Ultrafine-grained Samples Research Articles

High-cycle fatigue strength and small-crack growth behavior of ultrafine-grained copper with post-ECAP annealing

The effects of post equal channel angular pressing annealing on microstructure and high-cycle fatigue behavior of ultrafine-grained copper were examined. Grains with sizes up to a few tens of micrometers surrounded by fine grains were formed after the annealing as a result of discontinuous recrystallization. The fatigue life of annealed samples in the short- and medium-life fields was found to decrease, while in the long-life field, the fatigue strength at 3×107 cycles was about 9% greater than that of ultrafine-grained copper without annealing. Long-term stressing for as-received ultrafine-grained samples produced large grains over 100μm as a result of dynamic recovery and recrystallization. For annealed samples, there were no significant differences in grain sizes between pre- and post-stressing, showing the higher stability of bimodal microstructure. This stability was related to the slightly increased fatigue strength in long life fields. The effect of bimodal microstructure on the growth behavior of small surface-cracks was discussed.

Tailoring Microstructure and Mechanical Properties of 6063 Aluminium Alloy for Lightweight Structural Parts

In this work, an attempt has been made to improve the mechanical strength of 6063 aluminium alloy and thus its lightness via combination of severe plastic deformation (grain size refinement) and heat treatment (precipitation hardening). 6063 aluminium alloy was chosen as the best material for lightweight structures, where mass reduction is important, because of its high extrudability. Samples were hydrostatically extruded (HE) in supersaturated condition and subsequently subjected to an aging process. HE brings about significant grain size refinement well below 1 micron. The influence of aging parameters such as time and temperature on mechanical properties evolution of extruded material was determined. The microstructure of ultrafine grained (UFG) alloy was investigated using transmission electron microscopy. The average grain diameter and grain boundary misorientation angles (using Kikuchi lines) were measured. Mechanical properties were examined in microhardness and tensile tests. The results have shown that it is possible to combine grain boundary and precipitation strengthening and obtain ultrahigh strength in 6xxx series alloys. Additionally, heat treatment of UFG samples causes an increase in ductility measured in tensile tests, which is rather poor in severely deformed materials. To prove advantages of UFG aged samples for lightweight applications, finite element modelling was performed to compare the mass of chair elements made of coarse and ultrafine grained material. Simulations were made for the same stresses applied. It has been shown that if the chair was made of UFG aluminium alloy the mass reduction would be approximately 30 %.

An improved long-term nanoindentation creep testing approach for studying the local deformation processes in nanocrystalline metals at room and elevated temperatures

Abstract

Service properties of ultrafine-grained Ti–6Al–4V alloy at elevated temperature

This study deals with investigation of mechanical properties and fatigue behavior of the ultra-fine grained (UFG) alloy Ti–6Al–4V at elevated temperatures. UFG samples were produced by means of combination of equal-channel angular pressing and thermomechanical treatments. Studies of the temperature dependence of mechanical properties of the UFG alloy demonstrated their thermal stability upto 175–350 °C. It was revealed that 100-hour creep rupture strength at 300 °C increased from 750 MPa in the conventional state to 890 MPa in the UFG state. The alloy demonstrates stability of the UFG structure at 300 and 370 °C in the conditions of long-term tests. The fatigue tests were conducted with axial loading applied on a sample at 175 °C, the asymmetry factor of the cycle was 0.1. The fatigue endurance limit of the UFG alloy was almost 50 % higher than that of the CG alloy.

Ultrafine grain effect on pearlitic transformation in hypereutectoid steel

Abstract

Mechanical behaviour and underlying deformation mechanisms in coarse- and ultrafine-grained Zn over a wide range of strain rates

Spark plasma sintering was used to process bulk ultrafine-grained (average grain size of ∼250nm) samples from high purity nanocrystalline Zn powder. The microstructure of the consolidated samples was investigated and the mechanical behaviour was characterised by means of quasistatic and dynamic compression tests at room temperature and compared to that of coarse-grained counterparts. For both materials in the strain rate regime of 10−4–101s−1 a linear relationship between the logarithm of flow stress and strain rate values was observed since room temperature corresponds to a relatively high homologous temperature of 0.43. The strain rate sensitivity was higher for ultrafine-grained Zn (∼0.18) than for coarse-grained Zn (∼0.12) and twinning in the former sample was not observed in the entire strain rate range. For strain rates higher than 103s−1 the plasticity in coarse-grained Zn was controlled by dislocation drag but partial recrystallization was also observed. In the latter regime the mobile dislocation density was about 1011m−2. However, in ultrafine-grained Zn the relatively large dislocation density (∼1014m−2) and the small grain size limit the dislocation velocity yielding the lack of dislocation drag effects up to 104s−1.

