High Pressure Torsion Deformation Research Articles

The Effect of Processing Route on Properties of HfNbTaTiZr High Entropy Alloy.

High entropy alloys (HEA) have been one of the most attractive groups of materials for researchers in the last several years. Since HEAs are potential candidates for many (e.g., refractory, cryogenic, medical) applications, their properties are studied intensively. The most frequent method of HEA synthesis is arc or induction melting. Powder metallurgy is a perspective technique of alloy synthesis and therefore in this work the possibilities of synthesis of HfNbTaTiZr HEA from powders were studied. Blended elemental powders were sintered, hot isostatically pressed, and subsequently swaged using a special technique of swaging where the sample is enveloped by a titanium alloy. This method does not result in a full density alloy due to cracking during swaging. Spark plasma sintering (SPS) of mechanically alloyed powders resulted in a fully dense but brittle specimen. The most promising result was obtained by SPS treatment of gas atomized powder with low oxygen content. The microstructure of HfNbTaTiZr specimen prepared this way can be refined by high pressure torsion deformation resulting in a high hardness of 410 HV10 and very fine microstructure with grain size well below 500 nm.

Effect of the High-Pressure Torsion (HPT) and Subsequent Isothermal Annealing on the Phase Transformation in Biomedical Ti15Mo Alloy

Ti15Mo metastable beta Ti alloy was solution treated and subsequently deformed by high-pressure torsion (HPT). HPT-deformed and benchmark non-deformed solution-treated materials were annealed at 400 °C and 500 °C in order to investigate the effect of UFG microstructure on the α-phase precipitation. Phase evolution was examined using laboratory X-ray diffraction (XRD) and by high-energy synchrotron X-ray diffraction (HEXRD), which provided more accurate measurements. Microstructure was observed by scanning electron microscopy (SEM) and microhardness was measured for all conditions. HPT deformation was found to significantly enhance the α phase precipitation due the introduction of lattice defects such as dislocations or grain boundaries, which act as preferential nucleation sites. Moreover, in HPT-deformed material, α precipitates are small and equiaxed, contrary to the α lamellae in the non-deformed material. ω phase formation is suppressed due to massive α precipitation and consequent element partitioning. Despite that, HPT-deformed material after ageing exhibits the high microhardness exceeding 450 HV.

Tuneable Magneto-Resistance by Severe Plastic Deformation

Bulk metallic samples were synthesized from different binary powder mixtures consisting of elemental Cu, Co, and Fe using severe plastic deformation. Small particles of the ferromagnetic phase originate in the conductive Cu phase, either by incomplete dissolution or by segregation phenomena during the deformation process. These small particles are known to give rise to granular giant magneto-resistance. Taking advantage of the simple production process, it is possible to perform a systematic study on the influence of processing parameters and material compositions on the magneto-resistance. Furthermore, it is feasible to tune the magneto-resistive behavior as a function of the specimens’ chemical composition. It was found that specimens of low ferromagnetic content show an almost isotropic drop in resistance in a magnetic field. With increasing ferromagnetic content, percolating ferromagnetic phases cause an anisotropy of the magneto-resistance. By changing the parameters of the high pressure torsion process, i.e., sample size, deformation temperature, and strain rate, it is possible to tailor the magnitude of giant magneto-resistance. A decrease in room temperature resistivity of ~3.5% was found for a bulk specimen containing an approximately equiatomic fraction of Co and Cu.

Reduction of shear localization through structural rejuvenation in Zr–Cu–Al bulk metallic glass

The effect of high-pressure torsion (HPT) on the structure and mechanical behavior of Zr50Cu40Al10 bulk metallic glass was investigated using calorimetry, density measurement, high-energy X-ray scattering and nanoindentation. Increase in rotation number, and thus plastic deformation, lowers the hardness and elastic modulus, as well as the critical strain rate for suppressing the shear localization. Strain-softening induced by plastic deformation is attributed to the structural rejuvenation. Wide-angle X-ray scattering (WAXS) measurements suggest that HPT deformation reduces and broadens the peaks in the pair distribution function indicative of pronounced structural disordering and enabling structural rejuvenation. Measured excess free volume of 0.8% for samples deformed by HPT with a maximum structural relaxation enthalpy of 29.5 J/g in the sample with 20 and 50 revolutions.

