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.     

Effect of heat treatment on microstructure and microhardness evolution in a Ti–6Al–4V alloy processed by high-pressure torsion

A Ti–6Al–4V alloy was heat-treated to give two types of microstructures with different volume fraction of equiaxed α phase and lamellar (α + β) microstructure. Disks were cut from the heat-treated rods and processed by quasi-constrained high-pressure torsion (HPT) at room temperature with an applied pressure of 6.0 GPa and torsional straining from 1/4 to 20 turns. The results show that there is a gradual evolution of homogeneity in microhardness and grain size with increasing numbers of revolutions in HPT such that the microhardness values attain a maximum constant value across the disk after processing by HPT for 10 turns and the measured equilibrium grain sizes after 20 turns are ~130 nm in Ti64-1 and ~70 in Ti64-2. The results show also that a larger fraction of lamellar (α + β) in the microstructure of Ti–6Al–4V leads to a higher hardenability after processing by HPT.     

Microhardness and corrosion properties of hypoeutectic Al–7Si alloy processed by high-pressure torsion

High-pressure torsion (HPT) was used to produce hypoeutectic Al–7Si alloy samples having a range of microstructures to investigate the effect of the grain refinement on its corrosion behavior in 3.5 wt.% NaCl solution for the first time. Optical microscopy measurements reveal that with the HPT processing increased from 1/4 to 10 revolutions under an applied pressure of 6.0 GPa, brittle coarse silicon particles and intermetallic phases were effectively broken into ultrafine-grained particles and redistributed homogeneously into the Al-rich matrix. Open-circuit potential and polarization curves results exhibit that corrosion resistance of the Al–7Si alloy in NaCl solution was significantly enhanced upon high torsion strains, with corrosion rate reduced from 7.41 μm y⁻¹ for the as-received sample to 1.68 μm y⁻¹ for the 10-turn processed sample. Electrochemical impedance spectroscopy analysis combined with characterization of the corroded samples using scanning electron microscopy and energy dispersive X-ray spectroscopy indicates that the enhancement in corrosion performance of the Al–7Si alloy is due to the breakage of coarse silicon particles and intermetallic phases, the microstructure homogeneity and the increased HPT-induced active sites. It is demonstrated that microstructure refinement through HPT processing can significantly improve both microhardness and corrosion properties of the Al–7Si alloy.     

Effect of microstructure on fatigue behaviour of cast Al–7Si–Mg alloy

Abstract

The fatigue behaviour of a cast Al–7Si–Mg alloy, conforming to A356, has been studied. Specimens of this material were tested in both the as cast condition and a solution treated and aged condition. It was observed that the size, number, and position of casting defects influenced the fatigue life very strongly. This marked effect nearly hides that of the heat treatment. Nevertheless, if the analysis is carried out considering only results obtained from sound specimens it is revealed that the heat treatment causes an improvement in the fatigue resistance of the alloy.     

Microstructural and hardness evolution of additively manufactured Al–Si–Cu alloy processed by high-pressure torsion

Journal of Materials Science - Nanostructured Al-9%Si-3%Cu alloy was achieved by direct metal laser sintering (DMLS) and then processed using high-pressure torsion (HPT) processing, which resulted...     

Microstructural evolution and microhardness variations in a Cu–36Zn–2Pb alloy processed by high-pressure torsion

A coarse-grained Cu–36Zn–2Pb alloy with an initial grain size of ~54 μm was processed by high-pressure torsion (HPT) at room temperature under an applied pressure of 6.0 GPa through 1–10 turns, and the evolution of microstructure and microhardness was investigated. Analysis by X-ray diffraction (XRD) showed that in HPT processing the β′-phase transforms to an α-phase and a {111} texture is formed. Microscopic examination showed that dislocations were first formed at equivalent strains of not more than ~25 and when the equivalent strain increased to ~40 there was evidence for twins and secondary twinning. Fine grains were formed with an increase in equivalent strain to ~100 and with further straining these refined grains acted as precursors for additional grain refinement. The refined equiaxed grain size was ~250 nm after HPT through an equivalent strain of ~100 and the results show the microhardness reached a saturation value of ~220 Hv.     