Thermal stability of highly nanotwinned copper: The role of grain boundaries and texture

Abstract

Observations of unique plastic behavior in micro-pillars of an ultrafine-grained alloy

Abstract

Impact Toughness of Ultrafine-Grained Interstitial-Free Steel

Impact toughness of an ultrafine-grained (UFG) interstitial-free (IF) steel produced by equal-channel angular extrusion/pressing (ECAE/P) at room temperature was investigated using Charpy impact tests. The UFG IF steel shows an improved combination of strength and impact toughness compared with the corresponding coarse-grained (CG) one. The CG IF steel samples underwent a transition in fracture toughness values with decreasing temperature because of a sudden change in fracture mode from microvoid coalescence (ductile) to cleavage (brittle) fracture. Grain refinement down to the submicron (≈320 nm) levels increased the impact energies in the upper shelf and lower shelf regions, and it considerably decreased the ductile-to-brittle transition temperature (DBTT) from 233 K (−40 °C) for the CG steel to approximately 183 K (−90 °C) for the UFG steel. Also, the sudden drop in DBTT with a small transition range for the CG sample changed to a more gradual decrease in energy for the UFG sample. The improvement in toughness after UFG formation was attributed to the combined effects of grain refinement and delamination and/or separation in the heavily deformed microstructure. Although an obvious change from the ductile fracture by dimples to the brittle fracture by cleavage was recognized at 233 K (−40 °C) for the CG steel, no fully brittle fracture occurred even at 103 K (−170 °C) in the UFG steel.

Microstructure and mechanical behavior of UFG copper processed by ECAP following different processing regimes

Commercially pure (99.9%) copper was severe plastically deformed by equal-channel angular pressing (ECAP) following route-Bc in three different processing regimes in order to obtain ultrafine-grained (UFG) microstructures leading to improved mechanical properties. In regime-1, the billets were processed at room temperature up to eight passes. The billets were processed at 200°C up to eight passes in regime-2. Regime-3 is a hybrid regime by which the billets were deformed at 200°C up to four passes first, and these billets were then processed at room temperature for one, two and four passes. In all regimes, the ECAP processing results in a refinement of the conventionally grained (CG) initial microstructure of copper down to sub-micron level leading to a great improvement in the strength as compared to CG copper. Among the regimes applied, regime-3 was found to be the best regime for improvement in strength along with adequate ductility. The samples showed more than eight times increases in yield strength after processing in regime-3 for 4 + 4 passes, from 47 MPa for CG copper to about 408 MPa for the UFG sample. Such improvement in strength was accompanied by a 16.9% total elongation and 6% uniform elongation. The processing in regime-2 resulted in the best elongation to failure of about 22% after eight passes, but it gave the lowest strength values among others.

Influence of plastic deformation on evolution of defect structures, microhardness and electrical conductivity of copper

Samples of pure Cu cylinder were plastically deformed by the combination of cold forging (CF) and cold drawing (CD) at liquid nitrogen temperature. X-ray diffraction measurements indicate that an increase in deformation strain leads to a decrease in crystallite size and an increase in twin densities for the CF and CD processed ultrafine grained samples. Dynamic recovery is suggested to start during the deformation process and leads to a decrease in dislocation density at large deformation strains. The increase in twin density could compensate the loss of microhardness because of the decrease in dislocation density. The electrical conductivities of CF+CD samples were tested through standard four-probe method, all of which are higher than 92% the International Annealed Copper Standard. The results suggest that the strength of pure Cu could be improved and still keep its relatively high electrical conductivity by introducing deformation twins into its microstructure.

Direct Experimental Determination of Grain Boundary Excess Volume in Metals

The grain boundary excess volume, i.e., the grain boundary expansion, e{GB}, was experimentally determined for high-angle grain boundaries in nickel using the direct technique of high-precision difference dilatometry. Values of e{GB}=(0.35±0.04)×10{-10} m and e{GB}=(0.32±0.04)×10{-10} m were obtained by measuring the removal of grain boundary volume upon grain growth for two different types of ultrafine-grained samples. The results are discussed in comparison to values obtained so far from indirect techniques and from computer simulations. It demonstrates the strength of the presented novel, direct approach for grain boundary expansion measurements.