Microstructure Evolution and Phase Transformation of Ti‐1.0 wt%Fe Alloy with an Equiaxed α + β Initial Microstructure during High‐Pressure Torsion and Subsequent Annealing

Herein, the microstructure evolution and phase transformation during high‐pressure torsion (HPT) and subsequent annealing of Ti‐1.0 wt%Fe alloy with an equiaxed α + β initial microstructure are investigated. Both α and β grains in the initial microstructure fragment into smaller and elongated areas during HPT. X‐ray diffraction (XRD) analysis reveals that the deformation‐induced α to ω and β to ω phase transformations occur from the onset of HPT deformation. The β phase totally disappears after 1.5 rotations, whereas some α phase still remains even after 4 rotations. The nano‐hardness of β grains is much larger than that of α grains in the initial microstructure, due to the nano‐sized athermal ω precipitates inside the β grains. With an increase in the HPT rotation, the nano‐hardness of prior α areas continuously increases, whereas that of the prior β areas gradually decreases. During the subsequent annealing of HPT‐deformed sample, both ω to α and ω to β reverse‐phase transformations occur, and the deformation‐induced ω phase totally disappears at annealing temperatures above 400 °C. The recrystallization of α phase completes at an annealing temperature of 500 °C. At higher annealing temperatures, secondary α precipitates with ultrafine grain sizes start to appear inside the prior β areas.

Miniature tensile testing of SPD processed fine-grained aluminum

Manufacturing of ultrafine-grained (UFG) or nanocrystalline (NC) metals via a single step top-down approach imposing severe plastic deformation (SPD) is one of the most promising ways to achieve superior properties such as high strength and superplastic forming capability. Nonetheless, the lack of relevant data on their post-SPD performance in different test environments makes it difficult to fully understand their mechanical behavior. While characterizing the tensile behavior, almost all of the previous reports are limited to the discussion on the plastic performance of the material in terms of elongation to failure and corresponding strength, with only a few studies discussing the effect of grain fragmentation on work hardening response of the material. In the present work, a comprehensive analysis is presented in terms of the uniform and post-necking mechanical behavior of the ultrafine-grained material. Commercially pure aluminum is subjected to high pressure torsion (HPT) deformation with strains ranging from very low levels (γ ≈ 2.1) to values well in the saturation regime (γ ≈ 25.1). When tested in uniaxial tension, the strength increases monotonously. The uniform elongation improves with the imposed HPT strain, though remains lower than the value of the initial material. Based on the slopes of the stress-strain curve, three distinct zones are identified, i.e. uniform deformation, post-necking-1, and post-necking-2. With accumulating SPD deformation, the material shows enhanced pre-necking strength and ductility; while post-necking material fails early and at lower strength levels. The post-necking response is observed to be highly microstructure dependent: a lower grain size augments the resistance for micro-crack propagation and thus the ductility, however, once initiated, a crack propagates much faster in fine-grained than in coarse-grained HPT processed material.

High-pressure torsion deformation process of bronze/niobium composite

Abstract The deformation process in the material volume under high-pressure torsion (HPT) was studied. As a model object for the observation of deformation process, we used a composite comprising a bronze matrix and niobium filaments. The arrangements of the niobium filaments in the bronze matrix and their size have regular geometry. This allows us to monitor and measure the displacement of the niobium filaments in the sample volume, which results from HTP. The bronze/niobium composite samples were subjected to HPT at room temperature and 6 GPa, and the number of revolutions N=1/4, 1/2, 1, 2, 3 and 5. It was shown that HPT with revolution number of 1 leads to the 360° rotation of entire sample volume without sample slippage. Similar deformational behavior during HPT can be expected for high-ductility metallic materials. The increase in the number of revolutions more than 2 leads to twisting the niobium filaments in the sample volume and the formation of “vortex” multilayer structure. The mechanisms for the formation of such structures were discussed.

