Homogeneous Ultrafine-grained Structure Research Articles

Crystal-to-amorphous phase transformation in ternary shape memory alloys

This systematic study evaluates the role of important factors in crystal-to-amorphous phase transformation during severe plastic deformation in ternary shape memory alloys. Experiments coupled with thermodynamic calculations to obtain comprehensive interpretation on the amorphization in ternary shape memory alloys. Equiatomic binary NiTi together with ternary Ni–45Ti–5Hf and Ti–45Ni–5Cu (at%) alloys as alloy models were processed by high-pressure torsion (HPT) at room and cryogenic temperatures. The binary and Hf-added alloys revealed an amorphous structure with a few remaining nanocrystals, however, Cu-added ternary alloy showed resistance to the amorphization after HPT processing. The results revealed two key factors of preventing martensite-to-austenite transformation during deformation and the intrinsic potential of the material for amorphization drive crystal-to-amorphous phase transformation. Higher transformation temperatures, enthalpy and thermal hysteresis prevented martensite-to-austenite transformation, which therefore encouraged amorphization. In addition, a large negative heat of mixing, Gibbs energy for amorphization and a large atomic size mismatch are important factors in implementing crystal-to-amorphous phase transformation. This investigation is a step forward to a fundamental understanding of ternary or even compositionally complex shape memory alloys for microstructural engineering by providing a homogeneous ultrafine-grained structure due to amorphization and subsequent heat treatment to achieve superior shape memory effect and superelasticity.

The influence of the main alloying elements on the formation of the initial grain structure, on phase transformations and on structural changes that occur during superplastic deformation of alloys Al - 4.1 wt.% Mg - 0.5 wt.% Zr, 1420T, 1421, 1423

The article presents the results of research aimed at revealing the influence of the main alloying elements on the formation of the initial grain structure, on phase transformations and on structural changes that occur during superplastic deformation of several aluminum alloys. It was possible to form a homogeneous ultrafine-grained structure in samples of alloys Al - 4.1 wt.% Mg - 0.5 wt.% Zr and 1423 due to dynamic recrystallization during their superplastic deformation. It is revealed that the initial microstructure of the 1420T alloy samples is bimodal. The average grain size is approximately 5 μm, in some areas of the working part of the samples there are large elongated grains, the average size of which is approximately equal to 25 μm. The initial structure of alloy 1421 samples is fine-grained, and the initial structure of alloy 1423 samples is multi-grained and coarse-grained. Metallographic studies showed that the grain structure of samples of alloys 1420T, 1421 and 1423 increases slightly during superplastic deformation at high homologous temperatures. Cavitation accumulates in the samples and structural changes occur, which are probably associated with local melting at grain boundaries and at interphase boundaries. It was established that the presence of zirconium and scandium additions in the composition of the samples ensures the formation of an ultrafine-grained structure in them and counteracts the grain growth during superplastic flow. Magnesium and lithium, which are included in the samples of the studied alloys 1420T, 1421 and 1423, form several intermetallic phases with aluminum. These phases are part of mixtures of crystals of peritectic origin, which are localized in the form of layers between some grains. The occurrence of peritectic reactions at high homologous temperatures can be one of the reasons for the partial melting of samples of alloys 1420T, 1421, and 1423 during their superplastic deformation. Partial melting of samples of alloys 1420T, 1421 and 1423 can probably be carried out due to the presence of segregations of magnesium and lithium at the grain boundaries, which lower the melting temperature of the aluminum-based solid solution. Partial melting of samples of alloys 1420T, 1421, and 1423 during their superplastic deformation, which is performed at high homologous temperatures, leads to the formation of cells of a metastable liquid-solid phase at the grain boundaries, the viscous flow of which leads to the formation of fibrous structures due to the development of grain boundary sliding.

