Ultrafine-grained Structure Research Articles

Quantitatively evaluating the huge Lüders band deformation in an ultrafine grain stainless steel by combining in situ neutron diffraction and digital image correlation analysis

In the present study, a hybrid in situ neutron diffraction and digital image correlation measurement was performed on tensile deformation of an ultrafine grain (UFG) stainless steel exhibiting a huge Lüders band deformation to evaluate the individual contribution of the austenite matrix and the deformation-induced martensite to the strain hardening during the propagation of the band. Quantitative analysis revealed that the strain hardening of the austenite matrix was insufficient to maintain a uniform deformation when the flow stress was greatly enhanced by the UFG structure. The strain hardening required for the Lüders band to propagate was mostly provided by the formation of martensite and the high internal stress within it.

Suppression of mechanical instability in bioabsorbable ultrafine-grained Zn through in-situ stabilization by ZnO nanodispersoids

The issue of intrinsic microstructural and mechanical instability of Zn-based metals limits their expansion in potential applications of bioresorbable stents and orthopedic fixators. A new concept of stabilization of Zn microstructure by a small fraction of nontoxic nanometric ZnO dispersoids is proposed for the first time and demonstrated on the particular bioabsorbable model material. The effect of the ZnO dispersoids on post-processing microstructural stability, deformation and strengthening mechanisms, corrosion, and in-vitro biological behavior are pursued. The ZnO dispersoids arise in situ within deformed Zn structure during the consolidation of fine atomized Zn 99.99wt.% powder by hydro-extrusion. ZnO nanodispersoids (4.75 vol.%; ∼136 nm) form from passivating films present on Zn. They allow formation of ultrafine-grained Zn structure with an average grain size of ∼750 nm and its retention by Zener pinning action during annealing held at 100 °C. The model Zn + ZnO composite shows the superior mechanical properties than those reported for pure Zn materials. The utilized stabilization concept doesn't compromise corrosion and biological responses. Immersion of the Zn + ZnO in DMEM results in a corrosion rate, which complies with the desirable standard rate for biodegradable materials. Electrochemical tests suggest that the Zn + ZnO reaches a similar degradation rate after the first week of immersion and a more uniform corrosion behavior compared to the cast Zn reference. In-vitro cyto/genotoxicity assays performed using DMEM diluted extracts of the Zn + ZnO and cast Zn incubated with L929 cells yield in comparable and non-toxic responses. The presence of ZnO dispersoids induces a small but still significant bacteriostatic activity.

Microstructure evolution and mechanical properties of friction stir welded joint of ultrafine-grained dissimilar AA2024/AA5083 composite fabricated by accumulative roll bonding

In this investigation, the dissimilar ultrafine-grained (UFG) composite of AA2024 and AA5083 was made using four cycles of accumulative roll bonding (ARB). The composite was then welded by the friction stir welding (FSW) method. Using the Taguchi technique, welding parameters of tool rotational speed, welding speed, pin geometry, and tool tilt angle were optimized. Optical microscopy, Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) were used to study microstructural evolution in the ARB processed specimen, as well as in the stir zone (SZ), thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). Taguchi optimization showed that a Square pin, with a rotational speed of 655 rpm, welding speed of 110 mm/min, and tool tilt angle of 3° were the optimum welding process parameters. The microstructural assessment showed that the SZ consisted of a recrystallized microstructure with an average grain size of about 900 nm. Tensile strength was reduced from 667 MPa in the ARB-processed sample to 403 MPa in the optimized FSW condition. However, yield strength was slightly reduced due to the ultrafine-grains of the weldment. The relatively high strength of the composite was due to solid solution, grain boundaries strengthening, and nano-sized Al20Cu2Mn3 precipitates (T-phase) formation. The presence of precipitates led to a proper level of ductility of up to two times in the FSW structure compared to the UFG structure. The highest mean hardness of the weldment, ∼132 HV, was obtained which was due to the grain refinement, dislocation generated by the pin movement, and small-size precipitates.

