Coarse-grained Alloy Research Articles

Tensile and Corrosion Properties of Anodized Ultrafine-Grained Ti–13Nb–13Zr Biomedical Alloy Obtained by High-Pressure Torsion

Severe plastic deformation (SPD) is a popular group of techniques applied to achieve the nanostructuring of the metallic biomaterials and improvement of their mechanical characteristics. One of the most commonly used SPD methods is the high-pressure torsion (HPT) technique which enables the obtainment of the microstructure with small grains and high strength. In the present study, the influence of the plastic deformation and surface modification treatment on the tensile and corrosion properties of the Ti–13Nb–13Zr (wt%) alloy is investigated. In that purpose, the coarse-grained (CG) Ti–13Nb–13Zr (TNZ) alloy was subjected to the HPT processing by applying a pressure of 4.1 GPa with a rotational speed of 0.2 rpm and 5 revolutions at room temperature to obtain the ultrafine-grained (UFG) microstructure. The alloy microstructure before and after HPT processing was analysed using the scanning electron microscopy (SEM) and the X-ray diffraction (XRD). The homogeneity of the UFG TNZ alloy was determined by microhardness testing and microscopic observations. The nanotubular oxide layer on the surface of the TNZ alloy, both in CG and UFG condition, was formed by electrochemical anodization in 1 M H3PO4 + NaF electrolyte for 90 min. SEM analysis was used to characterise the morphology of the anodized surfaces, while energy dispersive spectroscopy was applied to determine the chemical composition of the nanostructured layers formed at the alloy surfaces. Mechanical properties of the TNZ alloy, before and after HPT processing and electrochemical anodization, were determined by tensile testing. After tensile testing, the fractographic analysis was conducted to identify the fracture mechanisms. The potentiodynamic polarization technique was used to determine the corrosion resistance of the alloy before and after plastic deformation and surface modification treatment. The obtained results showed that the alloy is reasonably homogeneous after the HPT processing. The XRD analyses reviled the presence of α′ and β phases in the CG TNZ alloy microstructure, while the additional ω phase was detected in the microstructure of the UFG TNZ alloy. The HPT obtained alloy exhibits higher hardness and improved tensile properties than the alloy in the as-received CG condition, while the electrochemical anodization leads to a decrease of its mechanical properties. Both CG and UFG alloys show excellent corrosion stability in Ringer’s solution. Moreover, electrochemical anodization leads to a decrease or an increase of the corrosion resistance of these materials, depending on the morphology of the formed nanotubular surface layers. The results indicate that the anodized CG TNZ alloy is characterized by a lower modulus of elasticity and better corrosion resistance properties than the anodized UFG TNZ alloy.

Tensile deformation behavior of coarse-grained Ti-55 titanium alloy with different hydrogen additions

The effect of hydrogen addition on the deformation behavior of coarse-grained Ti-55 alloys (~ 20 μm) was studied by uniaxial tension tests at high temperature. The elongation of hydrogenated Ti-55 titanium alloy firstly increases and then decreases with hydrogen content increasing at 875 °C. The highest elongation of 243.8% is obtained in the hydrogenated alloy with 0.1 wt% H, and the peak stress reaches a minimum value of 29.0 MPa in the hydrogenated alloy with 0.3 wt% H. Compared with that of the unhydrogenated alloy, the elongation of the hydrogenated alloy with 0.1 wt% H increases by 41.3% and its peak stress decreases by 40.6% at 875 °C. Hydrogen addition can promote the transformation of β phase and the dislocation movement. Appropriate hydrogen content can evidently improve the deformation properties of coarse-grained Ti-55 titanium alloy.

Microstructure and fracture toughness characterization of three 9Cr ODS EUROFER steels with different thermo-mechanical treatments

Ferritic martensitic ODS steels are one of the candidate structural materials for future Gen-IV nuclear fission and fusion reactors. The dependence of fracture toughness on microstructure was investigated by comparing three 9Cr ODS EUROFER steels manufactured through different thermo-mechanical processing routes. Quasi-static fracture toughness testing was performed with sub-sized C(T) specimens and microstructural characterization was carried out using scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. It was found that at lower test temperatures (−100 – 22 °C), the fracture toughness was primarily controlled by crack initiation at sub-micron particles and by production of secondary cracks during fracture. At higher temperatures (above 100 °C), fracture toughness was predominantly controlled by the matrix ductility and the grain boundary strength with a relatively ductile coarse-grained alloy demonstrating higher fracture toughness compared to high-strength fine-grained alloys. These results and discussion show that variations in thermomechanical treatments can produce significant differences in microstructure and fracture toughness behavior of ferritic martensitic ODS steels.

