Impact Toughness of Ultrafine‐Grained Commercially Pure Titanium for Medical Application

This study aims at achieving the best combination of strength, ductility, and impact toughness in ultrafine‐grained (UFG) Ti Grade 4 produced by equal‐channel angular pressing via Conform scheme (ECAP‐C) with subsequent cold drawing. UFG structures with various parameters (e.g., size and shape of grains, dislocation density, conditions of boundaries) are formed by varying the treatment procedures (deformation temperature and speed at drawing, annealing temperature). The tensile and impact toughness tests were performed on samples with a V‐shaped notch and different structures of commercially pure Ti Grade 4 in the coarse‐grained and UFG states. The results demonstrated that grain refinement, higher dislocation density, and their elongated shape were obtained as a result of drawing at 200 °С, which led to a decrease in both the uniform elongation at tension and the impact toughness of Ti Grade 4. Short‐term annealing at 400–450 °C could improve the impact toughness of UFG Ti with a non‐significant decrease in strength. This short‐term annealing contributes to the dislocation density decrease without considerable grain growth as a result of the recovery and redistribution of dislocations. The dependence of impact toughness on the strain hardening ability of UFG Ti was discussed.

     

Ultra-fine grained bulk CP-Ti processed by multi-pass ECAP at warm deformation region

In order to refine the grain size of commercially pure titanium (CP-Ti) to a submicrometer scale, equal channel angular pressing (ECAP) was attempted at a temperature range of 200–300 °C. The experiments revealed that, 250 °C was the minimum temperature at which ten passes of ECAP could be performed in a 105° die without the cracking of billets. An ultrafine-grained (UFG) microstructure with a mean grain size of 183 nm was achieved after 10 passes. The processed CP-Ti displayed high tensile strength of 892 MPa and high elongation to failure of 20.5%. The enhancement in mechanical properties is explained in terms of grain refinement and dislocation density increasing. The high ductility of UFG pure Ti with the absence of strain hardening behavior is attributed to its enhanced strain rate sensitivity.     

Microstructures and properties of ultrafine-grained pure titanium processed by equal-channel angular pressing and cold deformation

Equal-channel angular pressing (ECAP) has been used to refine the grain size of commercially pure (CP) titanium as well as other metals and alloys. CP-Ti is usually processed at about 400 degrees C because it lacks sufficient ductility at lower temperature. The warm processing temperature limits the ability of the ECAP technique to improve the strength of CP-Ti. We have employed cold deformation following warm ECAP to further improve the strength of CP-Ti. Ti billets were first processed for eight passes via ECAP route Bc, with a clockwise rotation of 90 degrees between adjacent passes. The grain size obtained by ECAP alone is about 260 nm. The billets were further processed by cold deformation (cold rolling) to increase the crystalline defects such as dislocations. The strength of pure Ti was improved from 380 to around 1000 MPa by the two-step process. This article reports the microstructures, microhardness, tensile properties, and thermal stability of these Ti billets processed by a combination of ECAP and cold deformation.     

Improved properties of Ti-6Al-4V by combining isothermal equal-channel angular processing and hot-extrusion

The aim of this work was to study effects of hot extrusion on the microstructure of Ti-6Al-4V (wt-%) alloy processed by ECAP. Firstly, an isothermally Ti–6Al–4V alloy processed by Equal channel angular pressing(ECAP) was preheated at 950°C for 6?min and then hot extruded at 900°C. The hot extrusion minimised the grain size and maximised the mechanical strength. Therefore, it was demonstrated that hot extrusion of Ti-6Al-4V alloys that processed by ECAP could be performed without compromising any mechanical properties. Therefore, it is possible to use the ability to apply a reduced cross-section in hot extrusion for an Ti-6Al-4V processed by ECAP without concern about the reduction of properties.     