Severe plastically deformed commercially pure aluminum: Substructure, micro-texture and associated mechanical response during uniaxial tension

Severe plastic deformation (SPD) of metals to obtain ultra-fine or even nano-sized grains has proven to be an interesting concept explored over the last few decades. However, the mechanical behavior of SPD metals and the underlying microstructural phenomena are not fully understood yet. In present work, commercially pure aluminum was subjected to high pressure torsion (HPT) deformation with strains ranging from very low levels to values well in the steady-state microstructure regime. The mechanical properties of the HPT processed samples were determined using tensile tests on miniature samples using full-field strain mapping. Orientation imaging microscopy (OIM) was utilized to follow the progression of grain refinement and texture as a function of imposed SPD. Local orientation based misorientation gradients helped to perform statistical boundary analysis and determine the fractions of incidental and geometrically necessary dislocation (GND) boundaries and local GND densities.From probability density distributions of the misorientation gradients two different stages of microstructural evolution, namely, fragmentation and saturation, could be discerned. The strength increased monotonously and the uniform elongation, though lower than the value of the annealed material, enhanced with the imposed strain in HPT. The post-necking response was observed to be highly microstructure dependent, where a lower grain size augmented the resistance for micro-crack propagation and enhanced the elongation-to-failure. In addition, the work hardening response corresponding to the yield point displayed maxima coinciding with the onset of the saturation stage. Anisotropy in fracture strain, observed between the axial and radial directions in a disk-like HPT sample, reduced with the randomization of shear texture, while higher intensities of the C {100}<110> orientation was considered responsible for the lower elongation-to-failure along the radial direction.

High-Pressure Torsion Deformation Induced Phase Transformations and Formations: New Material Combinations and Advanced Properties

Heavy plastic shear deformation at relatively low homologous temperatures is called high-pressure torsion (HPT) deformation, which is one method of severe plastic deformation (SPD). The aim of the paper is to give an overview of a new processing approach which permits the generation of innovative metastable materials and novel nanocomposites by HPT deformation. Starting materials can be either coarse-grained multi-phase alloys, a mixture of different elemental powders or any other combination of multiphase solid starting materials. After HPT processing, the achievable microstructures are similar to the ones generated by mechanical alloying. Nevertheless, one advantage of the HPT process is that bulk samples of the different types of metastable materials and nanocomposites are obtained directly during HPT deformation. It will be shown that different material combinations can be selected and materials with tailored properties, or in other words, materials designed for specific applications and the thus required properties, can be synthesized. Areas of application for these new materials range from hydrogen storage to materials resistant to harsh radiation environments.

In-Situ Synchrotron Profile Analysis after High-Pressure Torsion Deformation

The presence of hydrostatic pressure is a general crucial characteristic of severe plastic deformation methods for reaching high strains and for introducing large quantities of lattice defects, which are necessary to establish new grain boundaries. Insights into the processes occurring during deformation and the influence of hydrostatic pressure are necessary to help better understand the SPD methods. A special experimental procedure was designed to simulate the hydrostatic pressure release: High pressure torsion (HPT)-deformed microstructure changes related to the release of hydrostatic pressure after the HPT deformation of copper and nickel were studied by freezing the sample before releasing the pressure. High-resolution in-situ X-ray diffraction of the heating process was performed using synchrotron radiation in order to apply X-ray line profile analysis to analyze the pressure release. The results on copper and nickel generally indicated the influence of hydrostatic pressure on the mobility and interaction of deformation-induced defects as well as the resulting microstructure.

Tuning of the magnetic properties of Hf2Co11B alloys through a combined high pressure torsion and annealing treatment

The evolution of crystalline structure induced by heat treatment and high pressure torsion (HPT) deformation and its influence on the magnetic properties of amorphous and partially crystalline Hf2Co11B ribbons were investigated. Slight improvement in thermal stability of the as-quenched sample after deformation was observed as indicated by the shift of the first crystallization peak from 592 to 598 °C. Plastic deformation of the initially annealed partially crystalline alloy led to its amorphization, as confirmed by X-ray diffraction (XRD). However, the presence of small volume fraction of needle-like nanocrystals was indicated by transmission electron microscopy (TEM). The annealed sample subjected to high pressure torsion was characterized by the reduced coercive field, from 0.7 to 0.2 kOe, while the subsequent reannealing of the deformed sample enhanced the coercivity up to 1.3 kOe. Transmission electron microscopy analysis revealed the existence of nanocrystals with large lattice constant of 8 Å characteristic of Hf2Co11 phase. Magnetic measurements also confirmed that some nanocrystals of the hard magnetic phase were present in the sample after deformation, indicating their importance for maximization of coercivity. The isolation of this quite elusive phase is clearly linked to magnetic performance and the method combining severe plastic deformation (SPD) and heat treatment was found to allow tuning of the structure and improving the hard magnetic properties.