Effect of Ultrasonic Surface Rolling Treatment on Corrosion Behavior of Alloy 690

The nanograins (NG) on the top surface layer of alloy 690 were successfully prepared by ultrasonic surface rolling treatment (USRT). The average grain size of NG alloy 690 was 55 nm, and the thickness of the NG surface layer was about 1 μm. Meanwhile, the surface roughness was significantly reduced after surface nanocrystalliztion. The corrosion behavior of alloy 690 before and after USRT was studied in a secondary side environment containing chloride. Electrochemical corrosion experiments demonstrated that the passive film generated by USRT was denser than that formed on coarse-grained (CG) alloy 690. Pitting corrosion was prone to occur in grain boundaries of CG alloy 690, and the pits on the surface of NG alloy 690 were smaller than those generated on CG alloy 690 after 1000 h of an immersion experiment.

Mechanical properties and deformation mechanisms of a novel fine-grained Mg-Gd-Y-Ag-Zr-Ce alloy with high strength-ductility synergy

Grain boundary precipitation and segregation play an important role in determining mechanical properties of Mg alloys. In the present work, we studied work focuses on the strengthening and deformation mechanism of coarse-grained (CG) and fine-grained (FG) Mg-Gd-Y-Ag-Zr-Ce alloy. The CG alloy is strengthened by means of age-strengthening with the formation of both basal plate γ′′ and prismatic plate β′ precipitates in the grain interior. While the strengthening of FC alloy is completed by intergranular alloying segregation and intragranular precipitates γ′′ and β′. The segregation of alloying elements at the grain boundary and formation of sub-micron particles can stabilize the grain boundary and suppress the intergranular deformation. Consequently, dislocations could be trapped near γ′′ and β′ precipitates in the grain interior. Unlike CG alloys, the FG alloys exhibit a heterogeneous transition from elastic to plastic deformation via the Lüders plateau. The rapid gliding dislocation multiplications and fine-grained size are necessary and sufficient conditions for the Lüders strains. Our work provides the insights on the evolution of fine-grained microstructure and helps for the design of Mg alloys with good mechanical properties.

Grain boundary diffusion in CoCrFeMnNi high entropy alloy: Kinetic hints towards a phase decomposition

Grain boundary diffusion of the principal elements 57Co, 51Cr, 59Fe and 54Mn in a coarse-grained equiatomic CoCrFeMnNi high entropy alloy is measured in a wide temperature range of 643 to 1273 K in both C- and B-type kinetic regimes after Harrison’s classification. The results suggest that the product of the pertinent segregation factors, s, and the grain boundary width, δ, is about 0.5 nm for all elements at temperatures T > 800 K. Whereas one short-circuit contribution is observed at higher temperatures above 800 K, the penetration profiles in the C-type kinetic regime (643 – 703 K) reveal two distinct contributions that hint towards a phase decomposition at a fraction of high-angle grain boundaries at these temperatures and the existence of a structural multiplicity of high-angle grain boundaries. A correlative microscopy combining transmission Kikuchi diffraction and atom probe tomography manifests formation of neighboring Ni-Mn-rich and Cr-rich precipitates at high angle grain boundaries. Transmission electron microscopy revealed an increased dislocation density in the vicinity of such interfaces which is suggested to be a reason of the enhanced diffusion rates at low temperatures for such short circuits.

