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 (ε_vM = 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.     

Cold rolling and recrystallization textures of a Ni–5 at.% W alloy

The formation of recrystallization texture has been studied in a sintered Ni–5 at.% W alloy after heavy cold rolling (～95%) and annealing. Although the cold-rolled texture is a typical pure metal or Cu-type deformation texture on a global scale, variations in microstructure and microtexture are found in the deformed material between locally sheared regions and away those from these regions. The primary recrystallization texture consists of the cube ({1 0 0}〈0 0 1〉), a RD-rotated cube ({0 1 3}〈1 0 0〉) and twin-related orientations of these two components. The presence of both cube and the RD-rotated orientations are identified in thin bands of materials in the deformed matrix. However, predominantly cube-oriented grains nucleate and grow in regions away from the locally sheared regions. In contrast, the nucleation and growth of non-cube grains are observed in the vicinity of locally sheared regions. The formation of cube texture in Ni–5 at.% W alloy appears to occur primarily via the oriented nucleation of cube grains owing to the special properties of the cube bands.     

Distribution characterization of boundary misorientation angle of 7050 aluminum alloy after high-temperature compression

Compression tests of 7050 aluminum alloy have been conducted at different temperatures (340, 380, 420, and 460 °C) and different strain rates of 0.1, 1, 10, and 100 s^?1. The microstructure characteristics of the alloy after deformation are investigated using OM, electron backscatter diffraction (EBSD) technique and TEM. Results show that the volume fraction of recrystallized grains and the average misorientation angle increase with the increase of deformation temperature with the strain rate of 0.1 s^?1. When the 7050 aluminum alloys were deformed at 460 °C, the volume fraction of recrystallized grains and average misorientation angle decrease with increasing strain rate. The primary softening mechanism of the 7050 aluminum alloy deformed at 340, 380, and 420 °C with the strain rate of 0.1 s^?1 is dynamic recovery. Dynamic recrystallization is the main softening mechanism of the alloy deformed at 460 °C and different strain rates. The softening mechanism of the alloy is not sensitive to strain rate.     

Survey of computed grain boundary properties in face-centered cubic metals: I. Grain boundary energy

The energies of a set of 388 distinct grain boundaries have been calculated based on embedded-atom method interatomic potentials for Ni and Al. The boundaries considered are a complete catalog of the coincident site lattice boundaries constructible in a computational cell of a prescribed size. Correlations of the boundary energy with other boundary properties (disorientation angle, Σ value, excess boundary volume and proximity of boundary normals to 〈1 1 1〉) are examined. None of the usual geometric properties associated with grain boundary energy are useful predictors for this data set. The data set is incorporated as supplementary material to facilitate the search for more complex correlations. The energies of corresponding boundaries in Ni and Al are found to differ by approximately a scaling factor related to the Voigt average shear modulus or C₄₄. Crystallographically close boundaries have similar energies; hence a table of grain boundary energies could be used for interpolation.     

Spatial characterisation of the orientation distributions in a stable plane strain-compressed Cu crystal: A statistical analysis

Single copper crystals of the stable Goss orientation {0 1 1}〈1 0 0〉 were deformed in plane strain compression and the deformation-induced dislocation structures were investigated by high-resolution electron backscattered diffraction. Although the orientation maps exhibited an anisotropic dislocation boundary structure it was shown that the mean disorientation angle between point pairs saturated and became isotropic if their spacing was large enough (typically >30 μm). This saturation behaviour was interpreted as being a consequence of the anti-correlations between nearby dislocation boundaries and is discussed in terms of recent stochastic models of boundary formation. It was found that the disorientation boundaries, which were considered as being formed at relatively low strains, underwent rigid body-like rotations during deformation.     

