A low-cycle fatigue life prediction model of ultrafine-grained metals

A (high strain) low‐cycle fatigue (LCF) life prediction model of ultrafine‐grained (UFG) metals has been proposed. The microstructure of a UFG metal is treated as a two‐phase ‘composite’ consisting of the ‘soft’ matrix (all the grain interiors) and the ‘hard’ reinforcement (all the grain boundaries). The dislocation strengthening of the grain interiors is considered as the major strengthening mechanism in the case of UFG metals. The proposed model is based upon the assumption that there is a fatigue‐damaged zone ahead of the crack tip within which the actual degradation of the UFG metal takes place. In high‐strain LCF conditions, the fatigue‐damaged zone is described as the region in which the local cyclic stress level approaches the ultimate tensile strength of the UFG metal, with the plastic strain localization caused by a dislocation sliding‐off process within it. The fatigue crack growth rate is directly correlated to the range of the crack‐tip opening displacement. The empirical Coffin–Manson and Basquin relationships are derived theoretically and compared with experimental fatigue data obtained on UFG copper (99.99%) at room temperature under both strain and stress control. Good agreement is found between the model and the experimental data. It is remarkable that, although the model is essentially formulated for high strains (LCF), it is also found to be applicable at low strains in the high‐cycle fatigue (HCF) regime.     

超细晶纯钛疲劳裂纹的扩展行为

对纯钛进行2道次室温等径弯曲通道变形(ECAP)、等径弯曲通道变形加旋锻复合变形(ECAP+RS)并在旋锻后在300℃和400℃退火1 h,制备出4种具有不同组织的超细晶纯钛。对这4种超细晶纯钛进行疲劳裂纹扩展实验并观察分析超细晶纯钛的显微组织和疲劳断口的形貌,研究了裂纹的扩展行为。结果表明：显微组织对超细晶纯钛的疲劳裂纹扩展门槛值和近门槛区有显著的影响;超细晶纯钛的疲劳裂纹扩展门槛值随着塑性变形量的增大而增大,随着旋锻后退火温度的提高而降低;疲劳裂纹扩展速率曲线因超细晶纯钛晶粒尺寸和强度的影响出现转折,转折前ECAP+RS复合变形纯钛的抗疲劳裂纹扩展能力比ECAP变形强,且随着退火温度的提高而降低;转折后4种超细晶纯钛的疲劳裂纹扩展速率相差较小,呈现出相反的结果。疲劳裂纹扩展寿命中转折前近门槛区裂纹扩展寿命占绝大部分,因而转折前的门槛值与近门槛区的扩展速率对抗裂纹扩展能力更为重要。     

Cyclic deformation behavior and fatigue lives of ultrafine-grained Ti-6AL-4V ELI alloy for medical use

It was shown that introducing an ultrafine-grained (UFG) microstructure in pure metals as well as some alloys leads to strongly enhanced fatigue properties. The cyclic deformation behavior of UFG Ti-6Al-4V ELI (extra low interstitials) alloy is studied by both strain and stress controlled fatigue tests using plastic strain amplitudes between 3 × 10^?4 and 5 × 10^?3 and stress amplitudes ranging from 550 to 670 MPa. The UFG microstructures were obtained by equal channel angular pressing (ECAP) with different number of passes followed by a subsequent thermomechanical treatment (TMT). When compared to the conventional grain (CG) size counterpart, the UFG alloy exhibited a pronounced enhancement in the fatigue life in the S–N (Wöhler) diagram. It was also shown that additional UFG processing prior to TMT did not result in any further improvement of the fatigue resistance. Furthermore, microstructural investigations revealed a high cyclic stability of the UFG microstructure.     

Enhanced fatigue properties of ultrafine-grained Ti rods processed by ECAP-Conform

This study is focused on enhancement of the fatigue properties of Ultrafine Grain titanium (UFG Ti) processed by ECAP-Conform and subsequent drawing. By examining specific combinations of ECAP-Conform and drawing, we have shown that the size and shape of grains, dislocation substructure formation, and grain boundary state are among the main microstructural parameters that determine the strength and ductility in UFG Ti. The increase in fatigue resistance of UFG Ti to 610 MPa was attributed to the high values of strength and ductility (UTS = 1290 MPa with elongation = 13 %) This value is comparable with the fatigue resistance of the titanium alloy Ti–6Al–4V.     

