Improvement of strength and conductivity in Cu-alloys with the application of high pressure torsion and subsequent heat-treatments

Quenched and slowly cooled (annealed) Cu–0.7 %Cr, Cu–0.9 %Hf, and Cu–0.7 %Cr–0.9 %Hf alloys were processed by high pressure torsion (HPT). The microstructures of the alloys were studied immediately after HPT and subsequent annealing. It has been shown that the microhardness and the thermal stability of the severely deformed microstructure increase, while the average grain size decreases in the order of Cu–0.7 %Cr, Cu–0.9 %Hf, and Cu–0.7 %Cr–0.9 %Hf alloys. The microhardness in all alloys is higher after quenching and HPT, than after annealing and HPT. The largest dislocation density is achieved by quenching and HPT in Hf-containing samples. Cu₅Hf phase precipitations in Hf-containing alloys are more effective in retarding grain growth in comparison with Cr particles and lead to additional hardening during aging. It has been demonstrated that HPT-processing with subsequent heat-treatment might yield the combination of large hardness and high electrical conductivity in Cu alloys.     

Interpretation of hardness evolution in metals processed by high-pressure torsion

The processing of metals through the application of high-pressure torsion (HPT) provides the potential for achieving exceptional grain refinement in bulk disks. Numerous reports are now available describing the application of HPT to a range of pure metals and simple alloys. Excellent grain refinement was achieved using this processing technique with the average grain size often reduced to the nanoscale range. By contrast, the development of microstructure and local hardness is different depending upon the material properties. In order to make HPT processing more practical, it is indispensable to investigate the nature of the sample characteristics immediately after conventional HPT processing. Accordingly, this report demonstrates the different models of hardness evolution using representative materials of AZ31 magnesium alloy, high-purity aluminum, and Zn–22 % Al eutectoid alloy processed by HPT. Separate models are described for the evolution of hardness with equivalent strain, and the correlation between these models is suggested by the homologous temperature of HPT processing. A special emphasis is placed on examining the numerical expression of the level of strain hardening or softening of these metals with increasing equivalent strain.     

Structural and hardness inhomogeneities in Mg–Al–Zn alloys processed by high-pressure torsion

Experiments were conducted to evaluate the evolution of structure and hardness in processing by high-pressure torsion (HPT) of the magnesium AZ91 and AZ31 alloys. Both alloys were processed by HPT at room temperature for 1/4, 1, and 5 turns using a rotation speed of 1 rpm. Structure observations and microhardness measurements were undertaken on vertical cross-sectional planes cut through the HPT disks. The results demonstrate that the deformation is heterogeneous across the vertical cross sections but with a gradual evolution toward homogeneity with increasing numbers of revolutions.     

Mechanical properties and microstructure evolution in an aluminum 6082 alloy processed by high-pressure torsion

A commercial aluminum 6082 alloy was used to investigate the effect of the initial condition on subsequent processing by high-pressure torsion (HPT). The alloy was prepared in two different initial conditions: (i) in a T651 annealed condition and (ii) after a solution treatment followed by over-aging and subsequent processing by equal-channel angular pressing (ECAP). All samples were processed by HPT through 1/2, 1, 2, 5, and 10 turns and then the microstructures were examined using electron backscattered diffraction (EBSD). Significant grain refinement was achieved after processing by HPT through 5 turns with measured grain sizes of ~0.5 μm in both types of alloy. Microhardness measurements were conducted to evaluate the evolution of hardness after HPT for the two initial conditions. It is demonstrated that there is a difference in the hardness values between these two initial conditions, and this difference remains almost constant after processing by HPT.     

Evolution of microhardness and microstructure in a cast Al–7 % Si alloy during high-pressure torsion

Disks of a cast Al–7 % Si alloy were processed through high-pressure torsion (HPT) for 1/4, 1/2, 1, 5, and 10 revolutions under a pressure of 6.0 GPa and at temperatures of 298 and 445 K. The hardness of the samples after processing was significantly higher than in the cast sample, and the hardness profiles across the samples became more uniform with increasing numbers of turns. Processing at higher temperature gave lower hardness values. Experiments were conducted to examine the effects of HPT processing on various microstructural aspects of the cast Al–7 % Si alloy such as the grain size, the Taylor factor, and the fraction of high-angle grain boundaries. The results demonstrate that there is a correlation between trends in the microhardness values and the observed microstructures.     