Effect of Post-aging Heat Treatment on Strength and Local Corrosion Behavior of Ultrafine-Grained 2219 Al Alloy

The mechanical properties and local corrosion behavior of coarse-grained (CG) and ultrafine-grained (UFG) 2219 Al alloys with post-aging heat treatment have been investigated. The hardness and strength of UFG alloy were greatly higher and the susceptibility to intergranular corrosion was significantly lower compared with CG alloy. However, the exfoliation corrosion and electrochemical corrosion resistance of UFG alloy were lower than CG alloy. In addition, the local corrosion behavior of UFG alloys was less sensitive to post-aging conditions compared with CG alloy. The differences in mechanical properties and local corrosion behavior of UFG and CG 2219 Al alloy with post-aging heat treatment were mainly attributed to the complex interactions of grain size, dislocation and aging precipitates.

Effect of annealing on microstructure and mechanical properties of an ultrafine-structured Al-containing FeCoCrNiMn high-entropy alloy produced by severe cold rolling

Severe cold rolling (SCR) and annealing were applied to a face-centered cubic (FCC) Al0.3FeCoCrNiMn high entropy alloy. The crystal structure, microstructure, and mechanical properties of this SCRed Al-containing HEA were studied systematically. The coarse-grained alloy is substantially refined after SCR resulting in great improvement of the hardness and strength. With Al addition into the FeCoCrNiMn alloy, the thermal stability of this alloy is declined, and the precipitation of σ and B2 phases in the FCC matrix is greatly promoted. The microstructure evolution of the SCRed alloy during annealing can be divided into three stages: the combined precipitation of σ and B2 phases blow 1000 °C; the dissolution of σ phases and only B2 phase precipitates at 1000 °C; recovering to a single FCC phase at 1100 °C. The SCRed alloys annealed at 500–700 °C show a high strength but with a low ductility due to formation of a large number of σ and B2 phases. With increasing annealing temperature, the strength is gradually reduced that is attributed to grain growth of the matrix and dissolution of precipitates. In addition, these precipitates effectively suppress the grain growth of the matrix during annealing. A fully recrystallized and ultrafine-grained alloy with multiphase structure is obtained as the SCRed alloy annealing at 800 °C, which shows the YS, UTS and the elongation to failure of this alloy are 970 MPa, 1080 MPa and 8%, respectively. The high strength of this alloy is attributed to the grain refinement strengthening and precipitation strengthening.

Synthesis, structure, and mechanical response of Cr0.26Fe0.24Al0.5 and Cr0.15Fe0.14Al0.30Cu0.13Si0.28 nanocrystallite entropy alloys

The present research work has concentrated to synthesize nanocrystalline (NC) Cr0.26Fe0.24Al0.5 (medium entropy alloy, 3E-MEA) and Cr0.15Fe0.14Al0.30Cu0.13Si0.28 (high-entropy alloy, 5E-HEA) non-equiatomic (equal weight fraction) alloys through mechanical alloying (MA); which studied the influence of entropy effect on structural properties, microstructural characterization, and mechanical behaviour. Further, the same non-equiatomic ratio of two coarse grain alloys (CGAs) was manufactured by conventional powder metallurgy (PM) route (blending method, 3E-CGA, 5E-CGA) for comparison. All synthesized powders were hot-pressed (HPed) at 723 k for 30 min subsequently mechanical properties in terms of compressive stress-strain and hardness were examined. The samples of as-milled powders, HPed, and fractured were investigated using X-ray diffraction (XRD) and advanced electron microscopes. The HPed sample of 3E-MEA of Cr0.26Fe0.24Al0.5 produced 94% BCC and 6% FCC crystal structures due to more dissolution of Al atoms in the stronger bonding atoms of Cr-Fe lattice. Whereas 5E-HEA of Cr0.15Fe0.14Al0.30Cu0.13Si0.28 sample has exhibited 72.1% FCC phase and 27.9% BCC phase due to balance between the dissolution of FCC elements (Al, Cu, Si) and BCC elements (Cr, Fe). Further, 3E-MEA and 5E-HEA have exhibited the ultimate compressive strength (UCS) of 1278 ± 6.75 MPa and 2060 ± 2.8 MPa respectively whereas the corresponding conventionally blended alloys produced 268 ± 5 MPa and 615 ± 3 MPa for 3E-CGA and 6E-CGA respectively. Vicker’s hardness strength (VHS) of 5E-HEA of Cr0.15Fe0.14Al0.30Cu0.13Si0.28 has exhibited 68% more when compared to 3E-MEA of Cr0.26Fe0.24Al0.5, 3.26 times higher compared to blended alloys. Further, several strengthening mechanisms on the mechanical behaviour of MEA and HEA were investigated in which dislocation strengthening mechanisms followed by solid solution strengthening mechanisms have influenced more as compared to grain boundary strengthening mechanisms.

