Properties Of Ultrafine-grained Research Articles

Influence of decreased temperature on the plasticization effect in high-strength Al-Cu-Zr alloy

The influence of deformation temperature of tensile test on the mechanical properties of ultrafine grained (UFG) Al-Cu-Zr alloy in three different states: after high pressure torsion (HPT), subsequent low-temperature annealing as well as after annealing and a small additional deformation has been studied for the first time. The UFG structure was formed by HPT processing. The temperature dependences of the yield stress and elongation to failure under uniaxial tension in the temperature range of 77–293 K were obtained for all three states. The effect of increasing ductility from ∼3–5 to 11 % (the plasticization effect) at room temperature while maintaining a high strength of ∼465 MPa was observed after deformation-heat treatment (DHT), consisting of annealing and additional deformation. The plasticization effect decreases monotonically with decreasing temperature of tensile test and completely disappears at ∼223 K. In the temperature range 223–293 K, the temperature sensitivity of the yield stress and the value of activation energy (Q) of plastic flow differ in all three states, which is related to the changes in the grain boundary (GB) structure (dislocation density, size and shape of Al2Cu nanoprecipitates). In the temperature range of 77–223 K, the temperature sensitivity of the yield stress does not depend on the state of GBs. The plasticization effect at room temperature is preserved when the strain rate is changed from 10−4 to 10−3 s−1 and sharply decreases as strain rate is further increased. The strain rate sensitivity coefficient (m) for the UFG Al-1.47Cu-0.34Zr (wt%) alloy was determined in the states before and after DHT. The results and relevant mechanisms are analyzed by using the resulting m valuе and Q values as well as the observations of specific features of GB structure.

The Effects of Chemical Etching and Ultra-Fine Grain Structure of Titanium on MG-63 Cells Response

In this work, we study the influence of the surface properties of ultrafine grained (UFG) and coarse grained (CG) titanium on the morphology, viability, proliferation and differentiation of osteoblast-like MG-63 cells. Wet chemical etching in H2SO4/H2O2 and NH4OH/H2O2 solutions was used for producing surfaces with varying morphology, topography, composition and wettability. The topography and morphology have been studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The composition was determined by time of flight mass-spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS). The results showed that it is possible to obtain samples with different compositions, hydrophilicity, topography and nanoscale or/and microscale structures by changing the etching time and the type of etching solution. It was found that developed topography and morphology can improve spreading and proliferation rate of MG-63 cells. A significant advantage of the samples of the UFG series in comparison with CG in adhesion, proliferation at later stages of cultivation (7 days), higher alkaline phosphatase (ALP) activity and faster achievement of its maximum values was found. However, there is no clear benefit of the UFG series on osteopontin (OPN) expression. All studied samples showed no cytotoxicity towards MG-63 cells and promoted their osteogenic differentiation.

Effect of stored energy on corrosion fatigue properties of ultrafine grained Fe-20%Cr steel by equal channel angular pressing

Corrosion fatigue properties of ultrafine grained (UFG) Fe-20%Cr steel by equal channel angular pressing was investigated in term of grain refinement and stored energy. The corrosion fatigue life of the ECAP processed and the post-ECAP annealed sample was analysed. Under annealing temperature of 773 K, due to the recrystallization stage, there was a little difference in microstructure from the four passes ECAP. After the annealing process, the strength of the samples pressed four passes decreases, while the elongation showed more ductile. Two mechanisms for crack propagation have been well recognized for stainless steel. One is slipping step dissolution mode. In this model, the slip step formed by active dislocation at a crack tip is anodic dissolution reaction, so that crack tip advance to further distance. The other is stress adsorption mechanism. The corrosion fatigue cracks initiation and cracks propagation process of iron-chromium alloy can be characterized by corrosion pits and intergranular fracture, respectively. The corrosion fatigue behaviour of non-equilibrium and equilibrium grain boundary of iron-chromium alloy was occupied by crack initiation.

Investigation of Grain Refinement Mechanism of Nickel Single Crystal during High Pressure Torsion by Crystal Plasticity Modeling.