Effect of aging on microstructural development in an Al–Mg–Si alloy processed by high-pressure torsion

Experiments were conducted to evaluate the microstructural evolution in a commercial Al-0.6 % Mg-0.4 % Si alloy processed using high-pressure torsion for up to 20 turns. Disks of the alloy were tested in two different conditions: in a solution-treated condition and after a short aging treatment at 523 K. The results show that HPT processing introduces significant grain refinement through HPT processing including a reduction in grain size from ~150 μm to ~720 nm in 1 turn of HPT. The final grain size in this alloy was ~250 nm after 20 turns. Some tensile tests were conducted to evaluate the mechanical properties of the alloy at the solution treatment temperature. The results from these tests show that aging at 523 K leads to a small increase in ductility for all tensile samples with a maximum recorded elongation of ~230 %.     

Influence of solutionising and aging temperatures on microstructure and mechanical properties of cast Al–Si–Cu alloy

Abstract

This paper presents the influence of solution and aging temperatures on the microstructure and mechanical properties of 319 secondary cast aluminium alloy. Experimental alloy was subjected to different heat treatment cycles. Heat treatments were designed with two solutionising temperatures (504 and 545°C) at two solutionising times (4 and 8 h), followed by quenching in water at 60°C and artificial aging. The artificial aging was carried out at two temperatures (200 and 154°C) for 6 h. The improvement in mechanical properties was obtained with low solution temperature (504°C) for 8 h followed by quenching in water to 60°C and aging at low temperature (154°C). The increase in the solutionising temperature from 504 to 545°C was recommendable only for short solutionising time (4 h). Increase in the aging temperature from 154 to 200°C has led to the increase in hardness with the corresponding decrease in ductility. Aging under unfavourable conditions (prolonged aging at high temperature) caused coarsening of spheroidised eutectic silicon crystals and precipitated particles resulted in deleterious effect on the tensile strength.     

Compressive and impression creep behavior of a cast Mg–Al–Zn–Si alloy

Creep behavior of an Mg–6Al–1Zn–0.7Si cast alloy was investigated by compression and impression creep test methods in order to evaluate the correspondence of impression creep results and creep mechanisms with conventional compression test. All creep tests were carried out in the temperature range 423–523 K and under normal stresses in the range 50–300 MPa for the compression creep and 150–650 MPa for impression creep tests. The microstructure of the AZ61–0.7Si alloy consists of β-Mg₁₇Al₁₂ and Mg₂Si intermetallic phases in the α-Mg matrix. The softening of the former at high temperatures is compensated by the strengthening effect of the latter, which acts as a barrier opposing recovery processes. The impression results were in good agreement with those of the conventional compressive creep tests. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.009 < (σ/G) < 0.015 and 0.021 < (σ_imp/G) < 0.033 for the compressive and impression creep tests, respectively. Based on the steady-state power-law creep relationship, the stress exponents of about 4–5 and 10–12 were obtained at low and high stresses, respectively. The low-stress regime activation energies of about 90 kJ mol⁻¹, which are close to that for dislocation pipe diffusion in the Mg, and stress exponents in the range of 4–5 suggest that the operative creep mechanism is pipe-diffusion-controlled dislocation viscous glide. This behavior is in contrast to the high-stress regime, in which the stress exponents of 10–12 and activation energies of about 141 kJ mol⁻¹ are indicative of a dislocation climb mechanism similar to those noted in dispersion strengthening mechanisms.     

Simulation of material flow in friction stir processing of a cast Al–Si alloy

The objective of the current paper is using DEFORM-3D software to develop a 3-D Lagrangian incremental finite element method (FEM) simulation of friction stir processing (FSP). The developed simulation allows prediction of the defect types, temperature distribution, effective plastic strain, and especially material flow in the weld zone. Three-dimensional results of the material flow patterns in the center, advancing and retreating sides were extracted using the point tracking. The results reveal that the main part of the material flow occurs near the top surface and at the advancing side (AS). Material near the top surface was stretched to the advancing side resulting in a non-symmetrical shape of the stir zone (SZ). Furthermore, macrostructure and temperature rise were experimentally acquired to evaluate the accuracy of the developed simulation. The comparison shows that the stir zone shape, defect types, powder agglomeration, and temperature rise, which were predicted by simulation, are in good agreement with the corresponding experimental results.     