Microstructure and texture evolution during annealing a cryogenic-SPD processed Al-alloy with a nanoscale lamellar HAGB grain structure

The grain structure and texture evolution during annealing a Al–0.13% Mg submicron-grained alloy, deformed by plane-strain compression (PSC) at cryogenic temperatures, has been investigated by transmission electron microscopy and electron backscatter diffraction. After deformation the alloy contained a lamellar grain structure with a high-angle grain boundary (HAGB) spacing of 190 nm and an area fraction of ～80%. On annealing the grain structure coarsened and transformed from lamellar to equiaxed. Remarkably, the fraction of low-angle grain boundaries (LAGBs) progressively increased during annealing, to ～50% above 300 °C, leading to instability and discontinuous recrystallization at higher temperatures. This resulted in a “bimodal grain structure” comprised of bands of coarser grains and fine subgrains, arising as a result of the increase in proportion of lower-mobility LAGBs. The surprisingly large increase in LAGB fraction on annealing is shown to be related to orientation impingement, originating from the strong texture present after PSC in liquid nitrogen.     

Microstructural development and mechanical properties of reactive hot pressed nickel-aided TiB2-SiC ceramics

TiB₂-SiC composites with different amounts of Ni (0, 2 and 5 wt.%) added as sintering aid were fabricated by reactive hot pressing (RHP). The mechanical properties were assessed under ambient conditions and the flexural strength was further tested in the temperature range of 700–1000 °C. The microstructures of the composites were characterized by a scanning electron microscope (SEM), transmission electron microscope (TEM) and energy-dispersive spectrometer (EDS). The flexural strength degradation mechanism occurring at elevated temperatures was studied. Addition of a moderate amount of Ni led to an improvement of the mechanical properties at room temperature. For the investigated ceramic composites, TiB₂-SiC-5 wt.% Ni sample showed significantly enhanced mechanical properties, i.e., a flexural strength of 1121 ± 31 MPa, a fracture toughness of 7.9 ± 0.58 MPa·m^1/2, a hardness of 21.3 ± 0.62 GPa, and a relative density of 98.6 ± 1.2%. Ni distributed along grain boundaries improved the interface strength. The improved fracture toughness was ascribed to crack deflection, grain rupture and crack shielding effect of Ni. A substantial strength degradation occurred at elevated temperatures, which was attributed to softening of the grain boundaries, surface oxidation and sliding of grain boundaries. The elastic modulus was found to decrease with increasing temperature.     

Structure of martensite in sputter-deposited (Ni,Cu)-rich Ti–Ni–Cu thin films containing Ti(Ni,Cu)2 precipitates

The martensite structure in sputter-deposited thin films of Ti_48.6Ni_35.9Cu_15.5 was studied. The Ti(Ni,Cu)₂ phase precipitates during the annealing process. Fine Ti(Ni,Cu)₂ precipitates can be deformed by the shear deformation of martensitic transformation, but they obstruct the movement of the twin boundaries to some extent. Coarse Ti(Ni,Cu)₂ precipitates seriously impede the growth of martensite plates and lead to a rectangular-cell-like structure of martensite in the film annealed at 873 K. The resistance of Ti(Ni,Cu)₂ precipitates to the growth of the martensite plates enhances with the coarsening of Ti(Ni,Cu)₂ precipitates, which is one of the reasons for the decrease in the maximum recoverable strain with increasing annealing temperature. B19′ martensite with (0 0 1) compound twinning is frequently observed near coarse Ti(Ni,Cu)₂ precipitates and grain boundaries in films annealed at 873 and 973 K. The local stress concentration should be responsible for the presence of B19′ martensite.     

The formation of nanograin structures and accelerated room-temperature theta precipitation in a severely deformed Al–4 wt.% Cu alloy

The grain size achievable and long-term stability of a severely deformed aluminium copper alloy have been investigated when copper is used in solution to inhibit recovery. It is shown that copper is more effective than magnesium in inhibiting dynamic recovery. A grain width of only ～70 nm was obtained in an Al–4 wt.% Cu alloy, after processing by equal-channel angular extrusion to a strain of ε_eff = 10, resulting in a lamellar nanograin structure. However, post-processing, the severely deformed solid solution was found to be unstable at room temperature and copious precipitation of θ occurred at grain boundaries within the deformed state, leading to recovery of the deformation structure and a loss of strength. The solute level fell to equilibrium within ～9 months. The precipitation kinetics were shown to occur at many orders of magnitude higher than can be predicted by classical nucleation and growth theory. The reasons for this discrepancy are discussed.     