Analysis and Characterization on Dynamic Recrystallization in Casting AZ31 Mg Alloys Under Plane Strain Compression.

Studies on twinning, twin-induced dynamic recrystallization (TDRX), and their temperature and strain rate dependences are of considerable significance to the ultimate strength and plastic formability of the coarse-grained Mg alloys during severe plastic deformation. Plane strain compression tests were conducted on the parallelepiped samples of casting AZ31 Mg alloys. The twinning and recrystallization behaviors close to and away from the crack boundaries were characterized using electron backscatter diffraction. The results show: (1) with increasing strain rate for tests, the extension twin proliferates significantly. Due to the local stress concentration, the TDRX is more active in the area close to the crack tip and exhibits the positive strain-rate sensitivity as twinning; (2) the TDRX is not only stress-favored but also closely links to the temperature. However, the TDRX is not utterly proportional to the temperature. Compared to 400 °C, 300 °C is more beneficial to the TDRX, achieving the higher strength and plastic deformability. The main reason is that the higher strain-hardening rate and flow stress at the higher strain rate and lower temperature motivates the transformation from twinning to the fine twin-walled grains more efficiently, and the stress-favored TDRX is crucial to refine grains and continue plastic deformation for the casting Mg alloys with coarse grains.

A study on slip activation for a coarse-grained and single crystalline CoCrNi medium entropy alloy

The new CoCrNi medium entropy alloy (MEA) has emerged to be one of the most promising systems which provide extraordinary ductility and strength at cryogenic temperatures. In this study, utilizing both polycrystalline and single crystal specimens, as well as advanced optical strain measurements, the deformation mechanisms dictating the mechanical behavior at the onset of plasticity were detected and precisely quantified. Independent of deformation temperature, the accumulation of permanent strains at the microstructural level was attributed to plastic slip at the onset of yielding and at low levels of deformation (<10%). The resolved shear stress for slip activation was measured to be 78 MPa at 298 K and between 140 MPa and 160 MPa at 77 K. These unique measurements were used to provide an estimate of the temperature dependent resolved shear stress for slip at temperatures ranging from 77 K to 575 K. In addition, strain heterogeneities at the grain-scale were measured to study the nucleation of slip at the micro-scale in polycrystalline specimens. In summary, the present study aims to quantitatively assess the accumulation of plastic strain and reveal the underlying deformation mechanisms (i.e., slip or/twinning) leading to the buildup of plastic strains at the microstructural level.

Mechanical behavior at elevated temperatures of the ultrafine-grained titanium alloy VT8M-1 processed by rotary swaging

This paper investigates the mechanical properties at elevated operation temperatures of the Ti alloy VT8M-1(Ti-5.3Al-4.0Mo-1.2Zr-1.3Sn-0.2Si) in the coarse-grained (CG) and ultrafine-grained (UFG) states. The duplex UFG structure was produced by rotary swaging at a temperature of 750 °C. The UFG state was studied in terms of thermal stability during annealing at temperatures 500, 550, 600, 700 °C for a period of 1 hour. It was demonstrated that the recrystallization processes in the UFG structure start after annealing at 550 °C and above. We performed tensile mechanical tests at such temperatures. In a range of 300-450 °C, the ultimate tensile strength of the UFG alloy was stably above that of the CG alloy. Mechanical tests at 500 and 550 °C revealed a more significant decrease in the ultimate tensile strength of the UFG alloy as compared to the CG alloy, conditioned by the intensification of the recovery and recrystallization processes at these temperatures.

Influence of hydrogen content on ion nitriding of coarse-grained and ultrafine-grained VT6 titanium alloys

This work investigates low-temperature ion nitriding of coarse-grained VT6 titanium alloy performed at the temperature of 600°C with different hydrogen contents (0–30% H2). The optimum hydrogen content identified during the experiments with coarse-grained samples was used to further nitride ultrafine-grained VT6 titanium alloy at the temperature of 500°C. Microhardness distribution diagrams of and optical microstructure images were obtained; brittleness of the nitrated layer was estimated.