Microstructure and microtexture evolution with aging treatment in an Al–Mg–Si alloy severely deformed by HPT

Experiments were conducted on an Al–0.6 % Mg–0.4 % Si alloy to evaluate the effect of different preliminary thermal treatments on the evolution of microstructure and microtexture during processing by High-Pressure Torsion (HPT). Disks of the alloy were solution-treated, then some disks were briefly aged at 473 K, and other disks were briefly aged at 523 K before processing by HPT for up to 20 complete revolutions. The processing by HPT refined the microstructure to an average grain size as small as ~0.25 μm in the solution-treated alloy after 20 turns but preliminary aging led to slightly larger average grain sizes of ~0.35–0.40 μm after 20 turns. For all processing conditions, there was a high fraction of high-angle grain boundaries after HPT and it is shown that aging introduces changes in the microtexture intensities.     

Solid-state amorphization of Cu + Zr multi-stacks by ARB and HPT techniques

A series of CuZr binary alloys with wide composition range were fabricated through ARB and HPT techniques using pure Cu and Zr metals as the starting materials. Bulk alloy sheets with thickness of about 0.8 mm after ARB process and alloy disks with 0.30 mm in thickness and 10 mm in diameter after HPT process can be obtained, respectively. The structures of all the alloys were found to be gradually refined with the increase of ARB cycles or HPT rotations. As a result, nanoscale multiple-layered structure was formed for the 10 cycled ARBed specimens, which could partially transform into amorphous phase during subsequent low temperature annealing. While for the as-HPTed sample, the alloy was completely amorphized after 20 rotations without any heat treatment. The thermal stabilities of the amorphous alloys were studied. The deformation behavior and the amorphization mechanism during the ARB and HPT process were put forward and discussed.     

A comparison of microstructures and mechanical properties in a Cu–Zr alloy processed using different SPD techniques

Experiments were conducted to evaluate the microstructures and mechanical properties of a Cu–0.1 % Zr alloy processed using two different techniques of severe plastic deformation: equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). The samples were processed at room temperature through ECAP for eight passes or through HPT for 10 turns. The results show HPT is more effective both in refining the grains and in producing a large fraction of grain boundaries having high angles of misorientation. Both procedures produce reasonably homogeneous hardness distributions but the average hardness values were higher after HPT. In tensile testing at 673 K, the highest strength and ductility was achieved after processing by HPT. This is attributed to the grain stability and high fraction of high-angle grain boundaries produced in HPT.     

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.

     

Using ball indentation to determine the mechanical properties of an Al-7475 alloy processed by high-pressure torsion

A commercial Al-7475 alloy with an initial grain size of ~40 μm was processed by high-pressure torsion (HPT) for up to 2 turns at room temperature under a pressure of 6.0 GPa. The mechanical properties of the processed materials were evaluated by the ball-indentation technique to give information on the yield strength and the ultimate tensile strength. Following HPT, microhardness measurements revealed a steady increase in the hardness values from the centers of the samples towards the edges. After 2 turns, the ultimate tensile strength was ~1050 MPa at the edge of the disk and the measured grain size was ~70 nm. The results demonstrate the potential for using HPT to achieve excellent grain refinement in the Al-7475 alloy.     

The thermal stability of nanocrystalline cartridge brass and the effect of zirconium additions

The thermal stability of nanocrystalline cartridge brass (Cu–30 at.% Zn) and brass–Zr alloys were investigated. The alloys were produced by cryogenic ball milling and subsequently heat treated to a maximum temperature of 800 °C. The grain size of pure brass was found to be relatively stable in comparison to pure copper, and a high hardness was retained up to 600 °C. When 1 at.% zirconium was alloyed with the brass, the grain size was stabilized near 100 nm even at 800 °C. At the highest temperature, hardness was retained above 2.5 GPa for 1 and 5 at.% zirconium alloys, but the pure brass softened significantly. The stabilization is believed to be dominated by Zn–Zr interactions as a second phase of these two was observed in X-ray diffraction and transmission electron microscopy. Thermodynamic modeling indicates a zero grain boundary energy may be achieved depending on the mixing enthalpy value used (i.e., calculated vs. experimental) under ideal conditions, but microstructural features such as twinning and second phase particles are thought to be the dominant stabilization mechanism. Zr worked well in stabilizing the brass in the nanocrystalline state to nearly 90 % of its melting temperature.     

