Significance of temperature increase in processing by high-pressure torsion

Experiments and finite element simulations were conducted to measure the temperature increase in processing disc samples by high-pressure torsion. Aluminum, copper, iron and molybdenum were selected as model materials. The temperature increases at the early stages of straining but saturates to steady-state levels at large strains. The increase of temperature is proportional to the hardness and rotation speed and is higher at higher imposed pressures and is somewhat higher at larger distances from the disc center.     

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.     

Using finite element modeling to examine the flow processes in quasi-constrained high-pressure torsion

Finite element modeling was used to examine the flow processes in high-pressure torsion (HPT) when using quasi-constrained conditions where disks are contained within depressions on the inner surfaces of the upper and lower anvils. Separate simulations were performed using applied pressures from 0.5 to 2.0 GPa, rotations up to 1.5 turns and friction coefficients from 0 to 1.0 outside of the depressions. The simulations demonstrate the distribution of effective strain within the depressions is comparable to the prediction by ideal torsion, and the applied pressure and the friction coefficient outside the depressions play only a minor role in the distribution of effective strain. The mean stresses during processing vary linearly with the distance from the center of the disk such that there are higher compressive stresses in the disk centers and lower stresses at the edges. The torque required for rotation of the anvil is strongly dependent upon the friction coefficient between the sample and the anvil outside the depressions.     

A new type face-centered cubic zirconium phase in pure zirconium

So far,only two orientation relationships (OR) between hexagonal close-packed (HCP) (α phase) and face-centered cubic (FCC) structures in zirconium and titanium alloys have been reported.Here a new type FCC phase (named γphase) with OR:< 11(2)0 >αll< 100 >γ and {0001}αll{002}γ was observed for the first time in annealed pure zirconium by means of transmission electron microscopy (TEM) technique.The α→γphase transformation can be accomplished via expansion along[1(1)00]direction and slip of Shockley partial dislocation with 1/3[1(1)00]on (0001) basal planes.     

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.

     

Effect of dynamical heating on microstructure and microhardness of Ni processed by high-pressure torsion

The microstructures and microhardness of pure Ni processed by high-pressure torsion and their evolution upon dynamical heating from room temperature to 200 °C at a heating rate of 20 °C/min were investigated. The annealing resulted in further refinement of grains, formation of more fraction of high-angle grain boundaries (GBs), modification of state of GBs and ordering of dislocations. As a result of the microstructural optimization, the microhardness of the material was increased but not decreased after the dynamical heating.     

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.     

Thermal stability of pure bcc Zr fabricated by high pressure torsion

Recently pure omega plus bcc Zr was fabricated for the first time through the simultaneous application of compression and shear to pure alpha Zr by high pressure torsion. This phase was found to be stable under ambient conditions after processing. Here the thermal stability of the pure bcc Zr thus fabricated is analyzed using differential scanning calorimetry (DSC), in-situ X-ray diffraction at high temperature and transmission electron microscopy (TEM). Our results show that the temperature of the reverse transformation of the bcc phase is close to that of the omega phase. The presence of a mixed structure formed by alternating nanolaminates of the omega and the bcc phases might play a key role in the retention of these two phases at ambient pressure and temperature.     

Using high-pressure torsion for the cold-consolidation of copper chips produced by machining

High-pressure torsion (HPT) was used to consolidate chips machined from coarse-grained copper and from copper processed by ECAP. The results are compared with discs prepared by HPT and with discs processed by a combination of equal-channel angular pressing (ECAP) and HPT. It is demonstrated that the consolidated discs have exceptionally fine microstructures and high hardness. The results suggest the possibility of a saturation in grain refinement which may be related to a release in heat during high-pressure straining.     