Mechanical properties and biological responses of ultrafine-grained pure titanium fabricated by multi-directional forging

We aimed to investigate strengthening of pure titanium by ultra-grain refinement using multi-directional forging and to examine the biocompatibility after sulfuric acid treatment. Commercial titanium grade 2 was multi-directional forged to large cumulative strain to obtain homogeneous ultrafine-grained structure. The evolved microstructure of the obtained material was equiaxed and ultrafine-grained with high dislocation density. Average grain size was evaluated to be approximately 100 nm. The ultrafine-grained titanium showed higher tensile strength, Vickers hardness and a lower elastic modulus compared to conventional pure titanium grade 2. The surface features of the multi-directional forged titanium after acid treatments were characterized by a roughness of 1 μm composed of equiaxed fine dimples with fractal structure and a hydrophilicity of 15. Cell proliferation on multi-directional forged titanium was significantly promoted than on conventional titanium. The above results demonstrated the high potential of multi-directional forged titanium as a superior biocompatible implant material.

Evaluating the textural and mechanical properties of an Mg-Dy alloy processed by high-pressure torsion

Samples of an Mg-0.41Dy (wt. %) alloy were severely deformed by high-pressure torsion (HPT) at room temperature up to 15 turns and the texture, microstructure and microhardness values in the centres, mid-radial points and edges of the HPT-deformed discs were investigated using X-ray diffraction, Electron BackScatter Diffraction and Vickers microhardness measurements. The textures in the centres of discs were characterized by a typical weak basal fiber whereas at both the mid-points and edges of the discs there was a strong basal texture where the c-axis of most grains was shifted 15° away from the shear direction. An almost homogeneous ultrafine-grained structure with a grain size of about 0.75 μm was achieved after 15 HPT turns. The microhardness values in these three positions increased with increasing numbers of turns, reached a maximum and then decreased to a lower steady-state level at large strains. In addition to dislocation and grain size hardening, the results show that texture strengthening contributes significantly to the rapid increase in hardening in the early stages of deformation.

Enhanced Impact Toughness at Ambient Temperatures of Ultrafine-Grained Al-26 wt.% Si Alloy Produced by Equal-Channel Angular Pressing

Overcoming general brittleness of hypereutectic Al-Si alloys is in urgent need for expanding their application in automotive, aerospace and construction industries. A unique phenomenon was observed that bulk ultrafine-grained Al-26 wt.% Si alloy, produced by severe plastic deformation via equal-channel angular pressing, exhibited higher toughness at the impact temperature of − 196 ~ 100 °C than the coarse-grained casting alloy. The improvement in impact toughness at all testing temperatures was mainly due to the homogeneous ultrafine-grained structure with the breakage of brittle primary silicon crystals, which generated more and deeper fracture dimples that consumed much higher fracture energy. It indicates the advantage of bulk ultrafine-grained Al-Si alloys and spurs their application interest at various ambient temperatures.

Microstructure development of ultra fine grained Mg-22 wt%Gd alloy prepared by high pressure torsion

A hardenable lightweight Mg-22wt%Gd alloy with ultra fine grained (UFG) structure was prepared by high pressure torsion (HPT) at ambient temperature. The development of microstructure during HPT processing was investigated. A homogeneous UFG structure with grain size of 300nm was achieved after 15 HPT revolutions. The UFG alloy exhibits enhanced strength due to work strengthening by tangled dislocations forming a dense forest throughout grains. Dislocation density in the sample was determined by positron annihilation spectroscopy (PAS) and X-ray line profile analysis (XLPA). It was found that there is an additional source of X-ray profile broadening in addition to small crystallites and micro-strains caused by dislocations. The additional micro-strain component was attributed to lattice modulation by Gd-rich nano-wires formed by agglomeration of Gd solutes and to strains arising from boundaries of crystallite domains and inter-domain interactions. Analysis of the influence of the crystallite size on the strength of UFG Mg-22wt%Gd alloy revealed a breakdown in the Hall-Petch relationship when the crystallite size decreased below a critical value of ≈30nm.