The excellent properties of ultra-fine grained (UFG) materials are relevant to substantial grain refinement and the corresponding induced small grains delineated by high-angle grain boundaries. The present study aims to understand the grain refinement mechanism by examining the nickel single crystal processed by high pressure torsion (HPT), a severe plastic deformation method to produce UFG materials based upon crystal plasticity finite element (CPFEM) simulations. The predicted grain maps by the developed CPFEM model are capable of capturing the prominent characteristics associated with grain refinement in HPT. The evolution of the orientation of structural elements and the rotations of crystal lattices during the HPT process of the detected differently oriented grains are extensively examined. It has been found that there are mainly two intrinsic origins of lattice rotation which cause the initial single crystal to subdivide. The correlation between the crystallographic orientation changes and lattice rotations with the grain fragmentation are analyzed and discussed in detail based on the theory of crystal plasticity.

The microstructures and tensile mechanical properties of ultrafine grained and coarse grained Al-7Si-0.3Mg alloy rods fabricated from machining chips

Ultrafine grained (UFG) and coarse grained (CG) Al-7Si-0.3Mg (wt%) alloy rods were fabricated by hot extrusion of compacts of a nanocrystalline Al-7Si-0.3Mg alloy powder and coarse grained Al-7Si-0.3Mg alloy granules respectively. Both the nanocrystalline powder and the granules were prepared by processing Al-7Si-0.3Mg alloy (widely known as A356 cast alloy) machining chips produced in machining cast components of this alloy such as car wheels. Samples from the rods were also T6 heat treated with a condition of 535 °C/1 h-water quenching-165 °C/8 h. The as-extruded UFG rod exhibited a yield strength (YS) of 298 MPa, ultimate tensile strength (UTS) of 345 MPa and elongation to fracture (δf) of 5.9%, which changed to 223 MPa, 342 MPa and 10.2% respectively after the T6 heat treatment. In contrast, the as-extruded CG rod exhibited a YS of 101 MPa, UTS of 187 MPa and δf of 16.6%, which changed to 203 MPa, 275 MPa and 10.8% after the heat treatment. The UFG Al-7Si-0.3Mg alloy sample exhibits a dramatically different precipitation behavior from the CG Al-7Si-0.3Mg alloy sample during the T6 heat treatment, with the formation of fine platelet-shaped pre-β″ precipitates on Al {001} planes in the former sample in contrast of the formation of needle-shaped β″ precipitates along Al <001> directions in the latter sample. The change of precipitation behavior leads to a weaker precipitation hardening effect. This work demonstrated that a significantly higher tensile strength can be achieved with heat treated UFG Al-7Si-0.3Mg alloy than CG Al-7Si-0.3Mg without sacrificing tensile ductility.

Effect of chromium and zirconium content on structure, strength and electrical conductivity of Cu-Cr-Zr alloys after high pressure torsion

The effect of Cr and Zr contents on the properties of ultra-fine grained (UFG) Cu-Cr-Zr alloys subjected to high-pressure torsion (HPT) was studied. It is found that increasing in chromium content above the solubility limit does not cause additional hardening both after HPT and after subsequent aging. In contrast, excessive alloying with zirconium allows obtaining a structure with a smaller grain size, enhances structure stability during heat treatment and noticeably increases strength with high electrical conductivity of the alloy after HPT and additional aging.

Additivity of strengthening mechanisms in ultrafine grained Al–Mg–Sc alloy

In the light of unique and anomalous properties of ultrafine grained (UFG) alloys, an effort was made to develop a predictive capability of the yield strength (YS) of UFG Al–Mg–Sc alloy. UFG microstructure was introduced using friction stir processing. Microstructural characterization of grain size, dislocations, and nano-sized Al3(Sc,Zr) particles was carried out using electron backscatter diffraction and transmission electron microscope. The contribution from Peierls–Nabarro stress, solid solution strengthening, precipitation strengthening, grain boundary strengthening, and dislocation strengthening were assessed using existing strengthening models. Additivity law to predict the YS of the alloy was chosen based on the microstructural state of the alloy. The microstructural state of coarse grained and UFG alloy favored the use of linear additivity rule over Pythagorean. The use of a mixed (linear and Pythagorean) additivity rule was also carried out to assess its YS prediction capability. A difference in the range of 33–55% was observed between predicted and experimentally obtained YS for UFG alloy. The reason was related to the overprediction from grain boundary strengthening model. A smaller HP slope than the normal slope value was able to predict the YS of the UFG alloy more closely.