Structure and Hardening of Layered Steel/Copper/Steel Material Subjected to Co-Deformation by High-Pressure Torsion

The effect of high-pressure torsion (HPT) deformation on the structure and hardening of three-layered hybrid steel/copper/steel material and its components has been studied in this work. The HPT of the three-layered steel/copper/steel blank results in the formation of a strong boundary between layers, which is mainly smooth, and in an increase in their strength. However, no maximally possible values of microhardness of the outer steel layers of the hybrid are reached because of the absence of ultrafine-grained structure in them.

High strength nanocrystalline Cu\u2013Co alloys with high tensile ductility

A supersaturated single-phase Cu–26 at.% Co alloy was produced by high-pressure torsion deformation, leading to a nanocrystalline microstructure with a grain size smaller than 100 nm. The nonequilibrium solid solution decomposed during subsequent isothermal annealing. In situ high-energy X-ray diffraction was used to map changes linked to the separating phases, and the development of a nanoscale Cu–Co composite structure was observed. To gain further information about the relationship of the microstructure and the mechanical properties after phase separation, uniaxial tensile tests were conducted on as-deformed and isothermally annealed samples. Based on the in situ diffraction data, different isothermal annealing temperatures were chosen. Miniaturized tensile specimens with a round cross section were tested, and an image-based data evaluation method enabled the evaluation of true stress–strain curves and strain hardening behavior. The main results are as follows: all microstructural states showed high strength and ductility, which was achieved by a combination of strain-hardening and strain-rate hardening.

Influence of High Pressure Torsion on structure and properties of Zr-Ti-Nb alloy synthesized from TiH2, ZrH2 and Nb powders

The effects of High Pressure Torsion (HPT) deformation on the microstructure and mechanical properties of a corrosion-resistant, low modulus Zr-Ti-Nb alloy, obtained by vacuum sintering of cold pressed powder are studied. It is shown that HPT treatment leads to the following effects: elimination of the residual chemical heterogeneity of the alloy, significant reduction of the porosity of the material and the pore size and improvement of mechanical properties. The phase composition of the alloy does not change during processing. The results show the potential of the HPT method for the production of alloys with a favorable combination of properties.

High-pressure torsion driven mechanical alloying of CoCrFeMnNi high entropy alloy

Fully-dense bulk CoCrFeMnNi high entropy alloy (HEA) has been obtained by high-pressure torsion (HPT) deformation. Elemental, micrometer sized powders were successively mixed in equimolar proportions, consolidated and deformed at 5 GPa using imposed shear strain equivalent up to 100 rotations. Transmission electron microscopy, X-ray diffraction and 3D atom probe tomography were performed to study the microstructure evolution resulting from HPT-induced mechanical alloying. Single-phase, nanocrystalline (grain size of 50 nm) alloy with smaller chromium oxide precipitates (of 7–10 nm) exhibited a hardness of 6700 MPa, which is the highest one reported for as-processed bulk CoCrFeMnNi alloys to our best knowledge.

Anomalous re-ordering of Fe3Al disordered by high pressure torsion deformation

Severe plastic deformation of bulk DO3 ordered Fe-30 at.%Al was carried out using high pressure torsion to make a chemically disordered nanocrystalline alloy. To achieve re-ordering the alloy was heated to 250, 370 and 500 °C. Transmission electron microscopy investigations reveal that in contrast to the phase diagram B2 ordering occurs at low temperatures. Upon further heating, after the B2 order is already well restored, nanosized DO3 ordered domains grow within large B2 ordered regions. It is proposed that the anomalous re-ordering is triggered by deformation-induced vacancies and other defects leading to the formation of the high-temperature B2 ordered phase.

