Using Post‐Deformation Annealing to Optimize the Properties of a ZK60 Magnesium Alloy Processed by High‐Pressure Torsion

A ZK60 magnesium alloy with an initial grain size of ≈10 µm is processed by high‐pressure torsion (HPT) through 5 revolutions under a constant compressive pressure of 2.0 GPa with a rotation speed of 1 rpm. An average grain size of ≈700 nm is achieved after HPT with a high fraction of high‐angle grain boundaries. Tensile experiments at room temperature show poor ductility. However, a combination of reasonable ductility and good strength is achieved with post‐HPT annealing by subjecting samples to high temperatures in the range of 473–548 K for 10 or 20 min. The grain size and texture changes are also examined by electron back scattered diffraction (EBSD) and the results compared to long‐term annealing for 2500 min at 450 K. The results of this study suggest that a post‐HPT annealing for a short period of time may be effective in achieving a reasonable combination of strength and ductility.

     

Production of aluminum-matrix carbon nanotube composite using high pressure torsion

In this study, an Al-based composite containing carbon nanotubes (CNTs) was fabricated using a process of severe plastic deformation through high pressure torsion (HPT). Neither heating nor sintering was required with the HPT process so that an in situ consolidation was successfully achieved at ambient temperature with 98% of the theoretical density. A significant increase in hardness was recorded through straining by the HPT process. When the composite was pulled in tension, the tensile strength of more than 200 MPa was attained with reasonable ductility. Transmission electron microscopy showed that the grain size was reduced to 100 nm and this was much smaller than the grain size without CNTs and the grain size reported on a bulk sample. High resolution electron microscopy revealed that CNTs were present at grain boundaries. It was considered that the significant reduction in grain size is attributed to the presence of CNTs hindering the dislocation absorption and annihilation at grain boundaries.     

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.     

Consolidation of Cu-based amorphous alloy powders by high-pressure torsion

We applied high-pressure torsion (HPT) for consolidation of gas-atomized metallic glass Cu₅₄Zr₂₂Ti₁₈Ni₆ powders into high-density bulk disks. The effects of the number of revolutions (N = 1–5 turns), applied pressure (2.5–10 GPa), and temperature (298–473 K) on densification and structural changes were investigated. The consolidated glassy disks showed an excellent hardness of ~5.2 GPa although a mechanical softening effect along with fragmentation in the center of HPT disks occurred at N > 3 by a couple of branching cracks. The HPT process at higher applied pressures improved the bulk density and inter-particulate bonding, resulting in higher hardness. Increasing the temperature of HPT processing enhanced the densification and deep drawability of the consolidated metallic glass. Although the HPT process did not change the crystallization temperature of the metallic glass powders, it increased the crystallization enthalpy, suggesting the free volume increase and inhibition of a significant nanocrystallization during the HPT process.     

Microstructure,Phase Composition,and Thermal Stability of Two Zirconium Alloys Subjected to High‐Pressure Torsion at Different Temperatures

The structure, phase composition, and thermal stability of the industrial zirconium alloys, namely, E110 (Zr–1% Nb) and E635 (Zr–1% Nb–0.3% Fe–1.2% Sn), which are subjected to high‐pressure torsion (HPT) at room temperature (RT), 200, and 400 °С have been studied. HPT of Zr‐alloys at RT (10 revolutions) leads to the formation of grain–subgrain nano‐sized structure and to increase the microhardness by 2.1…2.8 times. The increase in the HPT temperature to 200–400 °С leads to the increase in the structural‐element average size. The structural‐element size in the complexly alloyed E635 alloy in all cases is lower compared with the E110 alloy. The hardening of the alloys after HPT at RT and 200 °С is close, and at 400 °С is much less. HPT initiates the α‐Zr → (ω‐Zr + β‐Zr) transformation, which is the main factor for alloys hardening. The α‐Zr → (ω‐Zr + β‐Zr) transformation in the E635 alloy occurs less quickly. The maximum amount (ω‐Zr + β‐Zr) phase in the structure of the alloys is observed after HPT at RT and 200 °C, and the minimum ? at 400 °C. During heating, the alloys undergo the reverse (ω‐Zr + β‐Zr) → α transformation which depends on both the alloy composition and HPT temperature.

     

Evolution of a martensitic structure in a Cu–Al alloy during processing by high-pressure torsion

A Cu-11.8 wt% Al alloy was quenched in iced water from a high temperature (850 °C) to introduce a martensitic phase and then the alloy was processed using quasi-constrained high-pressure torsion (HPT). The micro-hardness and the microstructures of the unprocessed and severely deformed materials were investigated using a wide range of experimental techniques (X-ray diffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy, and high- resolution TEM). During HPT, a stress-induced martensite–martensite transformation occurs and an $ \alpha^{\prime}_{1} $ martensite phase is formed. In the deformed material, there are nanoscale deformation bands having high densities of defects and twins in the $ \alpha^{\prime}_{1} $ martensite. It was observed that a high density of dislocations became pinned and accumulated in the vicinity of twin boundaries, thereby demonstrating a strong interaction between twin boundaries and dislocations during the HPT process.     

