Lamellar Boundary Spacings Research Articles

3D characterization of a nanostructured Al-Cu-Mg alloy

Three-dimensional (3D) characterization of variations in crystallography and chemistry of nanostructured metals will provide vital information to understand their mechanical and thermal behaviours. This study applied a surface sliding friction treatment (SSFT) at liquid nitrogen temperature to produce nanostructured surface layers in a peak-aged Al-Cu-Mg alloy. The nanostructured surface was characterized by means of 3D orientation mapping in the transmission electron microscope (3D-OMiTEM) and atom probe tomography (APT). 3D-OMiTEM results revealed a lamellar structure with an average lamellar boundary spacing of 26 nm at the topmost surface layer (depth < 20 μm), which is much finer than normally achievable in commercial purity Al deformed to high strain levels. Based on the 3D-OMiTEM data, a five-parameter grain boundary character analysis was carried out. It was found that low angle grain boundaries dominate the nanoscale structure and that the grain boundary plane distribution of high angle lamellar grain boundaries shows a preference around {101}. APT analysis showed segregation of Cu and Mg atoms at lamellar boundaries, which is believed to play a role in stabilizing the boundaries and enhancing the structural refinement during SSFT.

Segregation and precipitation stabilizing an ultrafine lamellar-structured Al-0.3%Cu alloy

Understanding the coarsening mechanisms and the role of solute atoms during recovery annealing of ultrafine lamellar-structured alloys produced by high strain deformation is crucial to tailor their microstructures and mechanical properties. In the present work, a lamellar-structured Al–0.3%Cu alloy with a boundary spacing of 200 nm was prepared by cold rolling to a von Mises strain of 4.5 (a thickness reduction of 98%), featuring Cu segregation to high angle lamellar boundaries. During recovery annealing in the temperature range of 100–175 °C, precipitation of fine Al2Cu particles occurred preferentially at lamellar boundaries. Recovery kinetics was analyzed based on measurements of lamellar boundary spacings in the annealed samples, showing an increase in the apparent activation energy from 77 kJ/mol at the beginning to 106 kJ/mol at the end of recovery. In situ observations of annealing in a transmission electron microscope revealed that the dominant coarsening process is the motion of Y-junctions formed by lamellar boundaries, which is subjected to various degrees of pinning from dislocations, dislocation boundaries and particles. Furthermore, it was found that this local pinning effect can be reinforced with the increase of misorientation angles of the attached dislocation boundaries, the coarsening of Al2Cu particles and the combined effect of interconnecting boundaries and particles. The results underpinned the importance of alloying elements in stabilizing finely spaced lamellar structures during deformation and annealing, providing guidelines for tailoring stable ultrafine structured alloys.

Microstructural Evolution and Mechanical Properties of Heavily Cold-Rolled and Subsequently Annealed Cu-3 wt.%Ti Alloys with Nano-Lamellar Structure

Changes in the microstructure and mechanical properties of Cu-Ti alloy sheets during cold rolling and subsequent annealing were studied with and without Cu-Ti precipitates in the initial microstructures. Pre-existing Cu-Ti precipitates plastically deformed and severely elongated in the rolling direction via cold rolling, accelerating the formation of a nano-lamellar structure. A mean lamellar boundary spacing of 20 nm was achieved at an equivalent strain of 6.7. Vickers hardness increased with decreasing lamellar boundary spacing. Subsequent annealing after cold rolling clearly enhanced the mechanical properties, and a maximum Vickers hardness value of 443 HV (4.35 GPa) was attained in the sample with pre-existing precipitates strained to 6.7. These results suggest that pre-existing precipitates promote microstructural refinement during heavy cold rolling, and that heavily cold-rolled Cu-Ti alloys exhibit anneal hardening behavior, leading to excellent mechanical properties.