Deposition of FeCoNiCrMn high entropy alloy (HEA) coating via cold spraying

High entropy alloys (HEAs) are of great interest in the community of materials science and engineering due to their unique phase structure. They are constructed with five or more principal alloying elements in equimolar or near-equimolar ratio. Therefore, HEAs can derive their performance from multiple principal elements rather than a single element. In this work, solid-state cold spraying (CS) was applied for the first time to produce FeCoNiCrMn HEA coating. The experimental results confirm that CS can be used to produce a thick HEA coating with low porosity. As a low-temperature deposition process, CS completely retained the HEA phase structure in the coating without any phase transformation. The characterization also reveals that the grains in the CSed HEA coating had experienced significant refinement as compared to those in the as-received HEA powder due the occurrence of dynamic recrystallization at the highly deformed interparticle region. Due to the increased dislocation density and grain boundaries, CSed HEA coating was much harder than the as-received powder. The tribological study shows that the CSed FeCoNiCrMn HEA coating resulted in lower wear rate than laser cladded HEA coatings.     

A comprehensive evaluation of parameters governing the cyclic stability of ultrafine-grained FCC alloys

The current paper presents results of a thorough experimental program undertaken to shed light onto the mechanisms dictating the cyclic stability in ultrafine-grained (UFG) alloys with a face-centered cubic structure. Cyclic deformation responses of several copper- and aluminum-based UFG alloys were investigated and the corresponding microstructural evolutions were analyzed with various microscopy techniques. The important finding is that a larger volume fraction of high-angle grain boundaries and solid solution hardening significantly improve the fatigue performance of these alloys at elevated temperatures and high strain rates, and under large applied strain amplitudes.     

Microstructure and mechanical properties of Cu and Cu-Zn alloys produced by equal channel angular pressing

Ultrafine-grained (UFG) Cu and Cu-Zn alloy were prepared using equal-channel angular pressing (ECAP) to investigate the effects of stacking fault energy (SFE) on microstructure evolution and mechanical properties. Combining with the previous researches, the grain refinement process of ECAP is divided into three stages based on the variation of tensile strength and plasticity. According to the influences of defects on strength and ductility during plastic deformation, the three stages are discussed in detail by considering the dislocation density, grain and twin boundaries. Besides, the impact of SFE on the strength and ductility of the UFG Cu-Zn alloys are evaluated, indicating that these two mechanical properties can be improved simultaneously in the whole ECAP process either through slightly or widely adjusting the SFE. This significant effect of SFE reflects in two aspects, one is in the microstructure evolution during ECAP processing and the other is in the subsequent tensile plastic deformation, both of which can be achieved through regulating the dislocation motion via changing the SFE.     

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.     

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.     

Three-dimensional numerical analysis and experimental investigation of grain refinement in multi-pass equal channel angular pressing for round-workpieces

Equal channel angular pressing (ECAP) has the capability of producing ultra fine-grained (UFG) materials bellow the dimension of 1 μm. At present, it is one of the most important methods to get bulk UFG materials. Multi-pass ECAP processes for round workpieces are investigated by using numerical simulations and experimental studies in this paper. The deformation mechanism of ECAP for grain refinement is obtained. Three processing routes A, B and C are simulated in order to study the influence of the processing routes to the deformation uniformity of the workpiece. The finite element (FE) analysis results of the multi-pass ECAP process show that the different processing routes result in the different deformation distributions. The grain in the workpiece is refined obviously after multi-pass pressing. The microstructures of the processed material are more different than that of the microstructure of the annealing initial equiaxed grains. The microstructure evolution of the workpiece can be changed via different processing routes. It is found that route B can get a high angle grain boundaries distribution in the workpiece than other routes. The results of the analysis show that the process of grain refinement can be described as a continuous dynamic recovery and recrystallization. The microstructure evolutions of the grain refinement mechanisms and micro-structural characteristics for different multi-pass ECAP processing routes are verified by using OM (optical model) and TEM (transmission electron microscope) analysis. In addition, the experimental microstructure results are also consistent with FE analysis results.     

