Fabrication and characterization of Ti–Al intermetallics through accumulative roll bonding

Accumulative roll bonding (ARB) imposes severe plastic strain on materials without changing the specimen dimensions. ARB process is mostly appropriate for practical applications because it can be performed readily by the conventional rolling process. An Al-2wt%Cu alloy was subjected to ARB process up to a strain of 4.8. Stacking of materials and conventional roll-bonding are repeated in the process. In this study, corrosion behavior of an Al-2wt%Cu alloy fabricated by ARB process was studied in 3.5%wtNaCl solution using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The morphology of structures was analyzed by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX). Also, the electrochemical experiments showed that corrosion resistance of samples decreases with increasing the number of ARB cycles due to the formation of oxide layer on defects and energetic regions such as grain boundaries with low/high angle and high density dislocations accumulated in sub-grains. According to the nyquist curves, by continuing the process, the diameters of semicircles decreased and the corrosion resistance and the polarization resistance subsequently decreased. After 6-cycle ARB, link up of small pits and micro crack were seen. Also, with increasing the number of the ARB cycles, the mean grain size of specimens decreased and it reached to 650 nm after 6 cycles of ARB process.

The Accumulative-Roll Bonding (ARB) process was applied to an aluminum based MMC fabricated by a sheath rolling method, in order to improve the mechanical properties. The 8 cycles of ARB were performed at ambient temperature under unlubricated conditions. The ARB process was also performed on the unreinforced material, for comparison with the composite sample. The tensile strength of the composite increases with the number of cycles up to 5 cycles of ARB, above which it decreases slightly. The tensile strength of the unreinforced material increases up to 2 cycles of ARB, above which it does not change. The strengthening in the composite by the ARB process is caused by the improvement in homogeneity of distribution of reinforcement particles with the number of cycles as well as work hardening of the matrix material, while the strengthening in the unreinforced material is only caused by work hardening. The drop in strength for both materials is caused by recovery or recrystallization. On the other hand, the elongation tends to greatly decrease after I cycle, but from 2 cycles it increases with the number of cycles for both materials. The unreinforced material shows an inhomogeneous microstructure along the thickness direction at lower cycles of ARB; having a finer grained structure near surface and bonded interfaces than the other area. However, the inhomogenity of microstructure vanishes gradually as the number of cycles increases. As a result, the microstructure after 8 cycles consists almost entirely of ultra-fine grains under 1 μm in diameter along the thickness direction. The ultra-fine grain initiate to develop at lower ARB cycle in composite than in unreinforced material. This is probably due to enhanced strain induced by locally accumulated deformation around reinforcement particles.

Commercial purity titanium was deformed by accumulative roll-bonding (ARB) process up to 8 cycles (equivalent strain of 6.4) at ambient temperature. This is the first study on ultra-high straining of h.c.p. metals by the ARB process. The microstructure of the ARB-processed specimens showed two kinds of characteristic ultrafine microstructures. One was the lamellar boundary structure elongated along RD, which has been also reported in the ARB-processed cubic metals. The lamellar boundary spacing decreased with increasing ARB strain and reached about 80 nm after 5 ARB cycles. The other microstructure was the equiaxed grains having mean grain size of 80–100 nm. Such a fine and equiaxed grain structure has not yet been reported in the as-ARB-processed materials before. The fraction of the equiaxed grains increased as the ARB process proceeded, and 90% of the specimen was filled with the equiaxed grains after 8 ARB cycles. As the number of the ARB process increased, the tensile strength increased and the total elongation decreased gradually. After 6 ARB cycles, the specimen exhibited almost the same mechanical properties as that of commercial Ti-6Al-4V alloy.