Effect of Severe Torsion Deformation on Structure and Properties of Titanium–Nickel Shape Memory Alloy

In the present work, the possibility of applying severe torsion deformation (STD) to a bulk near-equiatomic NiTi shape memory alloy in order to accumulate super-high strain and improve mechanical and functional properties was studied. STD was performed using the multidirectional test system “BÄHR MDS-830” at a temperature of 500 °C (the upper border temperature for the development of dynamic polygonization) in 14 and 30 turns with accumulated true strain values of 4.3 and 9.1, respectively. Structural phase state and properties were studied using differential scanning calorimetry, X-ray diffractometry, transmission electron microscopy, hardness measurements, and thermomechanical bending tests. STD at 500 °C allowed for the accumulation of high strain without failure. As a result of STD in 30 turns, a submicrocrystalline structure with an average grain/subgrain size of about 500 nm was formed. This structure ensured the achievement of high maximum completely recoverable strain values of 6.1–6.8%. The results obtained show the prospects of applying severe torsion straining deformation to titanium nickelide in terms of forming an ultrafine-grained structure and high properties.

Strength enhancement and uniform strain distribution through cross route-constrained groove pressing

Constrained groove pressing (CGP) is one of the severe plastic deformation techniques for sheet metals to impose large plastic strain and produce ultrafine-grained (UFG) structures without any change in dimensions. Cross route-constrained groove pressing (Cross-CGP), an advanced version of the conventional CGP process, involves two CGP cycles with an equivalent strain of 2.32 per cycle where the square sample is rotated by 90° after performing every cycle of CGP. The strain path in the sheet during conventional CGP and cross-CGP significantly affects the homogenization of mechanical properties. Sheets of low carbon steel are processed through conventional-CGP and cross-CGP routes. The ability to sustain the maximum strain has been increased in the cross-CGP process due to uniform strain distribution. The tensile strength and hardness in the sheet processed through cross-CGP are higher than that of conventional CGP, especially at higher pass numbers. Tensile behavior and hardness in two different orientations (parallel and perpendicular to the groove direction) have been found to be almost the same in the cross-CGP processed sheet.

Fabrication of equiaxed ultrafine-grained structures in TiB2/Al-Mg-Gd neutron shielding composites by powder metallurgy routines

Fabrication of equiaxed ultrafine-grained structures in TiB2/Al-Mg-Gd neutron shielding composites by powder metallurgy routines

Synergistic enhancement of strength, ductility, and toughness in a low carbon micro-alloy steel with an ultrafine-grained heterogeneous lamellar structure

Structural materials with heterostructure design have emerged as an efficient strategy to address the dilemma of balancing strength and ductility/toughness, resulting in increased interest from the scientific community. In this study, a novel ultrafine-grained heterogeneous lamellar (HL) structure has been architected in a common low-carbon micro-alloyed steel. The heterostructured steel exhibits a lamellar microstructure comprising alternating ultrafine/nano-grained lamellae and micrometer-grained lamellae, with a texture gradient along the thickness. Results show that the HL structure has a tensile strength comparable to quenched and tempered martensite (QTM) steel while retaining similar ductility as coarse-grained dual-phase (CG) steel. Furthermore, the HL specimen exhibits enhanced fracture toughness (KQ = 232.8 MPa m1/2), which is 11.3% and 28.8% higher than that in the CG and QTM specimens, respectively. Extrinsically, the macroscopical heterogeneous lamellar structure and microscopical elongated grain shape in ultrafine/nano-grained lamellae could provide substantial lamellar grain boundaries to hinder the connection of voids and make the crack propagation path more tortuous, which effectively increases the energy release rate to contribute to the exceptional fracture toughness. Intrinsically, not only micrometer-sized CG lamellae with high plastic deformation tolerance capacity can develop coarse (deep) dimples for energy consumption, but also the elongated ultrafine martensite grains can undergo significant plastic deformation together with the coordinated deformation of coarse-grained ferrite. We hope the strategy in this study may provide insights for microstructure optimization and synergistic strength-plasticity/toughness enhancement of conventional low-carbon micro-alloyed steels for structural applications.

