Microstructural evolution in copper subjected to severe plastic deformation: Experiments and analysis

The evolution of microstructure and the mechanical response of copper subjected to severe plastic deformation using equal channel angular pressing (ECAP) was investigated. Samples were subjected to ECAP under three different processing routes: B_C, A and C. The microstructural refinement was dependent on processing with route B_C being the most effective. The mechanical response is modeled by an equation containing two dislocation evolution terms: one for the cells/subgrain interiors and one for the cells/subgrain walls. The deformation structure evolves from elongated dislocation cells to subgrains to equiaxed grains with diameters of ∼200–500 nm. The misorientation between adjacent regions, measured by electron backscatter diffraction, gradually increases. The mechanical response is well represented by a Voce equation with a saturation stress of 450 MPa. Interestingly, the microstructures produced through adiabatic shear localization during high strain rate deformation and ECAP are very similar, leading to the same grain size. It is shown that both processes have very close Zener–Hollomon parameters (ln Z ∼ 25). Calculations show that grain boundaries with size of 200 nm can rotate by ∼30° during ECAP, thereby generating and retaining a steady-state equiaxed structure. This is confirmed by a grain-boundary mobility calculation which shows that their velocity is 40 nm/s for a 200 nm grain size at 350 K, which is typical of an ECAP process. This can lead to the grain-boundary movement necessary to retain an equiaxed structure.     

Ductilization of Mo–Si solid solutions manufactured by powder metallurgy

Mo–1.5 at.% Si alloys with additions of either Y₂O₃ or Zr were manufactured by mechanical alloying. The Y₂O₃ particles reduced the grain size and increased the room temperature strength, but did not alleviate the brittleness of previously investigated Mo–1.5 at.% Si without Y₂O₃. Additions of Zr, on the other hand, resulted not only in a fine grain size and an extremely high bend strength (～2 GPa), but also in limited bend ductility at room temperature. Zr additions are seen to be beneficial for three reasons. First, Zr reduces the grain size. Second, Zr getters detrimental oxygen by forming ZrO₂ particles (which in turn help to pin the grain boundaries). Third, in situ Auger analysis shows that Zr reduces the concentration of Si segregated at the grain boundaries. This is thought to enhance the grain boundary cohesive strength and thus leads to the observed ductility.     

Properties of nanocrystalline and nanocomposite WxZr1−x thin films deposited by co-sputtering

W_xZr_1?x thin films were deposited at room temperature on glass substrates by co-sputtering tungsten and zirconium targets in argon. The composition was found in the range 0 ≤ x ≤ 0.81. The grain size deduced from X-ray diffraction analysis ranged from 1.3 nm to 16 nm depending on the composition. The events in the resistivity, optical reflectivity and thickness evolutions were correlated with the X-ray diffraction analysis. Depending on the composition, the local organization can be attributed to a nanocrystalline solid solution of W in Zr, to a nanocomposite structure involving ZrW₂ nanograins embedded in an amorphous matrix, to ZrW₂ Laves phase nanograins and to a nanocrystalline solid solution of Zr in W. For 0 < x ≤ 0.72, the equivalent grain size is very small (less than 2 nm) and the evolution of the resistivity can be fitted by the estimated volume of the material perturbed by the grain boundaries.     

Atomic structural environment of grain boundary segregated Y and Zr in creep resistant alumina from EXAFS

Dopants Y and Zr at 100 p.p.m. level (Y or Zr to Al atomic ratio) in ultra-high purity polycrystalline alumina have been found to be mainly segregated to the alumina grain boundaries. The atomic structural environments around the Y and Zr segregants have been investigated by Extended X-ray Absorption Fine Structure (EXAFS). On average, Y ions in α-Al₂O₃ grain boundaries are coordinated by four oxygens, at a distance of 2.30 Å, which corresponds nearly to the Y–O bond length in cubic Y₂O₃, and Zr ions are coordinated by five oxygens at a distance of 2.14 Å, which is approximately the same as the average Zr–O bond length in monoclinic ZrO₂. However, in the EXAFS radial distribution function, the Y-cation and Zr-cation next nearest neighbor shell cannot be clearly identified. These results suggest that Y and Zr at 100 p.p.m. concentrations in α-Al₂O₃ occupy grain boundary sites with well defined nearest neighbor cation–oxygen bond lengths similar to those in their parent oxides but with the next nearest neighbor cation–cation distances varying considerably from site to site. From EXAFS, the Y grain boundary saturation concentration is estimated to be 6.0 atoms/nm², which is consistent with the estimate from STEM of 4.4 atoms/nm². The differences in the Zr–O and Y–O nearest neighbor distances and coordination numbers in Al₂O₃ grain boundaries are related to the Y–Al and Zr–Al size mismatches.     