Bulk Nanolaminated Nickel: Preparation, Microstructure, Mechanical Property, and Thermal Stability

A bulk nanolaminated (NL) structure with distinctive fractions of low- and high-angle grain boundaries (f LAGBs and f HAGBs) is produced in pure nickel, through a two-step process of primary grain refinement by equal-channel angular pressing (ECAP), followed by a secondary geometrical refinement via liquid nitrogen rolling (LNR). The lamellar boundary spacings of 2N and 4N nickel are refined to ~ 40 and ~ 70 nm, respectively, and the yield strength of the NL structure in 2N nickel reaches ~ 1.5 GPa. The impacts of the deformation path, material purity, grain boundary (GB) misorientation, and energy on the microstructure, refinement ability, mechanical strength, and thermal stability are investigated to understand the inherent governing mechanisms. GB migration is the main restoration mechanism limiting the refinement of an NL structure in 4N nickel, while in 2N nickel, shear banding occurs and mediates one-fifth of the total true normal rolling strain at the mesoscale, restricting further refinement. Three typical structures [ultrafine grained (UFG), NL with low f LAGBs, and NL with high f LAGBs] obtained through three different combinations of ECAP and LNR were studied by isochronal annealing for 1 hour at temperatures ranging from 433 K to 973 K (160 °C to 700 °C). Higher thermal stability in the NL structure with high f LAGBs is shown by a 50 K (50 °C) delay in the initiation temperature of recrystallization. Based on calculations and analyses of the stored energies of deformed structures from strain distribution, as characterized by kernel average misorientation (KAM), and from GB misorientations, higher thermal stability is attributed to high f LAGBs in this type of NL structure. This is confirmed by a slower change in the microstructure, as revealed by characterizing its annealing kinetics using KAM maps.

Linking recovery and recrystallization through triple junction motion in aluminum cold rolled to a large strain

Recovery mechanisms and kinetics have been studied in commercial purity aluminum (AA1050) cold rolled to a true strain of 5.5 (99.6% thickness reduction) and annealed at low temperatures from 140 to 220°C. Transmission electron microscopy, electron backscatter diffraction (EBSD) and electron channeling contrast (ECC) are used to characterize the microstructural evolution during annealing. The microstructural characterization shows that a deformed lamellar structure coarsens uniformly during annealing by triple junction motion while maintaining the lamellar morphology, leading to a gradual transition into a more equiaxed structure, where recrystallization nuclei start to evolve. The apparent activation energy for the microstructural coarsening is estimated separately for different stages characterized by an increase in the lamellar boundary spacing measured by EBSD and ECC. The apparent activation energy increases during annealing, from 110kJmol−1 at the beginning to 230–240kJmol−1 at the end of uniform coarsening, linking the recovery stages to recrystallization. The increase in activation energy underpins operation of different diffusion mechanisms for migration of boundaries and their junctions during coarsening, and solute drag may become increasingly important as the structure coarsens. These findings form the basis for a discussion of the thermal behavior of a fine lamellar structure produced by cold rolling to a large strain of both scientific and applied interest.

Recovery and recrystallization in commercial purity aluminum cold rolled to an ultrahigh strain

Recovery and recrystallization were studied in commercial purity aluminum cold rolled to an ultrahigh strain (εvM=6.4) and isothermally annealed at 300°C. The deformed material consists of three layers with similar fractions of high-angle boundaries (HABs) and similar lamellar boundary spacings, but with different textures and different spatial arrangements of the rolling texture components. Annealing leads initially to a coarsening of the lamellar microstructure, accompanied by a reduction in the HAB fraction. Ex-situ experiments using very short annealing times indicate that such microstructural changes are consistent with a process of coarsening via triple junction motion. The recovery proceeds similarly in the center and subsurface layers, but because of the different initial spatial arrangement of the texture components in these layers, the loss of HABs is significantly greater in the subsurface compared with the center layer. Further annealing leads to discontinuous recrystallization, which occurs differently in the center and subsurface layers. In the center layer, recrystallization proceeds more rapidly and with a larger frequency of nuclei, resulting in a smaller recrystallized grain size. In contrast, pronounced recrystallization in the subsurface layers is delayed, and the recrystallized grain size is larger than in the center. It is concluded that the changes taking place during recovery are very significant in determining the subsequent recrystallization behavior in terms of the final grain size and texture.