Texture,microstructure and mechanical properties of equiaxed ultrafine-grained Zr fabricated by accumulative roll bonding

The texture, microstructure and mechanical behavior of bulk ultrafine-grained (ufg) Zr fabricated by accumulative roll bonding (ARB) is investigated by electron backscatter diffraction, transmission electron microscopy and mechanical testing. A reasonably homogeneous and equiaxed ufg structure, with a large fraction of high angle boundaries (HABs, ∼70%), can be obtained in Zr after only two ARB cycles. The average grain size, counting only HABs (θ > 15°), is 400 nm. (Sub)grain size is equal to 320 nm. The yield stress and UTS values are nearly double those from conventionally processed Zr with only a slight loss of ductility. Optimum processing conditions include large thickness reductions per pass (ε ∼ 75%), which enhance grain refinement, and a rolling temperature (T ∼ 0.3T_m) at which a sufficient number of slip modes are activated, with an absence of significant grain growth. Grain refinement takes place by geometrical thinning and grain subdivision by the formation of geometrically necessary boundaries. The formation of equiaxed grains by geometric dynamic recrystallization is facilitated by enhanced diffusion due to adiabatic heating.     

Development of a recovered/recrystallized multilayered microstructure in Al alloys by accumulative roll bonding

A high-purity Al alloy and a supersaturated Al–0.3 wt.% Sc alloy (Al(Sc)) were accumulative roll bonded at 200 °C to generate sheet material consisting of alternating layers of Al and Al(Sc). The deformation structure within these layers consisted of lamellar bands aligned parallel to the rolling direction. Compared with those bands in Al(Sc), the bands in the Al layers were less refined but contained a larger fraction of high angle grain boundaries (HAGBs). Subsequent annealing at 350 °C generated alternating layers of coarse grains (Al layers) and a recovered substructure (Al(Sc) layers); the latter were stabilized by the precipitation of Al₃Sc particles. Within the Al layers, annealing did not significantly alter the rolling texture (β-fibre), although the strong Brass component was largely eliminated; this behaviour has been explained using the “ReNuc” model of recrystallization whereby nucleation is deemed to occur on HAGBs between the β-fibre components of the lamellar bands in conjunction with orientation dependent recovery.     

Processing optimization,surface properties and wear behavior of HVOF spraying WC–CrC–Ni coating

In this work, the optimal coating process (OCP) designed by Taguchi program for high velocity oxy-fuel (HVOF) thermal spraying WC–CrC–Ni powder on Inconel 718 substrate (IN 718) is obtained by optimizing hardness (38 FMR oxygen flow rate, 53 FMR hydrogen flow rate, 25 g/min powder feed rate and 7 in. spray distance). Oxygen flow rate affects hardness mostly. The surface properties such as microstructure, crystalline phase, hardness, and porosity of WC–CrC–Ni coating have been investigated. The phase of coating has been changed during the OCP spraying because a portion of carbides, such as WC, Cr₇C₃, Ni₃C decomposes to W₂C, Cr, Ni and free carbon. Hardness (1150 ± 50 Hv) and porosity (1.2 ± 0.2%) of the OCP coating have been improved by optimization. The friction and wear behaviors of the WC–CrC–Ni coating, electrolytic hard chrome (EHC) plating and IN 718 have been studied comparatively. The lubrication due to free carbon and metal oxide debris results in a decrease of friction coefficients of the WC–CrC–Ni, compared to EHC and IN 718 at both 25 and 450 °C. It is concluded that HVOF WC–CrC–Ni coating performs more excellent anti-wear than others at both temperatures.     