Grain boundary statistics in nano-structured iron produced by high pressure torsion

The microstructure and the spectrum of grain boundary misorientations were studied in Armco iron, following high pressure torsion (HPT) deformation, by means of transmission electron microscopy (TEM) and orientation imaging microscopy (OIM). It was found that HPT deformation results in the formation of an equiaxed grain structure with a mean grain size of 270 and 130 nm using a shear strain of γ = 210 and 420, respectively. The misorientation spectra in HPT iron have a bimodal character with maxima in low (at 1–2°) as well as in high misorientation angle ranges. A marked increase in the fraction of special boundaries (Σ3–Σ45) was revealed as a result of HPT. The microstructural changes due to HPT are discussed and compared with those obtained during conventional deformation modes.     

Wear properties of high pressure torsion processed ultrafine grained Al–7%Si alloy

In this paper, Al–7 wt% Si alloy was processed via high pressure torsion (HPT) at an applied pressure 8 GPa for 10 revolutions at room temperature. The microstructure and hardness of the HPT samples were investigated and compared with those of the as-cast samples. The wear properties of as-cast and the HPT samples under dry sliding conditions using different sliding distances and loads were investigated by reciprocated sliding wear tests.The HPT process successfully resulted in nanostructure Al–7 wt% Si samples with a higher microhardness due to the finer Al matrix grains and Si particles sizes with more homogeneous distribution of the Si particles than those in the as-cast samples.The wear mass loss and coefficient of friction values were decreased after the HPT process. The wear mechanism was observed to be adhesive, delamination, plastic deformation bands and oxidization in the case of the as-cast alloy. Then, the wear mechanism was transformed into a combination of abrasive and adhesive wear after the HPT process. The oxidization cannot be considered as a mechanism that contributes to wear in the case of HPT samples, because O₂ was not detected in all conditions.     

Optimization of Strength‐Electrical Conductivity Properties in Al–2Fe Alloy by Severe Plastic Deformation and Heat Treatment

High‐pressure torsion at room temperature followed by two processing routes, either 1) annealing at 200 °C for 8 h or 2) elevated temperature (200 °C) high‐pressure torsion, are employed to obtain simultaneous increase in mechanical strength and electrical conductivity of Al–2 wt%Fe. The comparative study of microstructure, particle distribution, mechanical properties, and electrical conductivity for both processing routes gives the optimal combination of high mechanical strength and high electrical conductivity in Al–2Fe alloy. It is shown that while the mechanical strength is approximately the same for both processing routes (>320 MPa), high‐pressure torsion at elevated temperature results in higher conductivity (≥52% IACS) due to reduction of Fe solute atom concentration in Al matrix compared to annealing treatment. High‐pressure torsion at 200 °C has been demonstrated as a new and effective way for obtaining combination of high mechanical strength and electrical conductivity in Al–Fe alloys.

     

Fatigue reliability assessment of turbine discs under multi‐source uncertainties

Hot section components of aircraft engines like high pressure turbine (HPT) discs usually operate under complex loadings coupled with multi‐source uncertainties. The effect of these uncertainties on structural response of HPT discs should be accounted for its fatigue life and reliability assessment. In this study, a probabilistic framework for fatigue reliability analysis is established by incorporating FE simulations with Latin hypercube sampling to quantify the influence of material variability and load variations. Particularly, variability in material response is characterized by combining the Chaboche constitutive model with Fatemi‐Socie criterion. Results from fatigue reliability and sensitivity analysis of a HPT disc indicated that dispersions of basic variables must be taken into account for its fatigue reliability analysis. Moreover, the proposed framework based on the strength‐damage interference provides more reasonably correlations with its field number of flights rather than the load‐life interference one.     

Allotropic phase transformation of pure zirconium by high-pressure torsion

Pure Zr is processed by high-pressure torsion (HPT) at pressures in the range of 1–40 GPa. A phase transformation occurs from α to ω phase during HPT at pressures above 4 GPa while the total fraction of ω phase increases with straining and saturates to a constant level at higher strain. This phase transformation leads to microstructural refinement, hardness and strength enhancement and ductility reduction. Lattice parameter measurements confirm that c for α phase is expanded about 0.6% by the presence of ω phase. The temperature for reverse transformation from ω to α phase increases with straining and thus, straining under high pressure increases thermal stability of ω phase. The ω phase obtained by HPT is stable for more than 400 days at room temperature.     