Triple Junction Motion – A New Recovery Mechanism in Metals Deformed to Large Strains

A phenomenologically new recovery mechanism – triple junction motion is presented. This recovery mechanism is found to be the dominant one at low and medium temperatures in highly strained aluminum, which has a very fine microstructure, composed of lamellae with the thickness of a few hundred nanometers. Triple junction motion leads to removal of thin lamellae and to a consequent increase of the thickness of neighboring lamellae. This recovery mechanism therefore increases the average lamellar boundary spacing and causes a gradual transition from a lamellar structure to a more equiaxed structure preceding recrystallization.

Microstructure and Mechanical Properties of Al-0.5 at.% X (=Si, Ag, Mg) Alloys Highly Deformed by ARB Process

Effect of solid solution elements on microstructure evolution and mechanical properties was investigated using a high purity Al (purity 99.99%) and Al-0.5 at.% X ( X = Si, Ag, Mg ) alloys deformed by accumulative roll bonding (ARB) process up to 7 cycles (equivalent strain of 5.6) at ambient temperature. The ARB-processed high purity Al showed the equiaxed microstructure having mean grain size of 750 nm. On the other hand, the microstructure of the ARB-processed Al-0.5at.%X alloys showed lamellar boundary structures elongated along RD. The mean lamellar boundary spacing significantly differed depending on the alloying elements, which suggested that solute atoms had a significant effect on microstructure evolution. The difference in the grain size was regarded to be caused by the difference in recovery processes in the alloys. The tensile strength of the alloys increased with increasing the number of ARB cycles. In the Al-Si and Al-Ag alloys, the post-uniform elongation increased with increasing the number of the ARB cycles. On the other hand, the elongation of the Al-Mg hardly changed during the ARB process.

Microstructure and mechanical properties of commercial purity titanium severely deformed by ARB process

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.

The tensile behavior and deformation microstructure of cryo-rolled and annealed pure nickel

The deformation microstructures of room temperature and cryo-rolled commercially pure nickel to a thickness reduction of 95% were studied by transmission electron microscopy (TEM). Lamellar boundary spacings and boundary misorientations were measured and analyzed. It was found that the only difference between these two microstructures was boundary spacing. After annealed under different annealing conditions, the tensile behaviors of rolled and annealed pure nickel were studied. It was found that the higher the strength after annealing, the lower the ductility. The combination of high strength and high ductility behavior found in copper by cryo-rolling and annealing was not found in the present research.

The mechanics of deformation–induced subgrain–dislocation structures in metallic crystals at large strains

We present a streamlined limiting case of the theory of Oritz & Repetto for crystals with microstructure in which the crystals are assumed to exhibit infinitely strong latent hardening. We take this property to signify that the crystal must necessarily deform in single slip at all material points. This requirement introduces a non–convex constraint that renders the incremental problem non–convex. We have assessed the ability of the theory to predict salient aspects of the body of experimental data compiled by Hansen et al. regarding lamellar dislocation structures in crystals deformed to large strains. Although the comparisons with experiment are somewhat indirect, the theory appears to correctly predict salient aspects of the statistics of misorientation angles and lamellar–boundary spacings, and the scaling of the average misorientation and spacing with increasing macroscopic strain.

Microstructural evolution during accumulative roll-bonding of commercial purity aluminum

The microstructure in commercial purity aluminum deformed from medium to high strain ( ε vM=1.6–6.4) by accumulative roll-bonding (ARB) at 473 K was quantitatively examined by transmission electron microscopy. It was found that a sub-micrometer lamellar structure characterizes the microstructure at high strains ( ε vM>1.6), and that the lamellar boundary spacing decreases and the misorientation across the lamellar boundaries increases with increasing rolling strain. This characteristic evolution has also been observed during conventional cold-rolling of commercial purity aluminum. However, a comparison between the two processes shows a significant difference in the evolution of the microstructural parameters. These differences are discussed based on the different processing conditions characterizing ARB and conventional rolling, respectively.