Effect of Ni content on the compression properties and abrasive wear behavior of the (TiB2–TiCxNy)/Ni composites

The (TiB₂–TiC_xN_y)/Ni composites were fabricated by the method of combustion synthesis and hot press consolidation in a Ni–Ti–B₄C–BN system. The effect of Ni content on the microstructure, hardness, compression properties and abrasive wear behavior of the composites has been investigated. The results indicate that with the increase in Ni content from 30 wt.% to 60 wt.%, the average size of the ceramic particles TiB₂ and TiC_xN_y decreases from 5 μm to ≤ 1 μm, while the hardness and the abrasive wear resistance of the composites decrease. The composite with the Ni content of 30 wt.% Ni possesses the highest hardness (1560.8 Hv) and the best abrasive wear resistance. On another hand, with the increase in the Ni content, the compression strength increases firstly, and then decreases. The composite with 50 wt.% Ni possesses the highest compression strength (3.3 GPa). The hardness and fracture strain of the composite with 50 wt.% Ni are 1251.2 Hv and 3.9%, respectively.     

The multi-scale microstructure and strengthening mechanisms of Mo-12Si-8.5BxZr (at.%) alloys

Mo-12Si-8.5B alloys with different Zr contents (0 at.%, 1 at.%, 2 at.%, 3 at.%, 4 at.%) were manufactured via a mechanical alloying process followed by hot pressing sintering technology. The microstructure of Mo-12Si-8.5B alloy exhibited a continuous submicro- and micro-scale α-Mo matrix in which the sub-micron Mo₃Si/Mo₅SiB₂ particles were distributed. Addition of Zr to Mo-12Si-8.5B alloy promoted to form spherical nano-scale intermetallic Mo₂Zr and ZrO₂ particles, which were mainly located at the grain boundaries (GBs) as well as partially within the grains. The microstructure of Mo-12Si-8.5B-xZr alloys was remarkably refined by these Mo₂Zr/ZrO₂ nanoparticles. Additionally, results of mechanical properties indicated that the Zr addition improved the hardness, compression strength, yield strength and flexure strength of alloys. In particular, the Mo-12Si-8.5B-2Zr alloy exhibited extremely high compression strength (3.38 GPa), yield strength (3.17 GPa) and flexure strength (1.15 GPa). Quantitative analyses indicate that both fine-grained strengthening and Zr-rich particle strengthening mechanisms play a significant role in strengthening the Mo-Si-B-Zr alloys, the strengthening is dominantly governed by grain size reduction. Furthermore, Zr getters detrimental oxygen by synthesizing ZrO₂ distributed at grain/phase boundaries, which contributes to increasing the GBs cohesion. Fracture surfaces revealed that the fracture mode transformed from intergranular to transgranular fracture owing to Zr addition.     

Recrystallization kinetics of ultrafine-grained Ni studied by dilatometry

The release of excess volume upon recrystallization of ultrafine-grained Ni deformed by high-pressure torsion was measured with a high-precision difference-dilatometer employing constant heating rates in the range from 0.3 to 10 K min^?1. The kinetics of the recrystallization process was analyzed according to the Johnson–Mehl–Avrami–Kolmogorov theory adapted to the case of constant heating rates. An effective Avrami exponent of 2 and a value of 1.20 eV for the activation energy of recrystallization was determined. Analysis by the Kissinger method yielded the same result for the activation energy.     

Recovery kinetics of nanostructured aluminum: Model and experiment

A model is suggested which, for a deformed and partially annealed material, can factor out the recovery component of the flow stress for conditions where recrystallization occurs simultaneously with recovery processes. The model is applied in an analysis of the isothermal rates of recovery of hardness upon annealing of commercial purity aluminum at temperatures between 140 and 220 °C of samples cold rolled to strains 2 and 4. This analysis is successfully validated by a microstructural characterization. The recovery rates are described by a first-order chemical reaction rate equation with an activation energy that varies from 89 kJ mol⁻¹ at the onset of recovery to 125 kJ mol⁻¹ at its conclusion. The recovery rate increases significantly with an increase in strain, thereby markedly reducing the thermal stability of the material deformed to the largest strain.     