Advancing nanograined/ultrafine-grained structures for metal implant technology: Interplay between grooving of nano/ultrafine grains and cellular response

Nanograined/ultrafine-grained (NG/UFG) metals provide surfaces that are different from conventional coarse-grained polycrystalline metals because of the high fraction of grain boundaries. In the context of osseointegration of metal implants, grooving of nanograins/ultrafine grains by electrochemical grooving is a potential approach to increase the biomechanical interlocking and anchorage with consequent enhancement of cellular response. The primary objective of the research described here is to advance science and technology of metal implants by making a relative comparison of osteoblast response of grain boundary grooved and planar NG/UFG surfaces. The NG/UFG substrates were obtained using an ingenious concept of controlled phase reversion and the grain boundaries were electrochemically treated to induce grooving of large fraction of grain boundaries of NG/UFG substrate. Experiments on the effect of grooving of grain boundaries of NG/UFG metal indicated that cell attachment, proliferation, viability, morphology, and spread are favorably modulated and significantly different from planar (non-grooved) NG/UFG substrates. Furthermore, immunofluorescence studies demonstrated stronger vinculin signals associated with actin stress fibers in the outer regions of the cells and cellular extensions on electrochemically grooved NG/UFG substrate. These observations are indicative of accelerated response of cell–substrate interaction and activity. The differences in the cellular response of planar and grain boundary grooved NG/UFG surface are attributed to favorable surface topography that accelerates the cellular activity.     

Microstructure and mechanical behavior of friction stir processed ultrafine grained Al-Mg-Sc alloy

Twin-roll cast (TRC) Al-Mg-Sc alloy was friction stir processed (FSP) to obtain ultrafine grained (UFG) microstructure. Average grain size of TRC alloy in as-received (AR) condition was 19.0 ± 27.2 μm. The grain size reduced to 0.73 ± 0.44 μm after FSP. About 80% of the grains were smaller than 1 μm in FSP condition. FSP resulted into 80% of the grain boundaries to have high angle grain boundary (HAGBs) character. Uniaxial tensile testing of UFG alloy showed an increase in yield strength (YS) and ultimate tensile strength (UTS) (by ∼100 MPa each) of the alloy with a very marginal decrease in total and uniform elongation (total - 27% in AR and 24% in UFG and uniform - 19% in AR and 14% in UFG). A theoretical model predicted that the grain refinement cannot take place via discontinuous dynamic recrystallization. Zener pinning model correctly predicted the grain size distribution for UFG alloy. From work hardening behaviors in both the conditions, it was concluded that grain boundary spacing is more important than the character of grain boundaries for influencing extent of uniform deformation of an alloy.     

Simultaneous enhancement in strength and ductility of Fe_(50)Mn_(30)Co_(10)Cr_(10) high-entropy alloy via nitrogen alloying

The effect of nitrogen on microstructural evolution and tensile properties of transformation-induced plasticity(TRIP)Fe₅₀Mn₃₀Co₁₀Cr₁₀HEAs was investigated.Nitrogen was fully introduced in solid solution by pressure-induced melting technique.Nitrogen addition turned the TRIP alloy to a twinning-induced plasticity(TWIP)alloy,and simultaneously improved the strength and elongation.For the nitrogen-doped HEA,the high yield strength is mainly resulted from the friction stress via interstitial strengthening effect,and the high ductility is originated from retained high strain-hardening capability via the successive onset of dislocation accumulation and deformation twinning.The strain-hardening behavior and microstructural evolution at specified strains were revealed.     