Influence of ultra-fine grain structure on corrosion behaviour of biodegradable Mg-1Ca alloy

Biodegradation studies of Mg-1Ca alloy with different microstructures in Ringer’s solution reveal corrosion mechanism based on the balance between the anodic dissolution of the secondary phase and hydroxide precipitation, controlled by the secondary phase size and distribution uniformity. Alloy with ultrafine-grain structure shows at least 5-fold improvement in corrosion resistance compared to the coarse-grained alloy due to the separation of the particles of the secondary phase and its continuous intragranular nano-precipitation. This promotes an unusual low-frequency impedance behaviour described by an inductive loop with the negative time constant, which leads to the increased impedance modulus signifying the improved corrosion resistance.

Overcoming the strength-ductility trade-off by the combination of static recrystallization and low-temperature heat-treatment in Co-Cr-W-Ni alloy for stent application

A process combining swaging, static recrystallization, and heat treatment at 873 K (low-temperature heat-treatment, LTHT) was developed for achieving both high ultimate strength and high ductility in Co-20Cr-15W-10Ni (mass%, CCWN) alloy for stent application. The alloys swaged to a sectional area reduction rate of 58.3% were annealed at 1373–1473 K for 30–300 s. Under annealing at 1373 K for 300 s, a fine grain structure with an average grain size of ~6 μm formed, while under annealing at 1473 K, a structure with an average grain size of 12 μm formed after 120 s. In the alloys annealed at 1373–1448 K, the formation of η-phase precipitates (M6X-M12X type, M: metallic elements, X: C and/or N) was observed, while no precipitates were observed in the alloys annealed at 1473 K. The improvement in ultimate strength by grain refinement was confirmed. Alloys annealed at 1473 K showed higher ductility compared to those annealed at 1373–1448 K even if the grain size was similar. It is considered that the η-phase precipitates deteriorated the ductility of the annealed alloys. LTHT suppressed the strain-induced martensitic γ-to-ε transformation to improve the ductility of the fine-grained as well as coarse-grained alloys. Thus, regardless of the grain size, it is newly evidenced that LTHT effectively improves ductility in CCWN alloy. By combining high-temperature short-time annealing and LTHT, both the ultimate strength and ductility of Co-20Cr-15W-10Ni (mass%) alloy improved, and it was possible to provide properties suitable for next-generation balloon-expandable stents with Co-20Cr-15W-10Ni (mass%) alloy.

Spark plasma sintering for high-speed diffusion bonding of the ultrafine-grained near-α Ti–5Al–2V alloy with high strength and corrosion resistance for nuclear engineering

The paper demonstrates the prospects of spark plasma sintering (SPS) for the high-speed diffusion bonding of the high-strength ultrafine-grained (UFG) near-α Ti–5Al–2V alloy. The effect of increased diffusion bonding intensity in the UFG Ti alloys is discussed also. The bonding areas of the UFG near-α Ti–5Al–2V alloy obtained by SPS are featured by high density, strength, and corrosion resistance. The rate of bonding in the UFG alloys has been shown to depend on the heating rate non-monotonously (with a pronounced maximum). At the stage of continuous heating and isothermic holding, the bonding kinetics was found to be determined by the exponential creep rate, the intensity of which in the coarse-grained alloys is limited by the diffusion rate in the crystal lattice α-Ti. In the UFG alloy, the exponential creep processes associated with gliding and climb of dislocations, the activation energy of which corresponds to the diffusion activation energy in the lattice dislocation nuclei, may take place simultaneously with the grain boundary sliding and Coble creep, the activation energy of which corresponds to the grain boundary diffusion activation energy.