Effects of Ni addition and cyclic sintering on microstructure and mechanical properties of coarse grained WC–10Co cemented carbides

Coarse grained WC–10(Co, Ni) cemented carbides with different Ni contents were fabricated by sintering-HIP and cyclic sintering at 1450 °C. The effects of Ni addition and cyclic sintering on the microstructures, magnetic behavior and mechanical properties of coarse grained WC–10(Co, Ni) cemented carbides have been investigated using scanning electron microscope (SEM), magnetic performances tests and mechanical properties tests, respectively. The results showed that the mean grain size of hardmetals increases from 3.8 μm to 5.78 μm, and the shape factor Pwc decreases from 0.72 to 0.54, with the Ni content increases from 0 to 6 wt.%. Moreover, the W solubility reaches the highest value of 10.33 wt.% when the Ni content is 2 wt.%. The hardness and transverse rupture strength of WC–8Co–2Ni are 1105 HV₃₀ and 2778 MPa, respectively. The cyclic sintering is conducive to increase the WC grain size of WC–10(Co, Ni) and improves the transverse rupture strength of WC–10Co without compromising the hardness of alloys.     

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 110 kJ mol^?1 at the beginning to 230–240 kJ mol^?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.     

Microstructure and tribological performance of NbC-Ni cermets modified by VC and Mo2C

The current study reports on the influence of the addition of 5–15 vol% VC or/and Mo₂C carbide on the microstructure and mechanical properties of nickel bonded NbC cermets, which are compared to cobalt bonded NbC cermets. The NbC, Ni and secondary carbides powder mixtures were liquid phase sintered for 1 h at 1420 °C in vacuum. The fully densified cermets are composed of a cubic NbC grains matrix and an evenly distributed fcc Ni binder. NbC grain growth was significantly inhibited and a homogeneous NbC grain size distribution was obtained in the cermets with VC/Mo₂C additions. The mechanical properties of the NbC-Ni matrix cermets are strongly dependent on the carbide and Ni binder content and are directly compared to their NbC-Co equivalents. The liquid phase sintered NbC-12 vol% Ni cermet had a modest Vickers hardness (HV₃₀) of 1077 ± 22 kg/mm² and an indentation toughness of 9.1 ± 0.5 MPa·m^1/2. With the addition of 10–15 vol% VC, the hardness increased to 1359 ± 15 kg/mm², whereas the toughness increased to 11.3 ± 0.1 MPa·m^1/2. Addition of 5 and 10 vol% Mo₂C into a NbC-12 vol% Ni mixtures generated the same values in HV₃₀ and K_IC when compared to VC additions. A maximum flexural strength of 1899 ± 77 MPa was obtained in the cermet with 20 vol% Ni binder and 4 vol% VC + 4 vol% Mo₂C addition, exhibiting a high fracture toughness of 15.0 ± 0.5 MPa·m^1/2, but associated with a loss in hardness due to the high Ni content. The dry sliding wear behaviour was established at room temperature and 400 °C from 0.1 to 10 m/s.     

On the effect of grain boundary segregation on creep and creep rupture

The present work investigates the effect of grain boundary chemistry and crystallography on creep and on creep damage accumulation in Cu–0.008 wt.% Bi and Cu–0.92 wt.% Sb at stresses ranging from 10 to 20 MPa and temperatures between 773 and 873 K. Small additions of Bi and Sb significantly reduce the rupture strain and rupture time during creep of Cu. High stress exponents (Cu–Bi) and high apparent activation energies for creep (Cu–Bi and Cu–Sb) are obtained. Sb promotes creep cavitation on random high-angle grain boundaries. Bi, on the other hand, causes brittle failure when small crack-like cavities cause decohesion. Both elements suppress dynamic recrystallization, which occurs during creep of Cu at high stresses and temperatures.