Evolution of microstructural parameters and flow stresses toward limits in nickel deformed to ultra-high strains

A quantitative analysis of microstructure and strength as a function of strain is presented for polycrystalline nickel (99.5%) deformed by high-pressure torsion in the strain range 1–300 (ε_VM, von Mises strain). Typical lamellar structures consisting of extended boundaries and short interconnecting boundaries have been found, with additional features at large strains which are equiaxed regions, small equiaxed subgrains and deformation twins. The evolution of microstructure and microstructural parameters falls in stages: (i) the first stage at ε_VM = 1–12; (ii) a transition stage at ε_VM = 12–34; and (iii) a saturation stage at ε_VM ⩾ 34. A scaling analysis of spacing between boundaries shows a universal behavior up to ε_VM = 300, indicating that the predominant deformation mechanism is dislocation glide whereas twin formation is of minor importance. A clear link is observed between the evolution in structure and flow stress, which can guide the development of strong metals with a structural scale extending below 50–100 nm.     

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.     

Effect of Zn additions on the age-hardening of Mg–2.0Gd–1.2Y–0.2Zr alloys

We have investigated the microstructures of age hardened Mg–2.0Gd–1.2Y–xZn–0.2Zr (x = 0, 0.3, and 1.0) (at.%) alloys to understand the remarkable age-hardening and unusual plastic elongation behavior. The age-hardening of the alloys occurs through the sequential precipitations of β′ and β₁ phases. The β₁ phase heterogeneously nucleates at the interface of the β′ phase, and relaxes strain fields around the β′. Although the addition of Zn degrades the age-hardening response, it causes the discontinuous precipitation of a 14H-type long-period stacking (LPS) phase at grain boundaries as well as within grains in the over-aged condition, which enhances the maximum tensile elongation. The composition of the β₁ phase was determined to be Mg–23.3 at.% RE–9.7 at.% Zn–2.0 at.% Zr (RE: rare-earth, Gd and Y), whereas that of the LPS is Mg–5.6 at.% RE–1.8 at.% Zn–1.0 at.% Zr.     

In situ observation of Si faceted dendrite growth from low-degree-of-undercooling melts

We directly observed the transition of crystal growth behavior of Si in a low undercooling region. We succeeded in observing the initiation of faceted dendrite growth from a portion of parallel twins with increasing degrees of undercooling. The critical undercooling for growing a faceted dendrite was experimentally determined to be ΔT = 10 K. We also confirmed that parallel twins associated with faceted dendrite growth were formed between grain boundaries and not at grain boundaries during melt growth. The parallel-twin formation was explained in terms of a model of twin formation on the {1 1 1} facet plane at the growth interface.     

Effect of the Zener–Hollomon parameter on the microstructures and mechanical properties of Cu subjected to plastic deformation

Pure Cu was deformed at different strain rates and temperatures, i.e. with different Zener–Hollomon parameters (Z) ranging within ln Z = 22–66, to investigate the effect of Z on its microstructures and mechanical properties. It was found that deformation twinning occurs when ln Z exceeds 30, and the number of twins increases at higher Z. The average twin/matrix lamellar thickness is independent of Z, being around 50 nm. Deformation-induced grain refinement is enhanced at higher Z, and the mean transverse grain size drops from 320 to 66 nm when ln Z increases from 22 to 66. The grain refinement is dominated by dislocation activities in low-Z processes, while deformation twinning plays a dominant role in high-Z deformation. An obvious increment in yield strength from 390 to 610 MPa was found in deformed Cu with increasing Z, owing to the significant grain refinement as well as the strengthening from nanoscale deformation twins.     

Nanocrystalline Fe–C alloys produced by ball milling of iron and graphite

A series of nanocrystalline Fe–C alloys with different carbon concentrations (x_tot) up to 19.4 at.% (4.90 wt.%) are prepared by ball milling. The microstructures of these alloys are characterized by transmission electron microscopy and X-ray diffraction, and partitioning of carbon between grain boundaries and grain interiors is determined by atom probe tomography. It is found that the segregation of carbon to grain boundaries of α-ferrite can significantly reduce its grain size to a few nanometers. When the grain boundaries of ferrite are saturated with carbon, a metastable thermodynamic equilibrium between the matrix and the grain boundaries is approached, inducing a decreasing grain size with increasing x_tot. Eventually the size reaches a lower limit of about 6 nm in alloys with x_tot > 6.19 at.% (1.40 wt.%); a further increase in x_tot leads to the precipitation of carbon as Fe₃C. The observed presence of an amorphous structure in 19.4 at.% C (4.90 wt.%) alloy is ascribed to a deformation-driven amorphization of Fe₃C by severe plastic deformation. By measuring the temperature dependence of the grain size for an alloy with 1.77 at.% C additional evidence is provided for a metastable equilibrium reached in the nanocrystalline alloy.     