Strengthening mechanisms of pure tungsten subjected to high pressure torsion: Deviation from classic Hall-Petch relation

In this work, the ultrafine grained (UFG) tungsten has been fabricated via high pressure torsion (HPT) of various turning numbers at a temperature of 823 K and a pressure of 1.5 GPa. The microstructure characteristics and mechanical properties of initial and high pressure torsion processed tungsten have been comparatively investigated by means of x-ray diffraction (XRD), electron backscatter diffraction (EBSD), transmission electron microscope (TEM) and microhardness tests. It is shown that high pressure torsion leads to microstructure refinement with an average grain size of ∼0.92 μm and the fraction of high angle grain boundaries (HAGB) increasing to 62.9 %. Moreover, the dislocation density increases from initial 1.17×10¹⁴ m⁻² to 3.89×10¹⁴ m⁻². The microhardness tests revealed that hardness increased gradually and its distribution became more homogeneous with torsion strain increasing. The strengthening model was established considering the grain boundary strengthening and dislocation strengthening mechanisms. Resultantly, deviation of Hall-Petch slope was found from the classic values, which is attributed to the easy movement of the extrinsic dislocations in the high angle grain boundaries with high distortion energy and high density of defects.     

Microstructure development and hardening during high pressure torsion of commercially pure aluminium: Strain reversal experiments and a dislocation based model

The effect of strain reversal on hardening due to high pressure torsion (HPT) was investigated using commercially pure aluminium. Hardening is lower for cyclic HPT (c-HPT) as compared to monotonic HPT (m-HPT). When using a cycle consisting of a rotation of 90° per half cycle, there is only a small increase in hardness if the total amount of turns is increased from 1 to 16. Single reversal HPT (sr-HPT) processing involves torsion in one direction followed by a (smaller) torsion in the opposite direction. It is shown that a small reversal of 0.25 turn (90°) reduces hardness drastically, and that decrease is most marked for the centre region. These behaviours and other effects are interpreted in terms of the average density of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs). A model is presented that describes the experimental results well. A key element of the model is the assumption that at the very high strains developed in severe plastic deformation processes such as HPT, the dislocation density reaches a saturation value. The model indicates that the strength/hardness is predominantly due to GNDs and SSDs.     

Microstructure and tensile behavior of Al and Al-matrix carbon nanotube composites processed by high pressure torsion of the powders

Carbon nanotubes (CNTs) are expected to be ideal reinforcements of composite materials used in aircraft and sports industries due to their high modulus and low density. In the present paper, severe plastic deformation by high pressure torsion (HPT) of powders at elevated temperature (473 K) was employed to achieve both powder consolidation and grain refinement of aluminum-matrix nanocomposites reinforced by 5 vol% CNTs. Before the HPT, the powders were ball milled using planetary ball mill in order to achieve molecular level mixing. Aluminum was treated by the same process for a reference. The HPT processed disk were composed of considerably equilibrium grain boundaries with high misorientation angles. The CNT-reinforced ultrafine grained microstructural features resulted in high strength and good ductility.     

The Control of Solidification of Ni‐Based Superalloy Single‐Crystal Blade by Mold Design Modification using Inner Radiation Baffle

This work is focused on an investigation of the directional solidification process of CMSX‐4 single‐crystal blades in the mold modified by application of inner radiation baffle (IRB). Micro‐ and macrostructure examination is carried out along the height of single‐crystal blades which are manufactured using standard and modified mold, at the withdrawal rates of 3 and 5 mm min^?1. It is established that application of modified mold allows better control over liquidus isotherm and leads to increase in temperature gradient, as compared to the manufacturing of blades using standard mold. The average value of temperature gradient in airfoil increases from 14 up to approx. 30 K cm^?1 and from 16 up to approx. 40 K cm^?1, for withdrawal rate of 5 and 3 mm min^?1, respectively. The curvature of liquidus isotherm diminishes and attains near‐flat profile along the airfoil for both values of withdrawal rate. The application of proposed technique results in reduction of primary dendrites arm spacing (PDAS) particularly in the inner and middle part of the blade. PDAS reaches average value of approx. 360 μm for withdrawal rate of 5 mm min^?1 and is lower as compared to the standard mold casting withdrawn at the rate of 3 (433 μm) and 5 mm min^?1 (468 μm).

     

Influence of Anvil Alignment on Shearing Patterns in High‐Pressure Torsion

A two‐phase duplex stainless steel was used as a model material in order to investigate the development of flow patterns when processing using high‐pressure torsion (HPT). The results show that double‐swirls are visible on the disc surfaces when processing with controlled amounts of anvil misalignment but not when the anvils are in an essentially perfect alignment. There are also shear vortices visible on the disc surfaces when processing with controlled amounts of misalignment but not when using perfect alignment. These results demonstrate the need for exercising significant care when processing discs by HPT. Prior to introducing torsional straining, it is important to ensure that the upper and lower anvils are in good alignment to within ≈25 µm.     