Investigation on nanocrystalline iron‐cobalt‐vanadium alloy processed by equal channel angular pressing (ECAP)

Nanocrystalline iron‐cobalt‐vanadium alloy was fabricated by Equal‐Channel Angular Pressing (ECAP). Microstructural evolution at different passes of ECAP and the effect of φ in the ECAP were researched. The results revealed that a phase slowly turned to γ phase and followed the form of dislocation cells in the iron‐cobalt‐vanadium alloy with the increase of severe plastic deformation. At last, it became reasonably finer bands of subgrains. The results with intersect at an angle φ of 90° was better than with at an angle φ of 120°. After three passes of ECAP, at an angle φ of 90°, the nanocrystalline microstructure could be obtained. The grain size was reduced from 30 μm in the initial state to 400nm.     

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。     

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.     

Nanoscale deformation of multiaxially forged ultrafine-grained Mg-2Zn-2Gd alloy with high strength-high ductility combination and comparison with the coarse-grained counterpart

Cold processing of magnesium(Mg) alloys is a challenge because Mg has a hexagonal close-packed(HCP)lattice with limited slip systems, which makes it difficult to plastically deform at low temperature. To address this challenge, a combination of annealing of as-cast alloy and multi-axial forging was adopted to obtain isotropic ultrafine-grained(UFG) structure in a lean Mg-2Zn-2Gd alloy with high strength(yield strength: ~227 MPa)-high ductility(% elongation: ~30%) combination. This combination of strength and ductility is excellent for the lean alloy, enabling an understanding of deformation processes in a formable high strength Mg-rare earth alloy. The nanoscale deformation behavior was studied via nanoindentation and electron microscopy, and the behavior was compared with its low strength(yield strength: ~46 MPa)-low ductility(% elongation: ~7%) coarse-grained(CG) counterpart. In the UFG alloy, extensive dislocation slip was an active deformation mechanism, while in the CG alloy, mechanical twinning occurred.The differences in the deformation mechanisms of UFG and CG alloys were reflected in the discrete burst in the load-displacement plots. The deformation of Mg-2Zn-2Gd alloys was significantly influenced by the grain structure, such that there was change in the deformation mechanism from dislocation slip(non-basal slip) to nanoscale twins in the CG structure. The high plasticity of UFG Mg alloy involved high dislocation activity and change in activation volume.     

Mechanical behaviour and texture of annealed AZ31 Mg alloy deformed by ECAP

Abstract

AZ31 Mg alloy samples were processed by equal channel angular pressing (ECAP) at 220°C for four passes. An average grain size of ～1·9 μm with reasonable homogeneity was obtained. The ECAP process imparted large plastic shear strains and strong deformation textures to the material. Subsequent annealing of the equal channel angular pressed samples produced interesting mechanical behaviours. While yield strength increased and ductility decreased immediately after undergoing ECAP, annealing at temperatures <250°C restored ductility significantly at a small decrease in of yield strength. Annealing at temperatures >250°C reduced yield strength without additional improvement in ductility. It is believed that the combination of stress relief via dislocation elimination, refined microstructure and the retention of a strong ECAP texture at low annealing temperatures enhance ductility. High temperature annealing breaks down the ECAP texture resulting in no further improvement in ductility. The results show that the mechanical properties of the alloy can be positively influenced by annealing after ECAP to achieve a combination of strength and ductility.     