Phase formation and thermal stability in Cu–Zr–Ti(Al) metallic glasses

In the present study we investigate the phase formation and the thermal stability of Cu₅₀Zr_50 ? xTi_x (0 ≤ x ≤ 10) and (Cu_0.5Zr_0.5)_100 ? xAl_x glass-forming alloys. Parameters indicating the glass-forming ability (GFA) are calculated from isochronal and isothermal calorimetric experiments. A high Ti content in the Cu–Zr–Ti alloys causes the precipitation of a metastable ternary Laves phase (C15), which does not form in Cu–Zr–Al. Accompanied with it is a significant drop in the activation energy of crystallization. Also the supercooled liquid region (ΔT_x = T_x ? T_g), the reduced glass transition temperature (T_rg = T_g/T_liq), and the γ parameter (γ = T_x/(T_g + T_liq)) (T_x: crystallization temperature, T_g: glass transition temperature and T_liq: liquidus temperature) are sensitive to the change in the crystallization sequence. The fragility values calculated are believed to overestimate the GFA of the investigated alloys. Careful selection of the alloy composition enables the targeted precipitation of different crystalline phases.     

Microstructure and grain boundary relaxation in ultrafine-grained Al/Al oxide composites

The microstructure and grain boundary relaxation in ultrafine-grained Al/Al oxide composites were studied by electron microscopy observation and internal friction measurement, respectively. Both the microstructure and the internal friction behavior of the composites were strongly influenced by the thermomechanical treatment parameters. All the Al particles were still covered by the native amorphous oxide shells in those composites sintered at T < 823 K, and no indication of Al grain boundary relaxation was detected. Some Al oxide shells were cracked, resulting in the formation of a few Al–Al grain boundaries between adjacent particles in the sample sintered at 823 K, and one internal friction peak centered at ～440 K was detected. All the oxide shells were broken into small fragments in those samples sintered at T ? 843 K, and two internal friction peaks were detected, one prominent peak at ～440 K and one weak peak at ～540 K. A microstructure with a bimodal grain size distribution of Al was formed via partial recrystallization after thermomechanical treatment of the sample sintered at 893 K, and two internal friction peaks with comparable intensity were detected. The internal friction peaks were associated with the relaxation of Al grain boundary in the composites.     

What is the theoretical density of a nanocrystalline material?

We prepared nanocrystalline Ni by a severe deformation method – high-energy ball milling – and collected neutron diffraction patterns during the annealing of nanocrystalline Ni. Analyzing the neutron diffraction patterns provides the lattice parameter, dislocation density and grain size of nanocrystalline Ni. We found that a low-temperature (T < 260 °C) anneal annihilates the statistically stored dislocations whereas a high-temperature (T > 260 °C) anneal grows the nanograins. For T < 260 °C, where nanocrystalline Ni has a constant grain size, the excess volume is proportional to the density of statistically stored dislocations. For T > 260 °C, where the statistically stored dislocations are completely annealed out, the excess volume is inversely proportional to the grain size. However, 80% of the excess volume in our severely deformed nanocrystalline Ni is due to the statistically stored dislocations. We finally used our experimental data to derive the grain size dependence of the theoretical density of a nanocrystalline material free from excess dislocations. The derived theoretical density agrees well with the experimentally measured density of nanocrystalline metallic materials that are relatively free from deformation-induced defects.     

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.     

Two-step hot pressing of bimodal micron/nano-ZrB2 ceramic with improved mechanical properties and thermal shock resistance

The densification and grain growth behaviors for micron- and nano-sized ZrB₂ particles were investigated. The densification on-set temperature (T_d-micron) and grain growth on-set temperature (T_g-micron) for micron-sized ZrB₂ particles were about 1500 °C and 1800 °C, respectively. And the densification on-set temperature (T_d-nano) and grain growth on-set temperature (T_g-nano) for nano-sized ZrB₂ particles were about 1300 °C and 1500 °C, respectively. A bimodal micron/nano-ZrB₂ ceramic was therefore prepared using a novel two-step hot pressing. A high relative density of 99.2%, an improved flexural strength of 580.2 ± 35.8 MPa and an improved fracture toughness of 7.2 ± 0.4 MPa·m^1/2 were obtained. The measured critical thermal shock temperature difference (ΔT_c) for this bimodal micron/nano-ZrB₂ ceramic was as high as 433 °C.     