Strength and fatigue of an ultrafine-grained Al-Cu-Mg alloy

The dependence of strength and fatigue on microstructure of the Al-Cu-Mg alloy has been investigated. Various microstructures of the alloy were produced: the one with a coarse-grained (CG) structure after T6 heat treatment; the one with a homogeneous ultrafine-grained (UFG) structure and the one with a bimodal (mixed) structure, both processed by equal-channel angular pressing (ECAP). The mean grain size and morphology of precipitates were studied by transmission electron microscopy. The ultimate tensile strength and the fatigue endurance limit were determined using the tensile and fatigue tests of standard specimens. It is established that the formation of a homogeneous UFG structure and of a bimodal (mixed) structure alloy contributes to a significant increase in microhardness by 16% and 60%, and an increase of the ultimate tensile strength by 20 and 52%, respectively, as compared to the samples subjected to T6 heat treatment. Fatigue tests show that the alloy with a bimodal (mixed) structure has the highest fatigue endurance limit, 45% higher than in the sample subjected to T6 heat treatment. In contrast, the formation of a homogeneous UFG structure enables increasing the fatigue endurance limit by 15% only.

Modeling the structure formation during hot deforming the billets of the parts of gas-turbine engines made of heat-resistant nickel alloy

The results of finite-element and physical modeling of deformation heating and structure formation during high-temperature isothermal upset of a cylindrical billet according to various modes (continuous and fractional deformation) have been presented. The results of numerical modeling, which agree with the experimental modeling data, showed that, in order to minimize the deformation heating and formation of a homogeneous ultrafine-grain structure in bulk billets made of heat-resistant alloys, it is purposeful to perform the deformation fractionally (with a single degree no larger than 30–50%), which allows for the deformation rate and performance of post-deformation annealing. The formation of an ultrafine-grain structure in the billets made of nickel alloys (for example, EP742U) guarantees an increase in the quality of ready wares.

Improved surface integrity from cryogenic machining of Al 7050-T7451 alloy with ultrafine-grained structure

Materials with ultrafine-grained (UFG) structure become the prioritised selections for specific applications where high strength, hardness, wear/corrosion resistance, etc. are sought. Machining materials with UFG structure requires further research as their mechanical and material properties are different from the ones with regular coarse grains. In this study, Al 7050-T7451 aerospace alloy was first processed by friction stir processing (FSP) with UFG structure for use in subsequent machining experiments. Cryogenic machining using liquid nitrogen as the coolant was applied in the experimental work of machining Al 7050-T7451 alloy with UFG structure. Surface integrity characteristics including microstructure, grain size and hardness were investigated in order to understand the differences of these properties induced by dry and cryogenic machining. Smaller grains with the grain size of around 1.6 μm were observed in the machined surface generated by cryogenic machining compared to the grains of bulk materials and the ones acquired by dry machining. On the other hand, higher hardness measured from the surface and subsurface regions of the machined samples show a harder layer formation by cryogenic machining. Forces and temperatures were measured during dry and cryogenic machining. Cutting and thrust force components for both dry and cryogenic condition were found to be almost identical due to the homogeneous UFG structure achieved by the previous FSP while cryogenic machining generated lower cutting temperature. Also, microstructure and hardness of the chips formed by machining were investigated to understand the effects of cryogenic machining on materials with UFG structure from the dynamic recrystallisation and grain growth points of view. Grains with approximately 1 μm grain size existed in the machined chips, and the hardness of the chips was about 42% higher than the bulk material. Overall, the experimental findings demonstrate that the proposed approaches were beneficial to enhance surface integrity characteristics of this material.

Mechanical properties and microstructure evolution in ultrafine-grained AZ31 alloy processed by severe plastic deformation

Commercial MgAlZn alloy AZ31 was processed by two techniques of severe plastic deformation (SPD)—extrusion followed by equal channel angular pressing (EX-ECAP), and high pressure torsion (HPT). Processing by ECAP was conducted at elevated temperature of 180 °C for 1–12 passes following route BC. HPT was applied at room temperature, and the specimens of the diameter of 19 mm with different number of turns (N = ¼ − 15) were prepared. Mechanical properties and grain fragmentation with strain due to EX-ECAP and HPT were investigated by Vickers microhardness measurements and transmission electron microscopy, respectively. Variations in dislocation density were investigated by positron annihilation spectroscopy. Differences in microhardness, grain refinement and dislocation density evolution resulting from principal differences of straining were found in the specimens. EX-ECAP resulted in homogeneous microstructure throughout the specimen's cross section as early as after four passes. On the other hand, laterally inhomogeneous microstructure with gradual reduction of grain sizes from the centre towards the periphery of the disk was observed in specimens after HPT. This microstructure and microhardness inhomogeneities were continuously smeared out and almost homogeneous ultrafine-grained structure was observed in specimen subjected to 15 HPT turns. Variations in mechanical properties and dislocation density evolution were compared in conditions corresponding to the same equivalent strain imposed by both techniques of SPD.