Hardening by Annealing and Implementation of High Ductility of Ultra-Fine Grained Aluminum: Experiment and Theory

Abstract The influence of low temperature annealing and subsequent deformation on microstructure, strength and ductility was investigated for the first time for high pressure torsion (HPT) processed commercially pure Al. Extremely high increases in the conventional yield stress (up to 50%) and ultimate tensile strength (up to 30%) were obtained by annealing of the ultrafine grained (UFG) samples in the range 90-200 °C for 1 h. Such increases were accompanied by a sharp drop in ductility up to 1%. Implementation of high ductility at the level of coarse-grained Al, while maintaining high strength of the HPT-processed sample was demonstrated for the first time and achieved by repeating the low temperature annealing followed by subsequent additional HPT deformation. The key role of relaxation of non-equilibrium high-angle grain boundaries (GBs) in the strengthening effect of UFG-Al by annealing is shown. Two theoretical models are suggested to explain the hardening by annealing and the implementation of high ductility in UFG structures. Within the models, plastic deformation occurs through emission of lattice dislocations from triple junctions of GBs containing pile-ups of grain-boundary dislocations, glide of the lattice dislocations across neighboring grains, their accumulation at and climb along the opposite GBs. The energy characteristics and the critical stresses of dislocation emission are determined in two different cases, for UFG Al subjected to annealing only and to annealing with subsequent additional HPT deformation. The calculated theoretical dependences of the flow stress on the plastic deformation value well fit qualitatively and quantitatively to our experimental data.

Microstructural evolution and its effect on the mechanical behavior of Ti-5Al-5Mo-5V-3Cr alloy during aging

Development of different microstructures with α phase precipitates by a series of aging of Ti-5Al-5Mo-5V-3Cr (mass%, Ti-5553) alloy and their influences on the mechanical behavior were studied. Coarse acicular α phase formed in the solution treated (ST) samples, while ultrafine equiaxed α phase formed in the high-pressure torsion deformed (HPTed) samples. The strengthening by the precipitation of α phase could be explained by the Hall-Petch relationship between the yield strength and the α-to-α interphase spacing. The Hall-Petch coefficient of the HPTed samples was larger than that of the ST samples, because the equiaxed α phase was harder than the acicular α phase. The ductility of the HPTed samples was enhanced by the homogeneous distribution of equiaxed α phase.

Exceptional grain refinement in a Mg alloy during high pressure torsion due to rare earth containing nanosized precipitates

The influence of the nanosized rare earth (RE) containing precipitates on grain refinement during severe plastic deformation is investigated in detail through a study of high pressure torsion (HPT) processing of a solution treated and aged Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt%). In the early stages of HPT deformation, dislocation generation and pile-up is promoted by the nanosized RE containing β′ precipitates. With increasing strain, the precipitates are cut by the moving dislocations and gradually dissolve into the α-Mg matrix aided by dislocations serving as diffusion channels for solute atoms. After HPT for 2 turns, the hardness reaches a maximum and on further deformation the hardness decreases although the microstructure refinement continues and the dislocation density is increasing. This is due to the continuing dissolution of precipitates, which dominates the hardness evolution at this stage. After HPT for 16 turns, the precipitates have almost completely dissolved and the average grain size is ~33 nm, which is the smallest ever reported for a Mg- or Al-based alloy. The present peak-aged Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt%) alloy exhibits quite different microstructure evolution and hardening behaviour during HPT processing as compared to both the cast and the solutionized Mg-Gd-Y-Zn-Zr alloy.

An EBSD analysis of Fe-36%Ni alloy processed by HPT at ambient and a warm temperature

Electron backscatter diffraction (EBSD) was used to evaluate the evolution of microstructure and texture in an Fe-36%Ni (wt. %) alloy after processing by high-pressure torsion (HPT) up to 10 turns at ambient temperature and 250 °C. HPT processing led to a strong microstructural refinement with average grain sizes of ∼0.30 and ∼0.24 μm at ambient temperature and 250 °C, respectively. The high-angle grain boundary fraction was slightly higher after processing at 250 °C and gradually increased with increasing numbers of HPT turns to saturate after 5 turns. The crystallographic texture after HPT processing at ambient temperature was characterized by typical A and B fibers of a simple shear FCC texture with domination of the B component. By contrast, the A, B and C components developed during HPT processing at 250 °C with C as the dominant texture component. The microhardness values for both HPT deformation conditions were very similar with a continuous increase with increasing HPT turns and almost saturation after 5 HPT turns. The grain size dependence of the microhardness followed the Hall-Petch relationship. The explanation for the microstructure, texture and microhardness difference was attributed to a dynamic recovery process that operates during processing at 250 °C.