A new approach to designing a grain refiner for Mg casting alloys and its use in Mg–Y-based alloys

A totally new grain refiner, Al₂Y, for cast Mg alloys has been predicted using the recently developed edge-to-edge matching (E2EM) crystallographic model. An addition of 0.6–1.0 wt.% Al into the Mg–10 wt.% Y melt promotes the in situ formation of Al₂Y, which reduces the average grain size from about 180 to 36 μm. Active nucleation Al₂Y particles were reproducibly observed at the centres of many refined grains. The excellent grain-refining efficiency is comparable to that of Zr for the same alloy, but the thermal stability of the grains refined by Al₂Y is much higher than those refined by Zr up to a temperature of 550 °C for 48 h. The mechanisms of grain refinement and the superior thermal stability of the refined grains due to Al addition in the Mg–Y alloy are discussed based on the current experimental results.     

High-temperature mechanical spectroscopy of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) with different amounts of intergranular phase

Y-TZP ceramics of different grain sizes and with various content of amorphous intergranular phase have been studied by mechanical spectroscopy and transmission electron microscopy. In all grades both clean grain boundaries and grain boundaries wetted by an amorphous film are present. In the materials with higher amount of intergranular phase, large amorphous pockets are present. A thermally activated mechanical-loss peak is observed in all grades at 1600 K (0.1 Hz; ΔH_act=560±20 kJ/mol). This peak evolves into a background at low frequencies and high temperature. The peak position and height depend on the grain size; the background is grain-size independent. The peak is interpreted as the limited sliding of those grains that are separated by an amorphous film. The background is attributed to the beginning of microcreep. In the grades with amorphous pockets an additional peak, interpreted as the α-relaxation of these pockets, is observed at 1425 K (0.1 Hz; ΔH_act=570±20 kJ/mol).     

Continuous dynamic recrystallization during the transient severe deformation of aluminum alloy 7475

Effect of strain rate and its discontinuous changes on the deformation and microstructural behavior of a coarse-grained 7475 Al alloy were studied in multidirectional forging at 763 K. Deformation at a higher strain rate of 3 × 10^?2 s^?1, controlled by homogeneous dislocation motion, leads to partial grain refinement taking place only near the original grain boundaries. Deformation at a lower rate of 3 × 10^?4 s^?1, controlled mainly by grain boundary sliding, in contrast, results in full development of strain-induced grains through grain fragmentation due to microshear band formation. Under conditions of discontinuous changes in strain rate, the flow stresses and grain size developed by subsequent severe deformation do not approach those appearing during continuous change at a constant strain rate. The nature of such strain-induced events is irreversible and athermal. The mechanisms of continuous dynamic recrystallization operating during severe deformation are discussed in detail.     

Microstructural evolution and grain boundary sliding in a superplastic magnesium AZ31 alloy

Tensile experiments on a fine-grained single-phase Mg–Zn–Al alloy (AZ31) at 673 K revealed superplastic behavior with an elongation to failure of 475% at 1 × 10^?4 s^?1 and non-superplastic behavior with an elongation to failure of 160% at 1 × 10^?2 s^?1; the corresponding strain rate sensitivities under these conditions were ～0.5 and ～0.2, respectively. Measurements indicated that the grain boundary sliding (GBS) contribution to strain ξ was ～30% under non-superplastic conditions; there was also a significant sharpening in texture during such deformation. Under superplastic conditions, ξ was ～50% at both low and high elongations of ～20% and 120%; the initial texture became more random under such conditions. In non-superplastic conditions, deformation occurred under steady-state conditions without grain growth before significant flow localization whereas, under superplastic conditions, there was grain growth during the early stages of deformation, leading to strain hardening. The grains retained equiaxed shapes under all experimental conditions. Superplastic deformation is attributed to GBS, while non-superplastic deformation is attributed to intragranular dislocation creep with some contribution from GBS. The retention of equiaxed grain shapes during dislocation creep is consistent with a model based on local recovery related to the disturbance of triple junctions.     

Nanoscaled grain boundaries and pores,microstructure and mechanical properties of translucent Yb:[LuxY(1−x)O3] ceramics

Translucent ceramics of Yb:[Lu_xY_(1?x)O₃] system doped by ZrO₂ was sintered from nanopowder synthesized by laser evaporation. The relative density of the ceramics was 99.97%, residual pores had sizes from 8 nm to 20 nm, Young modulus was 200 GPa at the applied load of 2000 mN, the microhardness was 12.8 GPa. The grains of ceramics had sizes 1–10 μm, but the thickness of grain boundaries was about 1 nm. The transcrystalline type of the crack propagation was detected in the specially broken ceramics. The results indicated high strength of grain bonds and good perfection of grain boundaries in the studied ceramics but an increased content of pores (higher than 10^?3 vol.%) and stoichiometry deviation (Lu:Y:O = 0.21:0.79:3) from the required one (Lu:Y:O = 0.25:0.75:3).