Corrosion behavior of metastable AISI 321 austenitic stainless steel: Investigating the effect of grain size and prior plastic deformation on its degradation pattern in saline media

The role of grain size and strain rate on the corrosion behavior of plastically-deformed Ti-stabilized austenitic stainless steel (AISI 321) in saline media was investigated. The as-received coarse-grained alloy (CG: ~37 µm) was subjected to thermomechanical processing to develop fine (FG: ~3 µm) and ultrafine (UFG: ~0.24 µm) grained structures. These samples were deformed under high (dynamic) and low (quasi-static) strain-rate conditions to a similar true strain of ~0.86. Microstructural analyses on specimens after deformation prior to corrosion study suggests a shift from the estimated stacking fault energy value of the steel. Electrochemical tests confirm the highest corrosion resistance for UFG specimens due to the formation of the most stable adsorbed passive film. This is followed by FG and CG specimens in that order. For the three grain sizes, the corrosion resistance of specimen deformed under quasi-static loading condition is higher than that subjected to dynamic impact loading while the corrosion resistance of undeformed samples is the least. This work also confirms the non-detrimental effect of TiCs in AISI 321 austenitic stainless steel on its corrosion resistance. However, TiNs were observed to be detrimental by promoting pitting corrosion due to galvanic coupling of TiNs with their surrounding continuous phase. The mechanism of pitting in AISI 321 in chloride solution is proposed.

Initial β Grain Size Effect on High-Temperature Flow Behavior of Tb8 Titanium Alloys in Single β Phase Field

The high-temperature flow behavior of TB8 titanium alloys with two different grain sizes was investigated in this present work. Results show that a significant characteristic of stress drop is visible at the start stage of the hot deformation process when the strain rates are 100 and 10−1 s−1. With the further increasing of strain, the flow stress initially rises to a maximum value and subsequently attains a plateau for the strain rates of 100 s−1 and a slight decrease for the strain rates of 10−1 s−1. Only dynamic recovery occurs under these deformation conditions. When the strain rates drop to 10−3 s−1, the dynamic recrystallization takes place during hot deformation. The values of deformation activation energy and materials constants at different strains were calculated. The processing maps at different strains were established for the fine- and coarse-grained alloys. The optimal processing parameter for hot processing was attained to be 900 °C/10−3 s−1 for fine-grained alloys and 950 °C/10−3 s−1 for coarse-grained alloys, respectively.

Deformation Behavior, Microstructure and Microhardness of Mg–3Al–1Zn Microtubes Processed by Isothermal Micro-Backward Extrusion

In the present work, for the first time, the capability of isothermal micro-backward extrusion method for fabrication of microtubes of coarse-grained as-cast AZ31 magnesium alloy was confirmed. Macrographs of the fabricated microtubes showed no visible surface cracks and acceptable dimensional accuracy. Microstructural investigations of the microtube revealed inhomogeneous microstructure consisting of a non-uniform distribution of shear bands and dynamically recrystallized grains. Microhardness measurements showed considerable enhancement compared with the as-received material due to the formation of shear bands and grain refinement inside them. Moreover, microhardness measurements revealed inhomogeneous mechanical properties of the microtube. Deformation behavior was investigated by finite element simulation using conventional material model. True stress–strains data extracted from experimental micro-compression test were used as the material parameters for finite element simulation. As a result, the regions with the highest material flow were found to be in the vicinity of the radius of the punch. Comparing this result with microstructural investigations showed that these regions had the highest density of shear bands and dynamically recrystallized grains.

High-Pressure Torsion Deformation Induced Phase Transformations and Formations: New Material Combinations and Advanced Properties

Heavy plastic shear deformation at relatively low homologous temperatures is called high-pressure torsion (HPT) deformation, which is one method of severe plastic deformation (SPD). The aim of the paper is to give an overview of a new processing approach which permits the generation of innovative metastable materials and novel nanocomposites by HPT deformation. Starting materials can be either coarse-grained multi-phase alloys, a mixture of different elemental powders or any other combination of multiphase solid starting materials. After HPT processing, the achievable microstructures are similar to the ones generated by mechanical alloying. Nevertheless, one advantage of the HPT process is that bulk samples of the different types of metastable materials and nanocomposites are obtained directly during HPT deformation. It will be shown that different material combinations can be selected and materials with tailored properties, or in other words, materials designed for specific applications and the thus required properties, can be synthesized. Areas of application for these new materials range from hydrogen storage to materials resistant to harsh radiation environments.