Ultrafine-grained Ti–Nb–Ta–Zr alloy produced by ECAP at room temperature

To ascertain the influence of severe plastic deformation (SPD) on a Ti–Nb–Ta–Zr (TNTZ) alloy, we studied the room temperature mechanical behavior and microstructural evolution of an ultrafine-grained (UFG) Ti–36Nb–2Ta–3Zr (wt%) alloy prepared via equal-channel angular pressing (ECAP) of the as-hot-extruded alloy. The tensile behavior, phase composition, grain size, preferred orientation, and dislocation density of the UFG alloy, processed under different conditions, were analyzed and discussed. Compared to the as-hot-extruded alloy, the ECAP-processed TNTZ alloy (3 passes) exhibited approximately 40 and 88 % increase in average ultimate strength and yield strength, respectively. Moreover, as the number of ECAP passes increased from 3 to 6, the TNTZ alloy exhibited not only the expected increase in ultimate and yield strength values, but also a slight increase in elongation. Our results suggest that the deformation mechanisms that govern the behavior of the as-hot-extruded coarse grained (CG) TNTZ alloy during ECAP involve a combination of stress-induced martensitic transformation and dislocation activity. In the case of the ECAP-processed UFG TNTZ alloy, the deformation mechanism is proposed to involve two components: first, dislocation activity induced by the strain field imposed during ECAP; and second, the formation of α″ martensite phase during the early stages of ECAP which eventually transforms into β phase during continued deformation. We propose that the deformation mechanism governing the room temperature behavior of the TNTZ alloy strongly depends on the grain size of the β phase.     

Microstructure and tensile strength of grade 2 titanium processed by equal-channel angular pressing and by rolling

Commercially pure titanium strengthened by severe plastic deformation constitutes an alternative to the use of complex Ti alloys in many medical or industrial applications. In this research, rods of grade 2 Ti were processed by up to six passes using Equal-channel angular pressing (ECAP) at 573 K followed by cold rolling at room or subzero temperatures. After four passes of ECAP, the grain size was refined down to the submicrometer scale and subsequent rolling led to further refinement. The microstructure was characterized by taking Vickers microhardness measurements and tensile testing was performed both at room temperature and in the temperature range of 573–773 K. The results show that at all temperatures the tensile strength is significantly improved by means of these processing techniques. At room temperature, the ultimate tensile strength of pure Ti after ECAP plus subzero rolling is close to that of the traditional Ti-6Al-4V alloy while maintaining adequate levels of elongation to failure.     

Influence of the ECAP Processing Parameters on the Cyclic Deformation Behavior on Ultrafine‐Grained Cubic Face Centered Metals

Ultrafine‐grained (UFG) materials are widely known to exhibit significantly improved fatigue properties when the fatigue life is regarded in a Wöhler‐S–N‐plot. More detailed, the achieved improvements in fatigue life significantly depend on the processing conditions of these UFG materials. In this work the influence of several equal channel angular pressing (ECAP) processing parameters on the fatigue properties of the Al–Mg model system with up to 2% of magnesium and on the technical alloy AA5754, namely AlMg3 are investigated. Most surprisingly, it is found that the material produced with route A (no rotation between ECAP passes), exhibit a higher fatigue life than the material produced by route B_c (90°‐rotation between ECAP passes). It is found that the different textures are responsible for that behavior. Moreover, the number of ECAP passes or the application of backpressure also significantly influences the fatigue life. In this context, relevant differences in the cyclic deformation behavior, microstructure, and damage mechanisms were discussed in this paper.     

Effect of 1.5 wt% tin addition on the properties of AZ31 magnesium alloy

In this paper, 1.5 wt%Sn was added to the AZ31 magnesium alloy aiming at improving the mechanical properties by using a low cost alloying element. Both alloys were prepared in the cast/heat treated (HT), rolled at 350 °C, rolled/heat treated at 400 °C and extruded at 350 °C. The results indicate that with addition of tin an improvement was obtained in both tensile strength and ductility of the AZ31 alloy in the cast/heat treated and in the extruded conditions. The yield and ultimate tensile strengths reached 98 MPa and 224 MPa respectively with 14 % elongation in the cast/heat treated condition while in the extruded condition these values were 212 MPa and 286 MPa with 20 % elongation. The tensile strength was even higher after rolling reaching 315 MPa for AZ31 with tin addition; however, as the material temperature during the last passes has decreased to relatively low values, the % elongation decreased to 1 %. After heat treatment at 400 °C for 2 hours the % elongation was restored and reached 12 %; this was accompanied by a decrease in tensile strength which reached 276 MPa. The results are discussed in relation to the microstructure evolution including grain size, phase identification, and volume fraction of phases.     