Effect of internal oxidation on wear behavior of ultrafine-grained Nb–Zr

The influence of surface modification by internal oxidation on dry sliding wear behavior of ultrafine-grained (UFG) Nb–Zr was investigated using a pin-on-disc type tribometer. The results show that improvement in strength by grain refinement via equal-channel angular pressing/extrusion has no substantial effect on the wear resistance of the non-oxidized UFG samples as compared to the coarse-grained material. This was attributed to the complex wear mechanisms operating in this alloy, such as adhesion leading to smearing, tribo-chemical reactions resulting in strong oxidative wear and also abrasion bringing about scratches and deep grooves. However, internal oxidation by heat-treatment significantly improved the wear resistance of Nb–Zr, especially under low to medium applied pressures due to the hardened diffusion zone with ZrO2 nanoparticles formed in the subsurface layer. Moreover, the improvement was more pronounced in the UFG material, which is attributed to increased diffusion in the UFG microstructure. When the applied pressure was increased above 0.5MPa, however, the wear rate increased considerably due to the elimination of the hardened subsurface layer. Still, even under high pressures, the oxidized samples demonstrated lower weight loss as compared to non-oxidized samples. Based on the investigations of the worn surfaces, it was determined that internal oxidation mostly eliminates the complex wear mechanisms operational in the non-oxidized samples, especially under low/medium loads. Improvement in wear properties by internal oxidation along with enhanced mechanical properties and previously demonstrated good biocompatibility and superior fatigue performance make internally oxidized UFG NbZr a promising candidate for biomedical applications in the human body.

Structural homogeneity and low-temperature micromechanical properties of ultrafine-grained AZ31 magnesium alloy

The degree of microstructural homogeneity of ultrafine-grained (UFG) AZ31 magnesium alloy produced by a special thermomechanical process including four cycles of equal-channel angular pressing (ECAP, route Bc) is studied by the microindentation method. The defect structure of the UFG alloy is quite homogeneous. Texture owing to ECAP showed up as a lower average microhardness measured in a plane perpendicular to the extrusion axis than that measured in a plane containing that axis, and in a slight dependence of the microhardness on the location of the indent on the billet surface. The microhardness of the initial coarse-grained alloy is close to that of the UFG samples measured in a plane perpendicular to the extrusion axis. Tensile deformation of the samples over a wide temperature range of 4.2–295 K increased the microhardness in the region of the neck by more than 30%. The temperature dependence of the microhardness over 77–295 K is indicative of thermally activated plastic deformation in AZ31 alloy. The mechanism for deformation of the alloy appears to be interactions between dislocations and local defects of impurity origin.

Tensile properties of ultrafine grained Mg-3%Al alloy studied at elevated temperature

Abstract The aim of this paper is to present an overview of the tensile behavior of ultrafine grained Mg-3%Al alloy. The bulk ultrafine grained Mg alloy was synthesized via consolidation of mechanical alloying. The grain size of the sample was calculated from X-ray diffraction line profile analysis. Tensile properties of ultrafine grained samples tested at temperatures of 25°C to 250°C are compared with that of a coarse grain microcrystalline sample. Plastic deformation of the ultrafine grained sample exhibits a combination of work hardening (15–48% plastic strain) and perfect plastic behavior characterized by a constant flow stress. At room temperature, the sample showed a gain in strength of about two times and enhanced ductility compared to the microcrystalline sample. The strain rate sensitivity was observed to be three times higher in the present ultrafine grained sample. A stress drop with increasing test temperatures was observed in both the samples. The effect of temperature on flow behavior in terms of work hardening rate, temperature sensitivity, ductility, yield strength, and tensile strength are also examined precisely. The present results show important evidence of the temperature dependence of the mechanical properties of the ultrafine grained Mg-3%Al alloy in comparison to the microcrystalline sample.