Ultrafine-Grained HSLA Steel Processed Using MAF: Dry Sliding Wear and Corrosion Behaviour

As ultrafine grained (UFG) steels obtained by severe plastic deformation (SPD) are useful for structural purposes and micro devices, it is necessary to establish the relation of microstructure with wear and corrosion properties. Very few studies are available that correlate the microstructural changes and the mechanical properties of UFG steels to the degradation phenomena such as wear and corrosion. In present work a HSLA steel was severely deformed by warm (500°C) multiaxial forging (MAF) technique using up to nine strain steps. Submicron sized grain size was obtained after warm MAF. The hardness and strength improved significantly. The MAF processed UFG HSLA steel did not show any improvement in wear resistance. Low pull-off-work and fragmented nanosized pearlitic cementite particles were the two factors found responsible for it. The corrosion resistance of HSLA steel after warm MAF remained largely unaffected.

Microstructure and Properties of Ultrafine Grained Cu-0.73%Cr Alloy after High Pressure Torsion

Microstructure, mechanical properties and electrical conductivity in Cu-0.73%Cr alloy after HPT process and the subsequent aging treatment have been investigated. Ultrafine grained structure with the grain size ~150 nm has been achieved after the HPT and the subsequent aging treatment. Ultrafine grains with some growth twins were preserved in the overaged state, showing high thermal stability. The peak microhardness and tensile strength of Cu-0.73%Cr alloy after the HPT was found at 480 °C for 2 hours. Electrical conductivity shows an increase trend in the different aging states.

An Experimental Evaluation of Material Properties and Fracture Simulation of Cryorolled 7075 Al Alloy

This work presents an experimental evaluation of yield strength, tensile strength, and impact toughness of 7075 Al alloy. The extended finite element method (XFEM) has been chosen for quasi-static crack growth simulations using Charpy impact energy as the crack growth criterion for both Bulk and ultrafine-grained (UFG) 7075 Al alloy. The 7075 Al alloy is rolled for different thickness reductions (40 and 70%) at cryogenic (liquid nitrogen) temperature, and its mechanical properties are studied by performing the tensile and Charpy impact testing. The microstructural characterization of the alloy was carried out using field emission scanning electron microscopy (FE-SEM). The rolling of the Al alloy at cryogenic temperature suppresses dynamic recovery, and dislocation cells formed during processing, transformed into fully formed ultrafine-grains (600 nm) at 70% thickness reduction. The impact energy used as the crack growth criterion under quasi-static loading condition based on the Griffith energy concept. The elastic-plastic ductile fracture simulations are performed by XFEM using ABAQUS Software (Version 6.9). For crack modeling, two different types of functions are used to model a crack based on partition of unity concept. A discontinuous function is used to model the portion behind the crack tip, whereas crack tip is modeled by near-tip asymptotic functions. This permits the crack is to be represented explicitly without meshing the crack surfaces, thus crack propagation simulations can be carried out without a need of re-meshing. Strain energy release and stress distribution ahead of the crack tip is found for some practical crack problems. The numerical examples indicate a significant improvement in crack growth properties of UFG 7075 Al alloy as compared to its bulk form due to an effective grain refinement.

Strain hardening and microstructure evolution during uniaxial compression of ultrafine grained zirconium at temperatures of 4.2–300 K

The mechanical properties of ultrafine grained (UFG) zirconium (grain size 200 nm) obtained by a combination of extrusion, drawing, and annealing, are studied experimentally under uniaxial compression at temperatures of 4.2–300 K and compared with the mechanical properties of coarse grained (CG) Zr. The evolution of the texture and microstructure of Zr during strain is studied by x-ray diffraction and transmission electron microscopy. The volume fractions of twinned material are determined for UFG and CG Zr. It is found that at room temperature and below, twinning activity is lower in coarse grained zirconium, but at the very lowest temperatures (4.2 K) the opposite effect is observed, i.e., increased twinning activity with decreasing grain size. The influence of internal thermal anisotropy stresses on twinning in CG and UFG zirconium is discussed. The effect of twinning on the mechanical properties of UFG Zr is analyzed.

Microstructures and Properties of Ultrafine Grained Iron by Rolling

Microstructures and properties of ultrafine grained (UFG) iron by different rolling processing are investigated. By applying the asymmetric rolling (ASR), the equiaxed UFG iron with the grain size of 0.9 μm has been obtained due to the shear strain induced by the mismatch of the two roll during ASR. With the same rolling reduction, the ASR processed iron shows higher yield strength than that of the symmetrically rolled one. When the ASR processed iron is further symmetrically rolled, the grain morphology maintains equiaxed and the grain size decreases to about 0.3 μm.