The fatigue damage development in a cast Al–Si–Cu alloy

The experimental identification of fatigue damage mechanisms and evaluation of their development rate, based on changes in material respond on cycle loading, has been presented in the work. The research has been conducted on hyper-eutectic cast alloy AlSi8Cu3. The microstructure and fracture analyses were performed. The high cycle fatigue tests were conducted with frequency of 20 Hz under constant nominal stress amplitude with monitoring the strain response of material during the test. The ratcheting was found as the main mechanism of the fatigue damage. It was established that the linear fatigue accumulation law should not be used for fatigue life prediction in case of the tested cast aluminum alloy.     

Development of microstructure and mechanical properties of as cast Al–Mg–Mn alloy during high temperature ECAP

Abstract

A large scale billet with diameter of 58·5 mm of an as cast Al–Mg–Mn alloy was processed by equal channel angular pressing (ECAP) at 350°C up to six passes. A significant refinement of the grains was observed after six pressings to ～2 μm. And the selected area electron diffraction (SAED) pattern showed that almost all of the grains were separated by boundaries with high angles of misorientation. A banded substructure was not observed during the hot ECAP, and a reasonably equiaxed structure was obtained just after one single pressing. Both the strength and the elongation increased abruptly in a single passage through the die, but thereafter, the increase was more gradual and exhibited a saturation effect after the fourth pressing. The good combination of strength and ductility of the Al–Mg–Mn alloy attained by the hot ECAP appeared to be attractive properties for industrial applications. Moreover, hot ECAP could possibly be used as an alternative step to hot extrusion or hot rolling in industrial processing, to break down an initial coarse as cast structure in a quite large scale billet.     

Sliding temperature and wear behaviour of cast Al–Si base alloy

Abstract

The influence of sliding interface temperature on friction and wear behaviour of eutectic (LM13) and hypereutectic (LM28) Al–Si base alloy in as cast and heat treated condition has been investigated. LM13 and LM28 alloys having nominal composition Al–12Si–1Ni–0.8Cu–0.6Mg and Al–17Si–1Ni–0.8Cu–0.6Mg used in this study. Wear and friction tests were performed under dry sliding conditions using a pin on disc type of friction and wear monitor with the data acquisition system conforming to ASTM G99 standard. It was found that sliding interface temperature has a close relation with wear and friction response of these alloys. Initial rise in temperature reduces the wear rate and as soon as a critical temperature (CT) is crossed, wear rate abruptly increases. The friction coefficient of both alloys first decreases with rise in temperature then subsequently increases beyond a certain temperature. The influence of temperature on wear behaviour in particular was found to be a function of alloy composition and heat treatment. For as cast LM28 alloy, the critical temperature (140°C) was found to be lower than that in the heat treated condition (180°C). A temperature–wear mechanism is proposed for these alloys.     

Influence of high-pressure torsion on microstructural evolution in an Al–Zn–Mg–Cu alloy

A commercial age-hardenable Al-7136 alloy was successfully processed by high-pressure torsion (HPT) at room temperature through 1/8 to 4 turns. Microhardness measurements showed significant hardening even after 1/8 turn with the average hardness value reaching a maximum after 1 turn and then slowly decreasing. Higher hardness values were attained by processing the alloy through one pass of equal-channel angular pressing in a supersaturated condition at room temperature and then applying HPT for 1 or 2 turns. Microstructural observations revealed the possibility of achieving true nanometer grain sizes of <100 nm after processing at room temperature. There were variations in hardness with imposed strain due to the fragmentation and subsequent growth of precipitates during processing.     