Effects of aluminum and copper cover tube casing on the ECAP process of AM30 magnesium alloy

Aluminum (Al) and copper (Cu) thin tube casings were employed to facilitate the equal channel angular pressing (ECAP) process of wrought AM30 magnesium (Mg) alloy. We covered the Mg rods (diameter, 14?mm) with pure copper (Mg/Cu) and Al-1050 alloy (Mg/Al) sheaths in the form of tubes (thickness, 2.5?mm). We then performed the ECAP processes at 200, 250, and 275°C using the conventional ECAP of AM30 alloy and covered tube casing (CTC) methods with a single pass. We assessed the effect of CTC on ECAP temperature, grain refinement, uniformity of structure, hardness distribution, and strength. The results of Mg/Cu and Mg/Al bimetal sample tests showed that there was a significant decrease in the process temperature when compared with the conventional ECAP of AM30. The sheath resolved the segmented material flow that occurred in a conventional ECAP of material at 250°C, and transformed it to a uniform flow in CTC bimetal samples. Grain sizes of the CTC samples decreased by 20% compared with the conventional ECAP of AM30 samples. Furthermore, grain uniformity, strain distribution homogeneity, tensile yield strength (TYS), and elongation increased. However, we observed a similar trend of deformation twinning in compression test results for the ECAP and CTC methods, and there was no significant variation in material yield asymmetry between the ECAP and CTC samples.     

The influence of defect and temperature on the fatigue behaviours of Al‐Si‐Cu‐Mg‐Ni alloy

High‐cycle fatigue (HCF) properties of two Al‐Si‐Cu‐Mg‐Ni alloys with different defect sizes named as alloys A (smaller ones) and B (bigger ones) were investigated at 350°C and 425°C, respectively. The results indicate that fatigue strengths of both alloys decrease as the temperature increases. Fatigue cracks originated from pores and oxide films at both temperatures. They propagated preferentially through cracked matrix at 350°C and debonded interface and grain boundary at 425°C. Alloy A exhibits higher fatigue life and fatigue strength than alloy B at 350°C due to its smaller pore sizes. However, it has slightly worse fatigue properties than alloy B at 425°C because the fatigue crack initiation is controlled by oxide film at this temperature and is not affected by its size. This indicates that there is a transition of predominant initiation site from pores to oxide films when the temperature increases. The fatigue strength estimated through defect size is consistent with the experimental results at 350°C, while unsuitable at 425°C.     

Room temperature corrosion properties of AZ31 magnesium alloy processed by extrusion and equal channel angular pressing

The electrochemical properties of AZ31 magnesium alloy processed by extrusion and equal channel angular pressing (ECAP) were investigated. The electrochemical properties were evaluated using potentiodynamic tests and electrochemical impedance spectroscopy in corrosion solution of 0.1 M sodium chloride. The electrochemical changes of the sample surface were correlated with microstructure evolution. Material processed by extrusion and subsequently by 8 passes of ECAP shows similar or even inferior corrosion resistance to the extruded material after immersion time up to 96 h. However, corrosion resistance of material after extrusion and ECAP is significantly better than that of the extruded material for immersion time of 168 h. This sudden improvement is caused by different formation and fall off of protective corrosion products. Microstructure after extrusion is inhomogeneous and contains relatively large grains, whereas material after ECAP possesses homogeneous ultrafine-grained (UFG) microstructure. As a result, material after ECAP offers more corrosion nucleation sites, but UFG microstructure causes that only smaller clusters of corrosion products fall off the surface. The easier and faster corrosion protective layer restoration on the surface of UFG material after ECAP leads to enhanced corrosion resistance.     