Evolution of microstructure and texture was studied in severely plastically deformed (up to an equivalent strain of 6.4) high purity (99.99%) Ni sheets processed through Accumulative Roll Bonding (ARB). As received Ni plates (~ 10 mm in thickness) were cold rolled to ~ 80% reduction in thickness (~ 2 mm) and vacuum annealed at 600°C for one hour and these were used as the starting materials (average grain size ~ 25 μm) for the subsequent ARB processing. ND and TD plane normal sections of the ARB processed sheets were subjected to Electron Back Scatter Diffraction (EBSD) and Transmission Electron Microscope (TEM) studies. The ARB processed Ni sheets were found to be filled with ultrafine grains (average grain size ~ 400 nm) after 8 cycles of ARB. Extensive shear band formation was observed particularly in the high cycle ARBed materials. The deformation textures were found to be quite inhomogeneous at the low cycle regime of the ARB. However, the deformation texture achieved remarkable homogeneity after 6 and 8 cycles of ARB and S ({123} ) component of the deformation texture was found to be quite strong.

Feasibility study of producing Al–5Zn–1 Mg alloy by accumulative roll bonding process and subsequent heat treatment

In this research, Al-Ni particle composite strips are formed by accumulative roll bonding (ARB) process using Al strips and Ni powder. The rule of ARB cycles and volume percentage (Vol%) of Ni powder on the microstructure, wear resistance and mechanical properties of the formed composites are investigated. According to the tensile test results, the yield stress and tensile strengths of the Al -Ni (p) composites tend to increase with rising of the ARB cycles. Ductility of the ARB samples significantly decreased in the first cycle of the ARB process and then elevated lightly from the second pass of the ARB. Furthermore, the yield stress and tensile strengths of the Al - Ni (p) composites with different vol% of Ni powder, increased with increasing the amount of Ni Powder. Also the hardness and wear resistance of produced composites were investigated. Micro hardness and wear resistance of these composites increased with increasing the number of ARB cycles and the amount of Ni particles content during ARB Process.

Investigation into microstructural and mechanical properties of Al–Sn alloy fabricated using accumulative roll bonding combined with heat treatment

The fracture behavior of commercially pure titanium (CP-Ti) processed by accumulative roll bonding (ARB) was investigated in this study. Monolithic and Ti-SiC composite samples were first produced by ARB process and then subjected to uniaxial tensile testing at room temperature. Two different ductile fracture mechanisms including shear dimple rupture and equiaxed dimple rupture were observed in the initial and final ARB cycles, respectively. The difference in the rupture mechanism was attributed to different stress states at the crack tip and different densities of metallurgical defects. A non-uniform distribution of dimple size was obtained for fracture surfaces of the samples processed by a low number of ARB cycles. This was attributed to the heterogeneity of microstructure in the primary ARB cycles. The fracture surface of the samples processed by high ARB cycles represented more uniform dimples. This was attributed to more homogenous microstructure in the final cycles. Moreover, SiC particles showed a major role in fracture of samples, so that they affected the size and depth of the dimples, as well as the number of ARB cycles in which the transmission of the fracture mechanism occurred.

Elongation increase in ultra-fine grained Al–Fe–Si alloy sheets

Ultra-fine grained AA8011 alloy sheets manufactured by the accumulative roll-bonding (ARB) process exhibited unique tensile deformation behavior. Tensile strength of the ARB processed AA8011 sheets increased up to three cycles, but then showed nearly the same value after three cycles. Meanwhile, the total elongation grew significantly with an increasing nember of ARB cycles. It was found that the strain-rate sensitivities (m) of the AA8011 sheets increased up to 0.047 by the ARB process. A large number of high-angle boundaries were introduced by the ARB process and the fraction of high-angle boundaries reached 70% after eight ARB cycles. In this paper, we discusse the increase in total elongation on the basis of strain-rate sensitive deformation of the material, which is also correlated with dynamic recovery.