Re-examination and comparison of activation volume, strain rate sensitivity, and dislocation density for coarse grain and CGP-processed UFG materials

The frequently used metallurgical parameters such as activation volume, internal stress, strain rate sensitivity, and dislocation density are identified in ultrafine-grained (UFG) materials through erudite analytical relations. It is difficult to analyze such parameters optically since the material consists of highly distorted and inhomogeneous microstructure due to the large strain imposed during the constrained groove pressing (CGP) process. Hence, transient tests like stress relaxation tests are advantageous over the optical to use. Apart from the identification of parameters in low-carbon steel materials using uniaxial loading and stress relaxation tests, the relationship between the parameters is nobly investigated in as-received and UFG structure samples produced through the CGP process. In the relationship, the exponential-power law constitutive to describe the accurate stress–strain curves, identification of mobile and forest dislocation density during strain hardening, and exponential variation between grain size and activation volume are proposed. The replenishment or exhaustion of mobile and forest dislocation density significantly affects the metallurgical parameters during uniaxial tensile loading followed by stress relaxation testing. In this experimental and analytical work, the CGP process produces a UFG material that has lower activation volume and higher stress drop, mobile dislocation density, forest dislocations density, strain rate sensitivity, and internal stress.

The Effect of Radial-Shear Rolling Deformation Processing on the Structure and Properties of Zr-2.5Nb Alloy.

The rheological properties of the Zr-2.5Nb alloy by the strain rate range of 0.5-15 s-1 and by the temperature range of 20-770 °C was studied. The dilatometric method for phase states temperature ranges was experimentally determined. A material properties database for computer FEM simulation regards the indicated temperature-velocity ranges were created. Using this database and DEFORM-3D FEM-softpack, the radial shear rolling complex process numerical simulation was carried out. The contributed conditions for the ultrafine-grained state alloy structure refinement were determined. Based on the simulation results, a full-scale experiment of Zr-2.5Nb rod rolling a on a radial-shear rolling mill RSP-14/40 was carried out. It takes in seven passes from a diameter of 37-20 mm with a total diameter reduction ε = 85%. According to this case simulation data, the total equivalent strain in the most processed peripheral zone 27.5 mm/mm was reached. Due to the complex vortex metal flow, the equivalent strain over the section distribution was uneven with a gradient reducing towards the axial zone. This fact should have a deep effect on the structure change. Changes and structure gradient by sample section EBSD mapping with 2 mm resolution were studied. The microhardness section gradient by the HV 0.5 method was also studied. The axial and central zones of the sample by the TEM method were studied. The rod section structure has an expressed gradient from the formed equiaxed ultrafine-grained (UFG) structure on a few outer millimeters of the peripheral section to the elongated rolling texture in the center of the bar. The work shows the possibility of processing with the gradient structure obtaining and enhanced properties for the Zr-2.5Nb alloy, and a database for this alloy FEM numerical simulations are also presents.

Annealing hardening effect aroused by solute segregation in gradient ultrafine-grained Mg-Gd-Zr alloy

In this study, a remarkable annealing hardening effect was detected in gradient ultrafine-grained (UFG) Mg-0.32Gd-0.11Zr (at.%) alloy sheet fabricated by sliding friction treatment (SFT). Under the precipitation-free condition, the annealed UFG structure shows an obvious hardness increment from 1.40 GPa to 1.89 GPa after 200°C heating for 12 h, which exhibits a much higher hardening response than the annealed coarse-grained (CG) structure. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and elemental mapping reveal prominent segregation of solute Gd atoms along grain boundaries, which endows the UFG structure with excellent grain boundary stability. Moreover, Gd segregation is also found around the extrinsic stacking fault (E-SFs) and the low-angle grain boundaries composed of edge dislocations. The large-scale solute partitioning provides a significant segregation hardening effect, which completely resists the softening effect aroused by the grain coarsening and dislocation annihilation. This work realizes a good combination of surface mechanical processing for fabricating UFGs and subsequent heat treatment, which earns desirable segregation hardening effects.

The Effect of Casting Technique and Severe Straining on the Microstructure, Electrical Conductivity, Mechanical Properties and Thermal Stability of the Al-1.7 wt.% Fe Alloy.