Effect of Initial Annealing Temperature on Microstructural Development and Microhardness in High‐Purity Copper Processed by High‐Pressure Torsion

The effect of the initial annealing temperature on the evolution of microstructure and microhardness in high purity OFHC Cu is investigated after processing by HPT. Disks of Cu are annealed for 1 h at two different annealing temperatures, 400 and 800 °C, and then processed by HPT at room temperature under a pressure of 6.0 GPa for 1/4, 1/2, 1, 5, and 10 turns. Samples are stored for 6 months after HPT processing to examine the self‐annealing effects. Electron backscattered diffraction (EBSD) measurements are recorded for each disk at three positions: center, mid‐radius, and near edge. Microhardness measurements are also recorded along the diameters of each disk. Both alloys show rapid hardening and then strain softening in the very early stages of straining due to self‐annealing with a clear delay in the onset of softening in the alloy initially annealed at 800 °C. This delay is due to the relatively larger initial grain size compared to the alloy initially annealed at 400 °C. The final microstructures consist of homogeneous fine grains having average sizes of ≈0.28 and ≈0.34 µm for the alloys initially annealed at 400 and 800 °C, respectively. A new model is proposed to describe the behavior of the hardness evolution by HPT in high purity OFHC Cu.     

Composition-dependent dynamic precipitation and grain refinement in Al-Si system under high-pressure torsion

Understanding composition effects is crucial for alloy design and development. To date, there is a lack of research comprehensively addressing the effect of alloy composition on dynamic precipitation, segregation and grain refinement under severe-plastic-deformation processing. This research investigates Al-x Si alloys with x = 0.1, 0.5 and 1.0 at.% Si processed by high pressure torsion(HPT) at room temperature by using transmission electron microscopy, transmission Kikuchi diffraction and atom probe tomography. The alloys exhibit interesting composition-dependent grain refinement and fast dynamic decomposition under HPT processing. Si atoms segregate at dislocations and Si precipitates form at grain boundaries(GBs) depending on the Si content of the alloys. The growth of Si precipitates consumes most Si atoms segregating at GBs, hence the size and distribution of the Si precipitates become predominant factors in controlling the grain size of the decomposed Al-Si alloys after HPT processing. The hardness of the Al-Si alloys is well correlated with a combination of grain-refinement strengthening and the decomposition-induced softening.     

Understanding Melt Flow Behavior for Al-Si Alloys Processed Through Vertical Centrifugal Casting

The objective of this article is to investigate the appearance, microstructure, and hardness of Al-Si alloys Al-12Si and Al-17Si in vertical centrifugal casting process. During rotation of the mold, molten metal flow affects the formation of uniform cylinder. In this study, flow of molten metal for Al-Si alloys at different rotational speeds is focused. It is found that for Al-17Si alloy a uniform cast tube is observed for 1000 rpm, whereas for Al-12Si it is at 1200 rpm; above and below these speeds, irregular cast tubes are formed. Finally, fine structured grain size with high hardness value is found in a uniform cast tube in comparison with others.     

Strengthening of Al through addition of Fe and by processing with high-pressure torsion

Iron is a common impurity element in aluminum and is expected to be used in a controlled manner. In this study, high-pressure torsion (HPT) was applied to 10-mm diameter bulk disk-type samples of Al–Fe alloys with different Fe additions: 2 and 4 wt%, and different initial states: as-cast, extruded, and annealed. Intense strain was introduced to the materials by HPT processing at room temperature under a pressure of 6 GPa for up to 75 revolutions. Tensile tests showed that a significant increase in the UTS above 500 MPa occurs with 13 % elongation in the Al–2 % Fe sample processed by HPT from the as-cast state. Microstructural analyses revealed that a close-to nanograined microstructure with a size of 125 nm and dispersion of intermetallic particles below 50 nm was attained, along with a maximum supersaturation of Fe of ~0.67 wt%. The Al–4 % Fe sample reached even higher supersaturation of Fe to ~0.99 % and similar strength but lower elongation due to insufficient fragmentation of coarse intermetallics. It is concluded that the eutectic structures in the cast state are a major contributor to the enhanced strengthening and the retained elongation. The saturated states of the microhardness at equal Fe contents were shown to be similar regardless of the initial state upon sufficient straining by HPT.     

An investigation of flow patterns and hardness distributions using different anvil alignments in high-pressure torsion

A super duplex stainless steel was selected as a model material to evaluate the origins of unusual double-swirl flow patterns that have been reported on the upper surfaces of discs processed by high-pressure torsion (HPT). The experiments were conducted by making changes in the anvil alignment prior to HPT processing. Experiments were conducted under two different conditions: using essentially a perfect anvil alignment and with an initial anvil misalignment of 100 μm. The experimental results show that no double swirls are visible on the surfaces of discs processed under conditions of perfect anvil alignment but double swirls become visible when processing with a misalignment of 100 μm. The presence of double swirls also affects the measured hardness distributions, and for a misalignment of 100 μm the hardness distribution along a diameter may be non-symmetric with respect to the centre of the disc.     