Effect of ultragrain refinement on quenching and partitioning steels manufactured by a novel method

In this study, ultrafine grained (UFG) quenching and partitioning steels was achieved by using tempered and deformed martensite as the pre-microstructure of the quenching and partitioning treatment. Compared with those manufactured through the conventional routine, superior mechanical properties were realized in UFG steels by using tempered and deformed martensite as the pre-microstructure of the quenching and partitioning treatment. The grain subdivision mechanism during deformation and the microstructure evolution during heating were investigated. Effect of carbide on the grain subdivision and its pinning effect against grain growth is highlighted. The proposed method produced UFG steels with considerable amount of retained austenite, which contributed to the enhanced mechanical properties of investigated steels.     

Surface hardening of biocompatible ultrafine-grained niobium zirconium alloy by two-stage oxidation treatment

The present study reports on an optimized surface hardening process for biocompatible ultrafine-grained (UFG) niobium 2.3 wt% zirconium (NbZr) alloy, a promising candidate implant material. The as-received material of conventional grain size (CG) was processed using multipass equal channel angular processing at room temperature to obtain an UFG microstructure featuring high strength and ductility. Subsequent surface hardening was performed by a heat treatment leading to internal oxidation. Using a thermogravimetric system, the influence of temperatures, time, and partial pressure of oxygen ( $ p_{{\text{O}}_2} $ ) on the oxidation kinetics were investigated. Metallographic and microscopic analyses and hardness measurements were employed to evaluate maximum hardness, penetration-depth and scale formation under various conditions. Heat treatment at 620 °C for 6 h at a $ p_{{\text{O}}_2} $ of 0.2 hPa led to sufficiently rapid oxidation kinetics yielding a relatively high depth of penetration without formation of loose Nb₂O₅ on the surface, which was observed at higher $ p_{{\text{O}}_2} $ . As compared to CG material, improved hardness profiles were reached using the same heat treatment parameters, since the UFG structure significantly changes diffusion conditions and therefore oxidation kinetics. After a second heat treatment in high vacuum the high maximum hardness of 820 HV0.01 in the UFG material was reduced effectively and a less steep hardness gradient was achieved, both contributing to a less brittle behavior under mechanical loading. High-cycle fatigue tests performed on surface-hardened UFG NbZr samples showed a substantial improvement of fatigue life in tests conducted near the endurance limit. Especially when high fatigue and wear resistance are key issues for a given application, the internal oxidation process offers an effective way to further improve the properties of UFG NbZr.     

Plastic deformation behavior of ultrafine-grained Al–Mg–Sc alloy

Ultrafine-grained (UFG) Al–Mg–Sc alloy was obtained by friction stir processing. The UFG alloy was subjected to uniaxial tensile testing to study the tensile deformation behavior of the alloy. An inhomogeneous yielding (Lüdering phenomenon) was observed in the stress–strain curves of UFG alloy. This deformation behavior was absent in the coarse-grained alloy. The Lüdering phenomenon in UFG alloy was attributed to the lack of dislocations in UFG microstructure. A strong dependence of uniform ductility on the average grain size was exhibited by the UFG alloy. Below a critical grain size (0.5 μm), ductility was very limited. Also, with the decrease in grain size, most of the plastic deformation was observed to be localized in necked region of the tensile samples. The negative strain rate sensitivity (SRS) observed for the UFG alloy was opposite of the SRS values reported for UFG alloys in the literature. Based on activation volume measurement, grain boundary mediated dislocation-based plasticity was concluded to be the micro-mechanism operative during plastic deformation of UFG Al–Mg–Sc alloy.     

Fatigue crack growth resistance of ECAPed ultrafine-grained copper

This work presents the experimental results of fatigue crack growth resistance of ultrafine-grained (UFG) copper. The UFG copper has a commercial purity level (99.90%) and an average grain size of 300 nm obtained by a 8-passes route Bc ECAP process. The fatigue propagation tests are conducted in air, at load ratios R = K_min/K_max varying from 0.1 to 0.7, on small Disk Shaped CT specimens. Both stage I and stage II regime of growth rate are explored. Results are partially in contrast with the few experimental data available in the technical literature, that are by the way about high purity UFG copper. In fact, the present material shows a relatively high fatigue crack resistance with respect to the unprocessed coarse-grained alloy, especially at high values of applied stress intensity factor ΔK. At higher R-ratio a smaller threshold intensity factor is found, together with a lower stage II fatigue crack growth rate. The explanation of such crack growth retardation is based on a diffuse branching mechanism observed especially at higher average ΔK.     