Microstructure and mechanical properties of ultrafine grained copper processes by multiple isothermal forging

Microstructure and mechanical properties of ultrafine grained (UFG) Cu processed by multiple isothermal forging (MIF) are studied and compared to those after processing by equal channel angular pressing (ECAP). It is shown that when the final temperature of MIF and the temperature of ECAP are the same, similar microstructures and mechanical properties are obtained in this material. While MIF allows for obtaining reasonably small grain sizes in the submicrometer range in large scale samples, ECAP allows for processing smaller grain sizes in smaller samples.<br />

The influence of temperature on the evolution of functional properties during pseudoelastic cycling of ultra fine grained NiTi

In the present study we used uniaxial pull-pull fatigue loading to study the pseudoelastic properties of ultra fine grained (grain size: 50–100 nm) NiTi wires. The pseudoelastic behavior of shape memory alloys is not an ideal process. We show how pull-pull cycling changes the pseudoelastic properties, resulting in a decrease of plateau stresses (characterizing the forward and reverse transformation) and an accumulation of irreversible strain (indicating the increase of dislocation density in the material and/or the increase of the volume fraction of stabilized martensite). Moreover, pseudoelasticity is only observed within a limited temperature window. In our case, optimum initial pseudoelasticity was obtained in a region of 10–80 °C. While all this is well appreciated, the fact that the evolution of the pseudoelastic behavior shows a strong temperature dependence even within this ‘pseudoelastic window’ has received much less attention. After 120 pseudoelastic cycles at temperatures between 28 °C and 80 °C, our material showed variations of the vertical width of stress–strain hysteresis and accumulated irreversible strain by factors of two and four, respectively.

Influence of Mn Content on the Microstructure and Mechanical Properties of Ultrafine Grained C-Mn Steels

The effect of Mn content on the microstructure and mechanical properties of two ultrafine grained 0.2%C-Mn steels has been investigated. The ultrafine grained microstructure was produced by use of large strain warm deformation and subsequent annealing. The final microstructure consists of fine cementite particles within an ultrafine grained ferrite matrix. The increase in the Mn content leads to a decrease in the average ferrite grain size (from 1.3 to 0.8 μm for an increase in the Mn content from 0.74 to 1.52 mass%). This can be attributed to the enrichment of Mn in the cementite particles, which becomes finer in the steel with a higher Mn content. The increase in the Mn content results in an increase in strength at equal ductility and toughness.

Effect of Heat Treatment on Microstructures and Tensile Properties of Ultrafine Grained C-Mn Steel Containing 0.34 mass% V

An ultrafine grained (UFG) C-Mn steel containing a relatively large amount of vanadium was fabricated by equal channel angular pressing (ECAP) and its microstructures and tensile properties were examined. This investigation was aimed at demonstrating the effect of precipitation stage of vanadium precipitates in the course of the material processing on the tensile properties of the ultrafine grained C-Mn steel, especially strength and strain hardening capability. For this purpose, the two different heat treatments were carried out on the present steel: (a) conventional normalization for vanadium precipitation before ECAP, and (b) isothermal transformation for vanadium precipitation during ECAP and subsequent annealing. The results showed that the second heat treatment was more effective on improving the thermal stability and the overall tensile properties of the steel by better uniform distribution of nano-sized vanadium precipitates which yielded an extensive interaction with lattice dislocations inside ultrafine ferrite grains. In addition, in this report, the feasibility enhancing the strain hardening capability of the UFG C-Mn steel was explored by comparing the microstructure and stress-strain curve of the steel prepared by the second heat treatment with those of the UFG C-Mn steel without vanadium.

Effect of Chemical Composition on Structure and Properties of Ultrafine Grained Cu-Cr-Zr Alloys Produced by Equal-Channel Angular Pressing

The properties of multi-functional CuCr and CuCrZr alloys with ultra fine grains (UFG) produced by equal-channel angular pressing (ECAP) have been investigated in dependence of concentration of alloying elements. A special attention is paid to optimization of fatigue performance together with strength, thermal and electric properties after ECAP and subsequent aging. Substantial improvement of fatigue life is evidenced in ECAP-fabricated alloys when compared with conventional tempers.