Enhanced Thermal Stability and Mechanical Properties of Ultrafine-Grained Aluminum Alloy

The paper reports on microstructure, strength and fatigue of ultrafine-grained (UFG) samples of the Al-Cu-Mg-Si aluminum alloy processed by high pressure torsion (HPT) at various temperatures. Application of the HPT treatment led to strong grain refinement, as well as to a raise of the mean-root square strains and dynamic precipitation. In case of optimal HPT treatment the UFG samples have demonstrated the enhanced thermal stability, an increase in ultimate tensile strength in 2.5 times and enhancement in fatigue endurance limit by 20 % in comparison with coarse-grained alloy subjected to standard treatment. It is shown that the regime of the HPT treatment governs the volume fraction of precipitates and segregations, thereby affecting a grain size and thermal stability of ultrafine-grained structure.

Scratch-induced deformation in fine- and ultrafine-grained bulk alumina

The nanoscratch behavior of two bulk α-alumina samples with 1.3 μm and 290 nm average grain sizes, respectively, was investigated using a nanoindenter in scratch mode, in combination with atomic force and scanning electron microscopy. A ductile to brittle transition was observed in the fine-grained sample, while the ultrafine-grained sample exhibited predominantly ductile deformation with a fish-bone feature indicative of a stick–slip mechanism. These findings suggest that grain refinement can increase the potential for plastic deformation in ceramics.

Influence of Annealing on Strength of Ultrafine Grained Low Carbon Steels by ECAP

In the present study, ultra fine-grained low carbon steel samples were processed by equal channel angular pressing (ECAP). Mechanical properties of the specimens annealed statically at several temperatures were evaluated by tensile and hardness test. In addition, grain sizes of the specimens were measured by SEM-electron back scattering pattern (SEM-EBSP) and X-ray diffraction analysis. Differential scanning calorimetry (DSC) measurement also evaluated thermal reactions in anneal process of the specimen. As a result, the grain size was changed at the temperature between 550oC and 600oC drastically and the tensile strength also became lower at the same temperature. The relation between yield stress and averaged grain diameter of specimens obeyed the Hall-Petch relation except the normalized specimen. Behavior of grain growth and recovery in structural observation by EBSP corresponded to reaction signal of the DSC curve.

Factors Influencing Tensile Ductility of OFHC Cu Having Different Ultrafine Grained Structures

Tensile ductility of OFHC Cu with the different ultrafine grained (UFG) structures, which were fabricated by the different routes of equal channel angular pressing (ECAP), was associated in detail with the microstructural characteristics developed by ECAP. OFHC Cu having the lamellar and equiaxed UFG structures was prepared by ECAP of routes A and Bc, respectively up to 8 and 16 passes. Their microstructures were closely examined by transmission electron microscopy and orientation image mapping. Tensile tests at room temperature were conducted on the ECAPed samples under the quasi-static condition of 10 � 3 s � 1 and 1 s � 1 . Uniform elongation of the lamellar UFG samples decreased with increasing the ECAP passage while both uniform and total elongations of the equiaxed UFG samples increased. In the case of route A producing the lamellar UFG structure, the fractions of high angle grain boundaries and grains less than 0.5 mm increased significantly but an analysis revealed that the dislocation free length decreased with increasing the ECAP passage. For route Bc resulting in the equiaxed UFG structure, the fraction of high angle grain boundaries increased but the grain size distribution and the dislocation free length remained nearly unchanged with increasing the ECAP passage. From the present experiments and analyses, it was found that tensile ductility of lamellar UFG OFHC Cu is primarily controlled by the dislocation free length and that of the equiaxed one is mainly dependent on the fraction of high angle grain boundaries. [doi:10.2320/matertrans.M2010089]

Effect of initial grain size on the joint properties of friction stir welded aluminum

In this study, 2 mm-thick commercial 1050-Al plates with an ultrafine grained (UFG) structure were obtained by the accumulative roll bonding (ARB) technique after a 5 cycle process and were subsequently joined by friction stir welding (FSW) at various revolution pitches (welding speed/rotation speed) of 1 mm/r, 1.67 mm/r and 2.5 mm/r. To understand the effect of the initial grain size on the welding properties, ARB processed samples followed by annealing under H24 conditions as well as the as-received samples in the fully annealed state were also applied to the FSW process. The microstructure evolution and Vickers hardness in the stir zone of all the samples were investigated. It was revealed that the annealed samples with an intermediate grain size finally obtained the most refined grain size and highest value of Vickers hardness in the stir zone. However, for the UFG samples, significant grain growth and corresponding decrease in hardness can be found in the stir zone.