Anticorrosion behavior of ultrafine-grained Al-26 wt% Si alloy fabricated by ECAP

Ultrafine-grained (UFG) Al-26 wt% Si alloy was obtained through multipass equal-channel angular pressing (EACP) procedure and subsequently tested in 3.5 wt% NaCl solution for the evaluation of electrochemical corrosion. The results show that the ECAPed alloy with increased number of pressing passes obtain lower mass-loss ratios, nobler Ecorr and Epit, lower Icorr values, and higher anode polarization. The improved corrosion resistance of the ECAPed alloy results from the homogeneous UFG structure with the breakage of brittle large primary silicon crystals, which contributes to a higher pitting resistance. The oxidation product with improved adhesion force and protection efficacy can be formed with greater ease on UFG alloys. It implies that grain refinement through severe-plastic-deformation can enhance anticorrosion behavior of hypereutectic Al–Si alloys, besides the well-known strengthening and toughening effects.

Strength and high fatigue properties of ultrafine-grained titanium rods produced by severe plastic deformation

The formation of an ultrafine-grained structure (UFGS) in titanium by severe plastic deformation significantly increases its mechanical properties, among them fatigue strength. In this work, high mechanical properties of titanium long rods are achieved using a combination of equal-channel angular pressing and subsequent thermal and thermomechanical treatment. The formation of a homogeneous UFGS in a rod of commercial titanium is found to increase the fatigue limit by a factor of about 1.5. Moreover, the fatigue strength of UFGS titanium estimated on notched specimens is higher than that of coarse-grained titanium, which is important for its structural application in medicine.

Corrosion behavior of hypereutectic Al-23%Si alloy (AC9A) processed by severe plastic deformation

Ultrafine-grained(UFG) hypereutectic Al-23%Si (mass fraction) alloy was achieved through equal-channel angular pressing(EACP) procedure. And the electrochemical properties after various ECAP passes were investigated in neutral NaCl solution. Potentiostatic polarization curves show that the corrosion potential of the ECAPed sample after 4 passes decreases markedly, while the corrosion current density reaches 1.37 times that of the as-cast alloy. However, the φ corr and J corr values after 16 passes are improved and approach those of the as-cast alloy. Immersion tests also show that the mass-loss ratio of ECAPed alloy decreases with increasing the pressing pass, which is lowered to 28.7% with the increase of pass number from 4 to 16. Pitting susceptibility of the ECAPed alloy after initial 4 passes is boosted, due to the presence of biggish voids resulted from the breakage of brittle large primary silicon crystals during ECAP. Increasing ECAP pass makes the voids evanesce and results in the homogeneous ultrafine-grained structure, contributing to a higher pitting resistance. These results indicate that enough ECAP passes are beneficial to increasing corrosion resistance of the hypereutectic Al-23%Si alloy.