Grain-boundary diffusion and precipitate trapping of hydrogen in ultrafine-grained austenitic stainless steels processed by high-pressure torsion

This study was conducted to clarify the effects of grain boundaries and precipitates on room-temperature hydrogen transport in two types of austenitic stainless steels with ultrafine-grained structures produced by high-pressure torsion (HPT) and subsequent annealing. The grains in the Fe-25Ni-15Cr (in mass%) alloy containing Ti and the Fe-25Cr-20Ni alloy were refined by the HPT-processing to ∼150 and ∼85 nm, respectively. The high-temperature annealing after the HPT processing led to the precipitation of η-Ni₃Ti for the former and σ-FeCr for the latter. In the HPT-processed specimens, hydrogen diffusivity was enhanced through short-circuit diffusion because of the increased population of grain boundaries in comparison with the increased opportunity of hydrogen trapping on dislocations. As for the post-HPT-annealed specimens having the precipitates, the hydrogen diffusion was hindered by the hydrogen trapping on η-Ni₃Ti precipitates, but was not affected by σ-FeCr precipitation. This depends on the affinity between hydrogen and constituting elements.     

Influence of zirconium on mechanical properties and phase transformation in low carbon steel

The influence of zirconium on the mechanical properties and phase transformation was investigated in low carbon steel. First, the steels are subjected to a special thermomechanical regime, and the hot rolled plates were used to characterise the tensile properties and impact toughness. Second, the phase transformation behaviour of the steels with various Zr contents was evaluated by both dilatometry and metallography. Finally, to confirm the existence of Zr containing precipitates in the Zr added steels, transmission electron microscopy and energy dispersive spectroscopy were used. It was verified that plenty of fine spherical (Nb,Ti,Zr)C, which is identified to be nearly 10?nm, can be formed when the concentration of Zr is in the range of 0.015–0.030%. The effects of zirconium on the phase transformation, including proeutectoid ferrite and pearlite transformation, and mechanical properties evolution were also identified and discussed.     

Zirconium phase transformations observed by “in-situ” XRD analysis

The high temperature phase transformations of zirconium were investigated by “in-situ” X-ray diffraction analysis. The experiments were carried out in a high temperature chamber Anton Paar HTK 1200N mounted on powder diffractometer Panalytical X’Pert Pro. The chamber was evacuated by a turbo-molecular pump creating a pressure about 3 × 10⁻⁴ Pa at the temperature 1200 °C before the measurement. The experimental samples were uniformly heated without thermal gradients up to 1000 °C or 1200 °C and then exposed for 180 min. At high temperature four or six diffraction patterns of each sample were recorded for the illustration of the phase changes in the material structure. Chemical composition of some samples after the exposure was determined by secondary ion mass spectrometry and compared with composition of the material in its initial state.     

The second phase particles and mechanical properties of 2124 aluminum alloy processed by accumulative back extrusion

An age-hardenable 2124 aluminum alloy was severely deformed by accumulative back extrusion (ABE) method up to three passes at 100 and 200 °C. The characteristics of the second phase particles were studied using scanning electron microscopy. The results indicated that the size of primary particles had been reduced after the first ABE pass where even much finer particles was obtained as the successive passes were applied. In addition, the secondary particles were fragmented into finer pieces after ABE at 100 °C, whereas a particle coarsening was realized as the deformation temperature rose to 200 °C. The latter was attributed to the Ostwald ripening mechanism. However, the volume fraction of secondary particles was significantly decreased after three ABE passes at 200 °C due to the occurrence of deformation induced dissolution. Additionally, the tensile properties of the processed materials were measured utilizing a miniaturized tensile testing method. The results were justified considering the evolution of the second phase particles.     

Zr对非晶Co-Si薄膜晶化和相变的影响

利用X射线衍射分析技术结合差示扫描量热仪,研究了Zr在Co-Si合金非晶薄膜晶化和相变过程中的作用.结果表明:溅射态非晶Co-Si薄膜和Co-Si-Zr薄膜在加热到950℃的过程中,均主要析出Co-Si和Co-Si2两种晶化相;Zr的加入增大了晶化激活能,抑制了晶化相的形核和长大,使薄膜能够在更高的温度范围保持稳定,但在高温800℃以上加热时,却使晶化相Co-Si向Co-Si2的转变加快.     

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.     