This paper features the changes in microstructure and properties of an Al-Fe alloy produced by casting with different solidification rates followed by severe plastic deformation and rolling. Particularly, different states of the as-cast Al-1.7 wt.% Fe alloy, obtained by conventional casting into a graphite mold (CC) and continuous casting into an electromagnetic mold (EMC), as well as after equal-channel angular pressing and subsequent cold rolling, were studied. Due to crystallization during casting into a graphite mold, particles of the Al6Fe phase are predominantly formed in the cast alloy, while casting into an electromagnetic mold leads to the formation of a mixture of particles, predominantly of the Al2Fe phase. The implementation of the two-stage processing by equal-channel angular pressing and cold rolling through the subsequent development of the ultrafine-grained structures ensured the achievement of the tensile strength and electrical conductivity of 257 MPa and 53.3% IACS in the CC alloy and 298 MPa and 51.3% IACS in the EMC alloy, respectively. Additional cold rolling led to a further reduction in grain size and refinement of particles in the second phase, making it possible to maintain a high level of strength after annealing at 230 °C for 1 h. The combination of high mechanical strength, electrical conductivity, and thermal stability can make these Al-Fe alloys a promising conductor material in addition to the commercial Al-Mg-Si and Al-Zr systems, depending on the evaluation of engineering cost and efficiency in industrial production.

Ultrafine-grained Al–La–Mg–Mn alloy with superior thermal stability and strength-ductility synergy

Ultrafine-grained (UFG) structures are highly desirable in microstructural design for their substantial improvements in strength and ductility; however, they often face challenges such as thermal instability and high manufacturing costs. In this work, powder hot-extrusion was employed as a cost-effective method to fabricate an Al–La–Mg–Mn alloy with a UFG structure (∼0.8 μm). The as-fabricated alloy exhibited a high-volume fraction of bimodal sub-micron intermetallics, with coarse intermetallics (∼270 nm) distributed primarily along the grain boundaries and fine ones (＜80 nm) in the grain interior. This unique structure cooperatively retarded the grain growth, resulting in a UFG structure with excellent thermal stability. The grain size grew slightly to 1.08 μm after thermal exposure at 400 °C for 100 h. Moreover, the UFG structure exhibited an excellent combination of strength and ductility, with yield strength, ultimate tensile strength, and elongation of 365 MPa, 543 MPa, and 13%, respectively. The UFG structure, decorated with bimodally distributed intermetallics, presents a promising microstructural design for UFG-structured alloys with excellent thermal stability and mechanical properties.

Corrosion Behavior and Bio-Functions of the Ultrafine-Grained Ti6Al4V-5Cu Alloy with a Dual-Phase Honeycomb Shell Structure in Simulated Body Fluid.

Titanium alloys are widely used in biomedical applications. However, cases of implant failure due to fatigue fracture and bacterial infection are common. In addition, implants are susceptible to metal ions (Al, V) released by long-term exposure to human body fluids, which causes neuropathy, mental disorders, and other diseases. Thus, development of novel materials to achieve long-term safety of implants is currently a research hotspot. Recently, our research group has developed an ultrafine-grained Ti6Al4V-5Cu alloy with a unique "dual-phase honeycomb shell" (DPHS) structure, which possesses high fatigue strength and stability. This study further affirmed its higher corrosion behavior, antibacterial properties, and cytocompatibility compared to the coarse-grained Ti6Al4V and Ti6Al4V-5Cu alloys. The ultrafine-grained structure of Ti6Al4V-5Cu having DPHS increased the proportion of phases (Cu-rich phases, β-phase, and Ti2Cu intermetallic phase) with a lower surface potential. It was observed that the developed microstructure was conducive to a stable configuration of the oxide (passive) layer on the alloy surface. In addition, the low-phase interfacial energies of the ultrafine-grained structure with DPHS even facilitated the improvement of the denseness of the protective passive film and eventually enhanced the corrosion behavior. Besides, the fine-Cu-rich phases and the micro-galvanic couples formed between them and the matrix significantly increased the contact frequency of bacteria, thus increasing the contact sterilization efficiency of the ultrafine-grained Ti6Al4V-5Cu alloy. These results showed that the new ultrafine-grained Ti6Al4V-5Cu alloy has excellent corrosion resistance and biological functions for clinical application.