Effect of growth rate on microstructure parameters and microhardness in directionally solidified Ti–49Al alloy

Intermetallics Ti–49Al (at.%) alloy was directionally solidified with different growth rates (V = 5 μm/s–30 μm/s) at a constant temperature gradient (G = 12.1 K/mm) by using a Bridgman type directional solidification furnace. The primary dendritic spacing (λ), interlamellar spacing (λ_L), and microhardness (HV) were measured. Effect of V on HV, λ and λ_L was experimental investigated. The dependencies of λ, λ_L and HV on the growth rate were determined by using linear regressing analysis. According to the result, the values of λ and λ_L decrease with the increasing of V, and the values of HV increase with the increasing of V and with the decreasing of λ and λ_L. The results were compared with previous similar experimental results for TiAl-based alloys.     

Achieving homogeneity in a Cu–Zr alloy processed by high-pressure torsion

A copper alloy, Cu–0.1 %Zr, was subjected to severe plastic deformation at room temperature using quasi-constrained high-pressure torsion. Disks were strained through different numbers of revolutions up to a maximum of ten turns under an applied pressure of 6.0 GPa and then examined to evaluate the evolution in the Vickers microhardness, Hv, and the microstructure. The results show lower values of Hv in the center regions of the disks in the early stages of processing but a gradual evolution to a high degree of hardness homogeneity after five and ten turns. Under conditions of hardness homogeneity, the distributions of the grain boundary misorientations are essentially identical at the center and the periphery of the sample. Homogeneity was further confirmed by conducting tensile testing at elevated temperatures where similar stress–strain curves and similar elongations to failure were recorded after processing through five and ten turns of HPT.     

Strain path and microstructure evolution during severe deformation processing of an as-cast hypoeutectic Al–Si alloy

Microstructure evolution in an as-cast Na modified Al–7%Si (wt. pct.) alloy was examined during redundant and monotonic straining by repetitive equi-channel angular pressing (ECAP) under ambient temperature conditions, and during friction stir processing (FSP). Redundant straining during repetitive ECAP was accomplished by processing following route B_C while monotonic straining employed route A. Single- and multi-pass FSP was conducted on this same as-cast material using an FSP tool having a threaded pin. The as-cast microstructure comprises equiaxed primary α dendrite cells embedded in the Al–Si eutectic constituent. The evolution of this microstructure during repetitive ECAP can be described by idealized models of this process. The primary and eutectic constituents can still be discerned and the Si particle distribution is not homogenized even during ambient temperature processing involving von Mises strains >9.0. In contrast, the primary and eutectic constituents cannot be distinguished in the stir zone after even a single FSP pass. Strain estimates based on the shape change of the primary α constituent indicate that the Si particle distribution has become homogeneous at local von Mises strains of 2.5–3.0 during the FSP thermomechanical cycle. Mechanical property data are consistent with strain path during SPD processing by repetitive ECAP and FSP.     

Achieving homogeneity in a two-phase Cu–Ag composite during high-pressure torsion

A Cu–8 wt%Ag alloy was processed at room temperature either by high-pressure torsion (HPT) or using a two-step deformation mode of equal-channel angular pressing (ECAP) and HPT. After HPT deformation, different flow patterns were observed on the disk surface without any post-deformation treatment thereby indicating an inhomogeneous shearing deformation. The microhardness distributions throughout the disks were compared after the two different processing routes. It is shown that the microhardness remains very low near the center of the disk although saturated in the outer regions after 10 revolutions. By contrast, an intrinsically homogeneous microhardness may be attained throughout the disk after the two-step deformation of ECAP and HPT. This study suggests a convenient procedure for achieving full homogeneity in high-strength materials during HPT processing.     

Relationships between microstructure and fatigue crack propagation paths in Al–Si–Mg cast alloys

Fatigue crack propagation of long and small cracks was investigated for hypoeutectic and eutectic Al–Si–Mg cast alloys. Crack propagation behavior in the near-threshold regime and Regions II and III was related to microstructural constituents namely primary α-Al dendrites and volume fraction and morphology of eutectic Si. Long crack thresholds reflect combined closure effects of global residual stress and microstructure/roughness. The small crack threshold behavior is explained through closure independent mechanisms, specifically through the barrier effects of characteristic microstructural features specific to each alloy. In Regions II and III changes in fracture surface roughness are associated with different crack propagation mechanisms at the microstructure scale. The extent of the plastic zone ahead of the crack tip was successfully used to explain the observed changes in crack propagation mechanisms.