Continuous high-pressure torsion using wires

A newly developed severe plastic deformation method, continuous high-pressure torsion (CHPT), was modified for continuous processing of metallic wires. In this study, using the CHPT, wires of high-purity aluminum (99.99%) and copper (99.999%) with diameters of 2 mm and total lengths of 100 mm were successfully processed by employing the same features as conventional high-pressure torsion (HPT) technique. The results of hardness measurements, 35 Hv for Al and 116 Hv for Cu, after CHPT at an imposed equivalent strain of ~13 were consistent with those of conventional HPT using disk and ring specimens, as well as with those of CHPT using sheet specimens. Transmission electron microscopy (TEM) demonstrated that the microstructural elements are elongated in the shear direction after CHPT. The average grain size reaches the steady-state level, ~1.3 μm, in Al, but the microstructure is at the non-steady state in Cu with subgrain sizes in the range of 0.3–4 μm.     

Short and long crack data for fatigue of 2024‐T3 Al sheets: binariness of scale segmentation in space and time

Pedagogically speaking, crack initiation–growth–termination (IGT) belongs to the process of fracture, the modelling of which entails multiscaling in space and time. This applies to loadings that are increased monotonically or repeated cyclically. Short and long crack data are required to describe IGT for scale ranges from nano to macro, segmented by the SI system of measurement. Unless the data at the nano scale can be connected with the macro, IGT remains disintegrated. The diversity of non‐homogeneity of the physical properties at the different scale ranges results in non‐equilibrium. These effects dubbed as non‐equilibrium and non‐homogeneous are hidden in the test specimens and must be realized. They can be locked into the reference state of measurement at the mi‐ma scale range by application of the transitional functions and transferred to the nano‐micro and macro‐large scale ranges. The aim of this work is to convert the ordinary crack length data to those referred to as short cracks that are not directly measurable. All test data are material, loading and geometry (MLG) specific. The results obtained for the 2024‐T3 aluminium sheets hold only for the MLG tested. The differences are more pronounced for the short cracks. These effects can be revealed by comparing the incremental crack driving force (CDF) for the ma‐mi range the ma‐large range and the na‐mi range The CDF is equivalent to the incremental volume energy density factor (VEDF). The incremental mi‐ma CDF is found to be 10–10⁵ kg mm^?1 for cracks 3–55 mm long travelling at an average velocity of 10^?5 mm s^?1. The crack velocity rises to 10^?3 mm s^?1 when the incremental CDF is increased to 10⁵–10⁶ kg mm^?1, while the crack lengths are 49–260 mm. The crack velocity for the na‐mi range of 0.040–0.043 mm slowed down to 10^?8 mm s^?1, and the incremental CDF reduces further to 10^?8–10^?2 kg mm^?1. Note that changed several orders of magnitude while the crack advanced from 0.040 to 0.044 mm. Such behaviour is indicative of the highly unstable nature of nanocracks. All results are based on using the transitionalized crack length (TCL). The TCL fatigue crack growth increment Δa is postulated to depend on the incremental CDF ΔS or ΔVEDF. The form invariance of , and is invoked by scale segmentation to reveal the multiscale nature of IGT that is inherent to fatigue crack growth. While the choice of directionality from micro to macro is not the same as that from macro to micro, this difference will not be addressed in this work.     

Microstructure evolution of CuZr polycrystals processed by high-pressure torsion

The microstructure evolution of extruded Cu–0.18 wt% Zr polycrystals processed by high-pressure torsion (HPT) at room temperature at the pressure of 4 GPa and the different number of the HPT revolutions (i.e. different strain) was investigated using the combination of the electron back-scatter diffraction, microhardness measurements and the X-ray diffraction. A significant transition from the inhomogeneous microstructure after few HPT revolutions into the homogeneous equiaxed microstructure with increasing number of HPT rotations was observed. HPT straining leads to the grain size refinement by a factor more than 100 after the 25 HPT revolutions. Moreover, the EBSD revealed an increase in the fraction of high-angle grain boundaries (HAGBs) with increasing HPT straining reaching the value of 70% after 25 revolutions. Additionally, a slight increase of the twin-related CSL Σ3 grain boundaries occurred during the microstructure refinement. The microhardness measurements confirmed the billet radial inhomogeneity at early stages of the HPT straining, whereas with increasing number of the HPT rotations, causing the specimen fragmentation and homogenization, the microhardness values increased. The average crystallite size and the average dislocation density in individual specimens determined by the XRD diffraction were in the range of approximately 100–200 nm and 2 × 10¹⁵ m⁻², respectively. Moreover, XRD measurements confirmed the absence of residual stresses in all specimens.