Fracture Kinetics and Mechanisms of Ultrafine-Grained Materials during Fatigue Tests in the Low-Cycle Fatigue Region

In this paper, the fracture kinetics and mechanisms in the low-cycle fatigue region were analyzed for different ultrafine-grained (UFG) materials with body-centered cubic (bcc), hexagonal close-packed (hcp) and face-centered cubic (fcc) lattices. Three-point bending principle fatigue tests were performed. The tests show that the UFG structure formation in the investigated materials has an ambiguous effect on the total number of cycles to failure (life) of the samples. The number of cycles to fatigue crack initiation (Nin) is about 20% of the total life of the samples, irrespective of the material state and the crystal lattice type. At the same value of the stress intensity coefficient range (∆K), for the majority of the investigated UFG materials, the fatigue crack propagation rate (dl/dN) is close to or lower than that of the initial coarse-grained (CG) materials. For the UFG materials, the coefficient n in the Paris equation is, in most cases, lower than that for the CG materials, which indicates that the UFG materials are less sensitive to cyclic overload. The fatigue fracture mechanisms of the investigated CG and UFG materials are rather similar, although the fracture of the UFG materials is accompanied by the formation of many secondary cracks, irrespective of the crystal lattice type.

Enabling manufacturing of multi-axial forging-induced ultrafine-grained strong and ductile magnesium alloys: a perspective of process-structure-property paradigm

ABSTRACT Magnesium (Mg) alloys are difficult to cold/warm-process due to their hexagonal close-packed (HCP) lattice, which has restricted slip systems and makes plastic deformation at low temperatures a challenging task. Multi-axial forging (MAF) and annealing of as-cast alloys were combined to create a texture-free ultrafine-grained (UFG) structure with a high strength-high ductility combination to address this challenge. The study showed a clear and fundamental change from basal and pyramidal dislocation slip to twinning in the strong and ductile UFG Mg alloy compared to the low strength coarse-grained (CG) Mg alloy counterpart. This implied that the dislocation activity, grain orientation, and particular grain boundary states played an important role in controlling the deformation mechanisms. The potential impact of processing Mg alloys by MAF opens-up a new frontier of strong and ductile low-density materials for light and efficient solutions including energy absorption and formability and provides a perspective in terms of process-structure-property relationship.

Fatigue Behavior and Fracture Features of Ti-15Mo Alloy in β-, (α + β)-, and Ultrafine-Grained Two-Phase States

The influence of the ultrafine-grained structure formed by equal-channel angular pressing via the “Conform” scheme on the fatigue behavior of metastable β-alloy Ti-15Mo has been studied. It is shown that the alloy with a two-phase ultrafine-grained structure achieved the best mechanical properties and enhanced fatigue endurance limit (up to 710 MPa on the basis of 107 cycles) due to the total contribution of grain boundary, dislocation, and phase strengthening mechanisms. A fractographic analysis of the fracture surface of samples after fatigue tests showed the features of fatigue crack propagation depending on the type of alloy microstructure. The general and distinctive features of fatigue failure of alloy samples in the initial coarse-grained (α + β)-, single-phase coarse-grained β-, and ultrafine-grained (α + β)-states are revealed. In all of the samples, a fatigue crack nucleated on the surface and propagated downward, i.e., perpendicular to the direction of the applied pressures. It is shown that fracture surfaces of the ultrafine-grained samples had a high roughness and were characterized by the presence of a large number of secondary cracks, as compared to the coarse-grained analogues.

FINITE ELEMENT ANALYSIS OF TWIST CHANNEL ANGULAR PRESSING

The article deals with the analysis of the unconventional pressing process of metallic materials by twist channel angular pressing (TCAP) method. TCAP process is a modification of the equal channel angular pressing (ECAP) forming method, which can be used to prepare bulk materials with an ultra fine-grained structure. Simulation of the TCAP process is an important source of information on the effect of design and technological parameters on the grain refinement efficiency. The TCAP process was simulated using 3D finite element method (FEM).