High formability of glass plus fcc-Al phases in rapidly solidified Al-based multicomponent alloy

A multicomponent Al₈₄Y₉Ni₄Co_1.5Fe_0.5Pd₁ alloy was found to keep a mixed glassy + Al phases in the relatively large ribbon thickness range up to about 200 μm for the melt-spun ribbon and in the diameter range up to about 1100 μm for the wedge-shaped cone rod prepared by injection copper mold casting. The glassy phase in the Al-based alloy has a unique crystallization process of glass transition, followed by supercooled liquid region, fcc-Al + glass, and then Al + Al₃Y + Al₉ (Co, Fe)₂ + unknown phase. It is also noticed that the primary precipitation phase from supercooled liquid is composed of an Al phase instead of coexistent Al + compound phases, being different from the crystallization mode from supercooled liquid for ordinary Al-based glassy alloys. In addition, it is noticed that the mixed Al and glassy phases are extended in a wide heating temperature range of 588–703 K, which is favorable for the development of high-strength nanostructure Al-based bulk alloys obtained by warm extrusion of mixed Al + amorphous phases. The Vickers hardness is about 415 for the glassy phase and increases significantly to about 580 for the mixed Al and glassy phases. The knowledge of forming Al + glassy phases with high hardness in the wide solidification and annealing conditions through high stability up to complete crystallization for the multicomponent alloy is promising for future development of a high-strength Al-based bulk alloy.     

High-pressure torsion for fabrication of high-strength and high-electrical conductivity Al micro-wires

Iron (Fe) is commonly found in aluminum (Al), but its contents are usually kept as low as possible, because the formation of intermetallic phases may induce fracture. In this study, high-pressure torsion (HPT) was used to control the microstructure in an Al-2 %Fe alloy in conjunction with wire drawing and an aging treatment, in order to improve not only their mechanical properties but also the electrical conductivity. It is shown that HPT processing of ring-shaped samples produced ultrafine grains with a size of ~150 nm in the matrix, while intermetallic phases were fragmented to nanosizes with some Fe fraction dissolved in the matrix. Semi-rings were extracted from the HPT-processed samples and swaged to a round section with 0.4-mm diameter. The HPT-processed sample was successfully drawn to a final diameter of 0.08 mm (25:1 ratio, 96 % reduction in area), whereas the sample without HPT processing failed after drawing to 0.117-mm diameter (12:1 ratio, 91 % reduction in area). The electrical conductivity increased to ~65 IACS % in the HPT-processed rings and to ~54 IACS % in the wires by aging for 1 h after the drawing.     

Stability of thermal-induced phase transformations in the severely deformed equiatomic Ni–Ti alloys

Nickel–Titanium (Ni–Ti) alloys of two different near-equiatomic chemical compositions (Ni-rich and Ti-rich) are subjected to severe plastic deformation by means of high pressure torsion (HPT) by higher rotation speed and larger total number of rotations. Further, the as-received and severely deformed specimens are subjected to heat treatments at 300 and 350 °C. Phase transformations of the specimens under different conditions are analyzed by employing differential scanning calorimetry and by X-ray diffraction. The results obtained show that in Ti-rich Ni–Ti alloy the sequence of phase transformations is found to be stable against heat treatments and independent of previous HPT process. Also, in Ni-rich Ni–Ti alloy, when it is subjected to HPT, the sequence of phase transformations found to remain unaltered. However, with or without HPT, after the heat treatments at 300 and 350 °C, the sequence of the phase transformation is found to be affected.     

Effect of Zn addition on the precipitation behaviors of Al–Mg–Si–Cu alloys for automotive applications

Effect of Zn addition on the precipitation kinetics and age-hardening response of Al–Mg–Si–Cu alloys was investigated by differential scanning calorimetry (DSC), hardness measurements, tensile tests and microstructural characterization. The results show that, compared with the Zn-free alloy, both the starting and peak temperatures in the DSC curve, and activation energy of β″ precipitation of Zn-added Al–Mg–Si–Cu alloy decrease significantly, corresponding to the greatly improved precipitation kinetics and age-hardening response, i.e., a hardness increment of 70HV after aging at 185 °C for 20 min. Moreover, the peak hardness and tensile properties can also be greatly enhanced after adding 3.0 wt% Zn even exhibiting a ductile fracture feature in the peak-aged state. No precipitates of the Al–Zn–Mg alloy system appear in the Zn-added Al–Mg–Si–Cu alloys after aging at 185 °C, and pre-β″, β″, and L precipitates are still the main precipitates in the two alloys after peak aging treatment. Finally, based on the microstructural evolution, a schematic diagram of precipitation in the Al–Mg–Si–Cu–Zn alloy is put forward, and the relationship between mechanical properties and microstructure is also established.