Nanostructured Titanium for Biomedical Applications

Metallic materials, for example, stainless steel, titanium and its alloys, and tantalum, are widely used for medical implants in trauma surgery, orthopedic and oral medicine. Successful incorporation of these materials for design, fabrication and application of medical devices require that they meet several critical criteria. Paramount is their biocompatibility as expressed by their relative reactivity with human tissues. Another is their ability to provide sufficient mechanical strength, especially under cyclic loading conditions to ensure the durability of the medical devices made therefrom. Finally the material should be machinable and formable thereby enabling device fabrication at an affordable cost. In this paper we show that nanostructured commercial purity titanium produced by severe plastic deformation (SPD) opens new avenues and concepts for medical implants, providing benefits in all areas of medical device technology. Numerous clinical studies of medical devices fabricated from commercial purity (CP) titanium for trauma, orthopaedic and oral medicine has proven its excellent biocompatibility. However the mechanical strength of CP titanium is relatively low compared to other metals used in biomedical devices. Whereas the strength of this material can be increased by either alloying or secondary processing, for example rolling, drawing, etc., these enhancements normally come with some degradation in biometric response and fatigue behaviour. Recently it has been shown that nanostructuring of CP titanium by SPD processing can provide a new and promising alternative method for improving the mechanical properties of this material. This approach also has the benefit of enhancing the biological response of the CP titanium surface. This paper reports the results of the first developments and studies of nanostructured titanium (n-Ti), produced as long-sized rods with superior mechanical and biomedical properties and demonstrates its applicability for dental implants. The effort was conducted using commercially pure Grade 4 titanium [C – 0.052 %, O2 – 0.34 %, Fe – 0.3 %, N – 0.015 %, Ti-bal. (wt. pct.)]. Nanostructuring involved SPD processing by equal-channel angular pressing followed by thermo-mechanical treatment (TMT) using forging and drawing to produce 7 mm diameter bars with a 3 m length. This processing resulted in a large reduction in grain size, from the 25 lm equiaxed grain structure of the initial titanium rods to 150 nm after combined SPD and TMT processing, as shown in Figure 1. The selected area electron diffraction pattern, Figure 1(c), further suggests that the ultra fine grains contained predominantly high-angle non-equilibrium grain boundaries with increased grain-to-grain internal stresses. A similar structure for CP Ti can be produced in small discs using other SPD methods, for example – high pressure torsion (HPT) as studied in detail. In the present work it was essential to produce homogeneous ultrafine-grained structure throughout a three-meter length rod to enable the pilot production of implants and provide sufficient material for thorough testing of the mechanical and bio-medical properties of the nanostructured titanium. Table 1 illustrates mechanical property benefits attainable by nanostructuring of CP titanium, for example, the strength of the nanostructured titanium is nearly twice that of conventional CP titanium. Notably this improvement has been achieved without the drastic ductility reductions (to below C O M M U N IC A IO N S

The impact of compression with oscillatory torsion on the structure change in copper

The influence of compression with oscillatory torsion on the copper structure and force parameters are presented. The compression with oscillatory torsion method, developed in the Faculty of Materials Science and Metallurgy at the Silesian University of Technology, is used to achieve severe plastic deformation resulting in homogeneous ultrafine-grained structure of metals. The deformation resistance of copper for various torsion frequency and compression rate is presented. The results of microstructural observations by using LM (light microscope) and TEM (Transmission Electron Microscope) technique are displayed as well. The geometrical parameters of structure elements and their misorientation angles were characterized by using TEM method. Application of compression with oscillatory torsion was found to cause a remarkable decrease of deformation resistance as compared to compression without torsion. Plastic flow localized in shear bands was observed. Structures with large misorientation occur in microbands areas. The banded structure formed during compression with oscillatory torsion consists of well-formed, elongated subgrains.

Microstructure and Properties of Ti Rods Produced by Multi-Step SPD

This paper studies the effect of combined SPD treatment on microstructure and mechanical properties of semi-products out of CP Ti. The combined processing, consisting of equal-channel angular pressing and further thermomechanical treatment, produced ultrafine-grained rods out of Grade 2 CP Ti with a diameter of 6.5 mm and a length of up to 1 m. It was established that the formation of homogeneous ultrafine-grained structure in Ti rod with α-grain size of about 100 nm allowed to enhance yield stress by 200% in comparison with initial annealed state.

The Effect of Severe Plastic Deformation on the Structure and Mechanical Properties of Al–Mg–Li Alloys

The structural-phase state and mechanical properties of commercial aluminum alloys produced by severe plastic deformation are studied and compared to the initial polycrystalline state. This kind of treatment is found to give rise to the formation of a homogeneous ultrafine-grained structure with second-phase particles occurring predominantly along grain boundaries. With this structure, the strain-temperature and strain-rate intervals wherein the superplastic properties of the alloys under study are observed are shifted to lower temperatures and higher rates.