Effect of grain size on superplastic deformation behavior of Zn-0.033 Mg alloy

Zn alloys reveal excellent biological degradability and compatibility, suitable mechanical properties, and unique anti-atherosclerotic properties, which make them suited for miniaturized medical implants. Superplastic forming is a promising method to fabricate high-performance microparts with difficult-to-deform materials. However, the effect of grain size on superplasticity in Zn alloys and the underlying mechanisms still need to be determined. In this research, Zn-0.033 Mg (in wt.%) alloy with different grain sizes from 1.53 to 36.17 μm was obtained by equal-channel angular pressing and heat treatment, and its mechanical performance was evaluated by uniaxial tensile tests in the temperature range of 453–533 K and the strain rate range of 0.001–0.01 s−1. Experimental results indicated that the flow stress is significantly reduced with increasing deformation temperature, and the elongation is effectively improved with the reduction in strain rate. Furthermore, the Zn-0.033 Mg alloy with a grain size of 1.53 μm exhibited a superplastic deformation with an elongation of 376 ± 79% at 453 K and 0.001 s−1. Two distinguishing deformation stages were observed during the superplastic deformation. In the early stage, the ∼85 ± 5°/ <1‾1‾20> twins significantly emerged, leading to subsequent dynamic recrystallization by high-angle grain boundary migration. In the steady-state stage, the non-basal dislocation slip was significantly activated via changing the grain orientation from hard to soft orientation with the aid of continuous dynamic recrystallization. The fracture mechanisms were further discussed based on the fracture surface analysis, indicating that the ultrafine-grained structures can accommodate the nucleation and growth of cavities by grain boundary sliding, enabling low-temperature superplasticity. These findings can grant a new insight and understanding of the superplastic mechanism of Zn alloys and shed light on the manufacture of high-performance Zn alloys for medical implants.

Low-temperature superplastic deformation of EK61 and EP975 wrought nickel-based superalloys with an ultrafine-grained structure

It is shown that thermomechanical treatment (TMT) of the EK61 superalloy with a strain rate (ε∙) of 10−3 s−1 and a gradual decrease in the treatment temperature from (0.92 – 0.65)Tδ leads to the transformation of the initial coarse-grained structure into an ultrafine-grained (UFG) mixed type structure. Such a mixed UFG microstructure consists of γ-phase grains and δ-phase particles 0.3 μm in size and relatively large particles of the δ-phase up to 2 μm in size. The EK61 superalloy having such a microstructure exhibits low-temperature superplasticity in the temperature range of 700 – 900°C, the maximum characteristics of which are achieved at 850°C and ε∙ =10−3 s−1. Low-temperature TMT in the temperature range of (0.84 – 0.8)Тγ' also leads to the formation of an UFG structure in the EP975 superalloy. The UFG microstructure of the EP975 superalloy consists of γ-phase grains and γ'-phase particles 0.5 μm in size. And there are relatively large particles of the γ'-phase up to 3.5 μm in size of a globular-shaped form. The EP975 superalloy with such a structure exhibits low-temperature superplasticity in the temperature range of 900 –1000°C, the maximum characteristics of which are achieved at 1000°C and ε∙ =10−3 s−1.

Enhanced Fatigue Limit in Ultrafine-Grained Ferritic-Martensitic Steel.

The influence of the ultrafine-grained (UFG) structure on the fatigue endurance limit and the nature of fatigue failure have been studied. It is shown that the formation of the UFG structure containing carbides and the coincidence site lattice relationship (CSL) and twin boundaries leads to an increase in the fatigue endurance limit. To study the mechanisms of fatigue failure, scanning and transmission electron microscopy and X-ray diffraction analysis were used. Studies have shown that the formation of the UFG structure as a result of rolling and subsequent heat treatment above the temperature of the ferrite/austenite phase transition leads to an increase in the fatigue endurance limit by more than 70%, from 475 to 800 MPa, compared to coarse-grained samples. The dynamic aging observed during fatigue tests was more pronounced in materials with a UFG microstructure. The influence of the CSL and twin boundaries on the nature of the fatigue failure of ferritic-martensitic steel is discussed.