Effect of ultrasonic vibration during casting on microstructures and properties of 7050 aluminum alloy

The microstructures and properties of ultrasonic and conventional cast 7050 aluminum alloys have been studied. Compared with the conventional cast alloy, the ultrasonic cast ingot (UI) is characterized with a finer microstructure than that in the conventional cast ingot (CI). For the hot-rolled plates, the UI alloy can be aged faster and aging-strengthened easier than the CI alloy. When aged at 120 °C, the UI alloy reaches its peak strength after 8 h, with tensile strength of 602 MPa, yield strength of 547 MPa and elongation of 12.7%, respectively, whereas the CI alloy plate is with its tensile strength, yield strength and elongation of 536 MPa, 462 MPa and 15.0%, respectively, after peak aged for 12 h.     

Precipitation hardening and grain refinement in an Al–4.2wt%Mg–1.2wt%Cu processed by ECAP

The precipitation and the strength evolution during equal channel angular pressing performed at 180 °C in an Al–4.2wt% Mg–1.2wt%Cu alloy have been studied by room temperature compression tests and transmission electron microscopy. The age hardening behaviour of these AlMgCu alloys, in which the precipitation sequence involves the S-phase and its precursors, was investigated and revealed a yield strength peak after 8 days at 180 °C. The influence of the Severe Plastic Deformation on the microstructure and mechanical properties of under-aged and peak-aged samples are presented. Notably, in the under-aged sample, a gradual increase of the strength after each ECAP pass is obtained while, the peak-aged samples loose much of their strength during the first ECAP pass. TEM characterization of the microstructure before and after ECAP is presented and linked to the evolution of the mechanical properties.     

Hybrides Walzen am Beispiel von Titan‐Aluminium‐Verbunden

Hybrid rolling as exemplified by titanium‐aluminium laminates Triple layered titanium‐aluminium laminates composed of titanium alloys TiAl6V4 (ASTM grade 5) and Ti 99.8 ASTM grade 1) together with the aluminium alloys AlMgSi 0.5 (EN‐AW 6060) and AlCuMg 1 (EN‐AW 2017) are manufactured by hot rolling and the deformation behaviour is investigated subject to alternating deformation parameters. The focus is the investigation of the differences between stepwise and continuous increases in true strain. True strains of 20 … 60 % are tested at temperatures from 400 … 500 °C. The contact zone of the manufactured laminates is then metallographically examined and the interlayer bond strength is mechanically tested. Torsion tests are presented for qualitatively determining the bond strength of the laminate. Bond forming already initiates at true strains of 35 % and temperatures of 350 °C within the rolling gap.     

Microstructure of MgAl5Ca3Sr alloy after creep deformation and high‐temperature heat treatment

The paper presents results of microstructural investigations of MgAl5Ca3Sr magnesium alloys in the as‐cast condition, after creep tests at 180 °C, and after heat treatment at 450 °C for 4.5 hours. The microstructure of MgAl5Ca3Sr alloy is composed of α‐Mg solid solution, irregular shaped (Mg,Al)₂Ca phase with C36 crystal structure, bulky (Mg,Al)₁₇(Sr,Ca)₂ phase, fine lamellar Mg₂Ca phase with C14 structure, needle‐shaped Al₂Ca precipitates with the C15 crystal structure. The precipitation of the needle‐shaped Al₂Ca phase in the α‐Mg grains and spheroidization of the C14 phase were found after heat treatment at 450 °C in argon atmosphere. The (Mg,Al)₂Ca (C36) and (Mg,Al)₁₇(Sr,Ca)₂ phases seems to be stable at 450 °C, however, the increasing of aluminum content in C36 compound was observed suggesting the initial stage of C36 → C15 transformation. After creep deformation at 180 °C precipitates of the Al₂Ca phase were found in α‐Mg phase. The intermetallic compounds are stable at 180 °C. The MgAl5Ca3Sr alloy exhibits good creep resistance up to 75 MPa. Tensile properties are comparable to those of Mg‐RE‐Zn–Zr alloys.