Full density consolidation of pure aluminium powders by cold hydro-mechanical pressing

Gas atomized pure Al powders were successfully consolidated into full density at room temperature by newly developed cold hydro-mechanical pressing (CHMP). The effects of the configuration of pressures on the relative density and microstructure of consolidated specimens were investigated. The full density consolidation of metal powders at room temperature resulted from severe shear deformation of the particles which ruptured the surface oxide layers. Our works demonstrate that the novel CHMP provides an economical approach to consolidate particulate material into high quality bulk preforms for engineering applications.     

Analysis of phase transformation in orthotropic elastoplastic materials under tension and shear

This study aims to investigate the possibility and features of stress-induced phase transformation in orthotropic elastoplastic materials with strain-softening behavior under complex stress state. The theory on multi-phase equilibrium is applied. Two-phase deformations in strips under tension and shear are analyzed for sixteen loading states. The features of the solutions are described in detail. It is shown that locally plastically deformed bands can indeed form in such materials and can spread along the length of the strips whilst the stresses in both phases remain unchanged, but, for thin-walled tubes under torsion, no localized bands can be observed and only homogeneous phase transformation on the whole tubes can occur. Influence of the values of the shear stress on inclination angles of the bands is revealed. All the predicted results are consistent with experimental observations.     

Effect of magnetic fields on solid-melt phase transformation in pure bismuth

The differential thermal analysis (DTA) apparatus has been designed in order to investigate the effect of magnetic fields on solid-melt phase transformation in pure bismuth. The endothermic peaks of DTA curves show that melting is insensitive to magnetic fields, which can be verified from thermodynamics. However, the exothermic peak obviously shifts to higher temperature as the magnetic field strength increases, from which the magnetic field does not affect the crystal growth but nucleation. On the basis of the assumption that there is an intermediate state between a crystal nucleus and a liquid atom, one possible reason for the shift of exothermic peaks is that kinetic barrier of nucleation is lowered and nucleation is activated by magnetic fields.     

Solidification pathway and phase transformation behavior in a beta-solidified gamma-TiAl based alloy

The phase transformation behavior of an as-cast Ti-42Al-5 Mn(at.%) alloy after subsequent quenching from 1380 ℃ to 1000 ℃ was investigated based on the differential thermal analysis(DTA),electron probe micro analyzer-backscattered electrons(EPMA-BSE),transmission electron microscope(TEM) and X-ray diffraction(XRD).The results show that,the solidification path can be summarized as follows:Liquid→Liquid+β→β→β→α→β+α+γ→β_o+α_2+γ→β_o+γ+α_2/γ→β_o+γ+α_2/γ+β_(o,sec),with the phase transformation α→β temperature(T_β)=1311℃,phase transformation γ→β temperature of(T_(γsolv))=1231℃,phase transformation α_2→α or β_o→β temperature(T_(α2→α)/T_(β_o→β))=1168 C,eutectoid temperature(T_(eut))=1132 ℃ and T_(α_2/γ→βo,sec)≈1 120℃.In comparison with Ti-42 Al alloy,the T_(eut) and T_(γsolv)are slightly increased while both the Tp is decreased obviously by 5% Mn addition.When quenched from the temperature of 1380-1260 ℃,the martensitic transformation β→α' could occur to form the needlelike martensite structure in β area.This kind of martensitic structure is much obvious with the increase of temperature from 1260℃ to 1380 ℃.When the temperature is below T_(γsolv)(1231℃),the γ grains would nucleate directly from the β phase.For the temperature slightly lower than T_(eut)(1132℃),the dotted β_(o,sec) phases could nucleate in the lamellar colonies besides the γ lamellae precipitated withinα_2 phase.Finally,at room-temperature(RT),the alloy exhibits(p_o+α_2+γ) triple phase with microstructure of β_o+lamellae+γ,of which the lamellar structure consists of α_2,γ and β_(o,sec) phases.The phase transformation mechanisms in this alloy,involving β→α',β→γ,α_2→α_2/γ and α_2→β_(o,sec) were discussed.