Corrosion behavior of Al0.4CoCu0.6NiSi0.2Ti0.25 high-entropy alloy coating via 3D printing laser cladding in a sulphur environment

High-entropy alloys(HEAs) are of great interest in materials science and engineering communities owing to their unique phase structure.HEAs are constructed with five or more principal alloying elements in equimolar or near-equimolar ratios.Therefore,they can derive their performance from multiple principal elements ratherthan a single element.In this work,three-dimensional printing laser cladding was applied to produce an Al_0.4CoCu_0.6NiSi_0.2Ti_0.25 HEA coating.The experimental results confirmed that the laser cladding could be used to produce a thin coating of 120 μm in thickness.In the high-temperature laser cladding process,some Fe elements diffused from the substrate to the coating,forming a combination of face-centred cubic and body-centred cubic phase structures.The HEA coating metallurgically bonded well with the substrate.Owing to the increased dislocation density and number of grain boundaries,the HEA coating was harder and had a stronger hydrophobicity than X70 steel.The electrochemistry results showed that the HEA coating had better corrosion resistance than X70 steel.Aluminium oxides formed on the surface of the HEA coating had a certain protective effect.However,because of the laser cladding,the HEA coating generated cracks.In future work,the laser cladding technology will be improved and heat treatment will be implemented to prevent formation of cracks.     

Solid-state cold spraying of FeCoCrNiMn high-entropy alloy:an insight into microstructure evolution and oxidation behavior at 700-900℃

About 3 mm thick five-element equimolar high-entropy alloy(HEA) FeCoCrNiMn was successfully deposited by solid-state cold spraying(CS).The high-temperature oxidation behavior of the CSed HEA was investigated at 700-900℃.Heat treatment was performed on the CSed HEA before oxidation to heal the incomplete interfaces between the deposited particles.Results show that the microstructure of the CSed HEA is characterized by grain refinement and abundant interparticle incomplete interfaces.Post-spray heat treatment promotes recrystallization and grain growth in the CSed HEA.After oxidation testing,the oxide scales are composed of multi-layers:a Mn₂O₃(or Mn₃O₄) outer layer,a Mn-Cr spinel intermediate layer and a Cr₂O₃ inner layer.The CSed HEA exhibits higher parabolic rate constants and more favorable internal oxidation than the bulk HEAs that have similar compositions in the literature.Such a discrepancy becomes pronounced at higher temperatures.The grain refinement and numerous particle boundaries are responsible for such a distinctive performance of the CSed HEA.     

Thermal stability of ultrafine grained aluminium alloy prepared by large strain extrusion machining

Abstract

In the present study, ultrafine grained (UFG) Al alloy chips with average grain sizes of ～200 nm were successfully prepared by large strain extrusion machining (LSEM) process using a combined cutting tool with rake angle of 10° and chip compression ratio of 1.0. The tests showed that the Vickers hardness of the UFG Al alloy is significantly improved due to grain size reduction. To understand effect of heat on the microstructure and mechanical properties, the UFG chips were subjected to heat treatment at different temperatures and different annealing time durations. When annealed <100°C, most of fine grains within the UFG chips were found to be replaced by elongated grains whose grain sizes increased with a significant increase in the aspect ratio as the annealing time increased. Despite such increase in grain size, the Vickers hardness was not reduced as expected because of the precipitation of secondary phases. When annealed at temperatures up to 200°C, recrystallisation occurred, along with grain growth, but the Vickers hardness did not deteriorate because of precipitation of secondary phases, as before. However, annealing at temperatures of 300°C and above resulted in significant reduction in hardness of the chips due to dominance of grain growth over secondary precipitation. These results indicated that UFG Al alloy chips have a good thermal stability at temperatures <200°C.