher strength and lower ductility than the unprocessed material. Good superplastic properties were achieved in tensile testing at elevated temperatures with a maximum elongation of 750% for the sample processed at 473 K and tested in tension at 703 K under an initial strain rate of 1.0 × 10⁻² s⁻¹. The measured superplastic elongations are lower than in samples prepared by equal-channel angular pressing because of the use of very thin disks in the HPT processing.     

Fabrication and mechanical properties of silicon carbide–silicon nitride nanocomposites

A powder mixture of ultrafine –SiC–35 wt% –Si₃N₄ containing 6 wt% Al₂O₃ and 4 wt% Y₂O₃ as sintering additives were liquid–phase sintered at 1800°C for 30 min by hot–pressing. The hot–pressed composites were subsequently annealed at 1920°C under nitrogen–gas–pressure to enhance grain growth. The average grain–size of the sintered bodies were ranged from 96 to 251 nm for SiC and from 202 to 407 nm for Si₃N₄, which were much finer than those of ordinary sintered SiC–Si₃N₄ composites. Both strength and fracture toughness of fine–grained SiC–Si₃N₄ composites increased with increasing grain size. Such results suggested that a small amount of grain growth in the fine–grained region (250 nm for SiC and 400 nm for Si₃N₄) was beneficial for mechanical properties of the composites. The room–temperature flexural strength and fracture toughness of the 8–h annealed composites were 698 MPa and 4.7 MPa · m^1/2, respectively.     

Investigation of the kinetics of dissolution of metals in metallic melts by the rotating disk method (mechanism of dissolution of iron in aluminum)

Analysis of the mechanism of dissolution of an armco-iron disk rotating at 60–600 rpm in liquid aluminum at 700–800°C showed that, at 700–750° C in the initial stages (first 152–30 min) of the process, the rate of dissolution is reduced due to the formation of a protective -phase layer. When the disk rotation speed and the reaction time and temperature are increased, the rate of dissolution becomes faster than the rate of diffusional dissolution according to V. G. Levich. An expression was derived for the dissolution rate in the presence of a protective surface layer whose thickness increases with time.     

Texture evolution of AA3003 aluminum alloy sheet produced by accumulative roll bonding

The accumulative roll bonding process was carried out on an AA3003 aluminum alloy sheet up to eight cycles. The electron backscattering diffraction (EBSD) method was employed to investigate the microtextural development in the ARB processed sheets. The results indicate that with increasing the number of cycles, the overall texture intensity increases even up to the eighth rolling pass and a strong texture develops. The main textural components are the copper and Dillamore components of which the intensities increase with increasing number of cycles. Measurement of microhardness and lamellar spacing of grains in the processed sheets revealed that the presence of second phase particles in this aluminum alloy can promote the occurrence of dynamic recovery during the ARB process.     

The evaluation of hot-working characteristics of cobalt-based implant alloys

The hot-working characteristics of wrought Co-Ni-Cr-Mo implant alloy during ingot-to-billet conversion were evaluated using a Gleeble-2000A simulator. The hot tensile test at 700–1 320 °C was used to determine the optimum hot-working parameters at a strain rate equivalent to that of conventional press forging to ensure acceptable hot workability. Hot ductility and deformation resistance as a function of temperature can be clearly established. The fracture surfaces of the tensile specimens were examined to correlate them with the hot tensile ductility values at various temperatures. The poor ductility at temperatures above 1300 °C was attributed to the incipient melting of grain boundaries. The effect of temperature and strain rate on the flow-stress behaviour and microstructures were investigated by uniaxial compression testing in the temperature range 900–1200 °C and strain rate, , range of 0.01–10s^–1. The strain-hardening and steady-state behaviour were described from the measured true stress-true strain curves.     

Accumulative roll bonding of aluminum alloys 2219/5086 laminates: Microstructural evolution and tensile properties

The present study describes the course of microstructure evolution during accumulative roll bonding (ARB) of dissimilar aluminum alloys AA2219 and AA5086. The two alloys were sandwiched as alternate layers and rolled at 300 °C up to 8 passes with 50% height reduction per pass. A strong bonding between successive layers accompanied by substantial grain refinement (∼200–300 nm) is achieved after 8 passes of ARB. The processing schedule has successfully maintained the iso-strain condition up to 6 cycles between the two alloys. Afterwards, the fracture and fragmentation of AA5086 layers dominate the microstructure evolution. Mechanical properties of the 8 pas ARB processed material were evaluated in comparison to the two starting alloy sheets via room temperature tensile tests along the rolling direction. The strength of the 8 pass ARB processed material lies between that of the two starting alloys while the ductility decreases after ARB than that of the two constituent starting alloys. These differences in mechanical behavior have been attributed to the microstructural aspects of the individual layer and the fragmentation process.     

Spray deposition of an iron aluminide

High velocity oxyfuel (HVOF) spray forming of an iron aluminide [Fe–12.5 Al–2.93 Ni–0.02 B (wt%), containing 300 p.p.m. oxygen], followed by heat treatment for 24 h at 500°C, 18 h at 600°C and 20 min at 800°C, and multipass hot rolling at 800°C has been studied. Three different thicknesses (0.43, 0.93 and 1.33 mm) of sprayed deposit were produced by spraying for different times (approximately 10, 20 and 30 min). The spray-deposited layers exhibited some oxide and some porosity. This porosity was reduced by heat treatment. The as deposited layer had a high degree of B2 order, and a B2 antiphase domain size of 4.5 nm. On hot rolling this material to a reduction of 38%, it was found to be more susceptible to edge cracking than similar material processed by an ingot–extrusion–hot rolling route. In heat treatment, the aluminide-sprayed layer formed a non-protective Fe2O3 oxide, rather than the usual Al2O3 that forms on the binary alloy. This is attributable to the Ni content of the iron aluminide powder employed. © 1998 Kluwer Academic Publishers