It is known that the severe plastic deformation (SPD) induced by Accumulative Roll Bonding (ARB) results in more important grain refinement as compared to conventional rolling. Since ARB enables production of large amounts of ultra-fine grained (UFG) materials, its adoption into industrial practice is favoured. The paper presents the results of a study of high purity aluminium processed by ARB and cold rolling. Microstructure changes induced by both methods were studied by light and transmission electron microscopy. Dislocation density and arrangement were assessed by positron annihilation spectroscopy. Strength evolution was estimated by hardness measurements. Texture measurements were performed by X-ray diffraction. ARB processing results in over twofold increase in hardness. Hardness increases significantly after two ARB cycles and it raises only a little or decreases during subsequent cycles. The increase in hardness induced by conventional rolling is smaller. Positron lifetime measurements reveal a substantial increase of dislocation density at the first ARB cycle and a moderate increase or even a decrease at further cycles. The high fraction of positrons trapped at grain-boundary dislocations gives evidence for substantial grain refinement confirmed by TEM examinations. Grain size of 1.2 m in the rolling plane and as small as of 90 nm in the normal direction is obtained. The rolled samples have a typical rolling texture (-fibre). The - fibre of the sample ARB processed to strain of 2.4 is weaker as compared to its rolled counterpart and it presents through thickness variations. The surface layers do not have any -fibre orientations but they have ND-rotated cube texture formed by the shear strains induced by lubricant-free rolling.

On the microstructure and texture of intermetallics in Al/Mg/Al multi-layer composite fabricated by Accumulative Roll Bonding

Microstructure, texture and mechanical properties of AA 1060 aluminum alloy processed by cryogenic accumulative roll bonding

An ultra low carbon interstitial free (IF) steel was severely deformed by the six-layer stack accumulative roll-bonding (ARB) process for improvement of the mechanical properties. As-received material with 1 mm in thickness showed a recrystallization structure with average grain diameter of 27 μm. The ARB was conducted at ambient temperature after deforming the as-received material to 0.5 mm thick by cold rolling. The ARB was performed for six-layer stacked, i.e. 3 mm thick sheet, up to 3 cycles (an equivalent strain of ∼7.1). In each ARB cycle, the stacked sheets were, first, deformed to 1.5 mm thick by the first pass, and then reduced to 0.5 mm thick, equals to the starting thickness, by multipass rolling without lubrication. The specimen after 3 cycles of ARB was annealed for 1.8 ks at various temperatures ranging from 673 K to 1073 K. The tensile strength of the ARB processed materials increased largely with the number of ARB cycles, after 3 cycles it reached a maximum of 1.12 GPa, which is about 4 times larger than that of the initial material. The elongation dropped largely after the cold rolling prior to the ARB, however it remains almost constant during the subsequent ARB process. Transmission Electron Microscopy revealed that the ARB processed materials exhibited a dislocation cell and/or subgrain structure with relatively high dislocation density. The selected area diffraction (SAD) patterns suggested that the orientation difference between neighboring cells was very small. The annealing up to 873 K resulted in gradual decrease in the strength due to the static recovery. The annealing above 873 K resulted in recrystallization and normal grain growth, and thereby a significant drop in the strength and recovery in ductility.

Accumulative Roll Bonding (ARB) is a technique of grain refinement by severe plastic deformation, which involves multiple repetitions of surface treatment, stacking, rolling, and cutting. The rolling with 50% reduction in thickness bonds the sheets. After several cycles, ultrafine-grained (UFG) materials are produced. Since ARB enables the production of large amounts of UFG materials, its adoption into industrial practice is favoured. ARB has been successfully used for preparation of UFG sheets from different ingot cast aluminium alloys. Twin-roll casting (TRC) is a cost and energy effective method for manufacturing aluminium sheets. Fine particles and small grain size are intrinsic for TRC sheets making them good starting materials for ARB. The paper presents the results of a research aimed at investigating the feasibility of ARB processing of three TRC alloys, AA8006, AA8011 and AA5754, at ambient temperature. The microstructure and properties of the ARB were investigated by means of light and transmission electron microscopy and hardness measurements. AA8006 specimens were ARB processed without any problems. Sound sheets of AA8011 alloy were also obtained even after 8 cycles of ARB. The AA5754 alloy suffered from severe edge and notch cracking since the first cycle. The work hardening of AA8006 alloy saturated after the 3rd cycle, whereas the hardness of AA5754 alloy increased steadily up to the 5th cycle. Monotonous increase in strength up to 280 MPa was observed in the ARB processed AA8011 alloy.