Comparison of Microtexture Evolution During Cryo and Room Temperature Rolling of Ti–5Ta–2Nb Alloy

An α + β Ti–5Ta–2Nb alloy has been developed for applications under severe oxidizing conditions of boiling concentrated nitric acid in spent fuel reprocessing plant. The formation of nanocrystalline grains has been observed in the cryo-rolled sample accompanied by a significant improvement in yield strength whereas the grain refinement is not significant in case of room temperature rolled sample. Suppression of dynamic recovery plays a major role in the formation of nanostructured material. The evolution of micro-texture during room temperature rolling and cryo-rolling has been compared in this study. The rolling texture of room temperature and cryo-rolled samples are similar to the rolling texture predicted for hexagonal materials with c/a <1.6. XRD analysis indicates peaks shift to lower 2θ with increase in rolling strains indicating the presence of tensile residual strains. Peak broadening is also observed indicating an increase in defect density and decrease in grain size.     

Microstructural Characterization of Internal Welding Defects and Their Effect on the Tensile Behavior of FSW Joints of AA2198 Al-Cu-Li Alloy

Internal features and defects such as joint line remnant, kissing bond, and those induced by an initial gap between the two parent sheets were investigated in AA2198-T851 friction stir welded joints. They were compared with the parent material and to defect-free welds obtained using a seamless sheet. The cross-weld tensile strength was reduced by the defects by less than 6 pct. The fracture elongation was not significantly affected in view of experimental scatter. Fracture location, however, changed from the thermomechanically affected zone (retreating side) to the defect in the weld nugget for the welds bearing a kissing bond and for some of the gap welds. The kissing bond was shown by EBSD to be an intergranular feature; it fractured under a normal engineering stress close to 260 MPa during an in situ SEM tensile test. Synchrotron tomography after interrupted tensile testing confirmed opening of the kissing bond. For an initial gap of 23 pct of the sheet thickness, intergranular fracture of copper-enriched or oxide-bearing grain boundaries close to the nugget root was evidenced. The stress and strain state of cross-weld specimens loaded under uniaxial tension was assessed using a 3D finite element, multi-material model, determined on the basis of experimental data obtained on the same specimens using digital image correlation.     

Studies on ω Phase Formation in Ti-Mo Alloys Using Synchrotron XRD

This paper presents the results of synchrotron based x-ray diffraction experiments in Ti-Mo alloys, to examine the formation of metastable ω phase, which could not be detected using conventional XRD (x-ray diffraction). The study confirms the formation of non-equilibrium, athermal ω phase on quenching the β phase at different cooling rates in Ti-15 wt% Mo alloy and its absence in Ti-25 wt% Mo alloy. On ageing, isothermal ω is observed in Ti-15 wt% Mo. However, increase in amount of Mo stabilises the bcc β phase in Ti-25 wt% Mo alloy, leading to absence of ω phase, despite ageing in the α + β field. The threshold concentration of Mo to stabilize the β phase in the alloy is higher than 15 and lower than 25 wt%.     

固溶温度对Ti-62A合金拉伸性能和断裂韧性的影响

Ti-62A合金是一种新型高强高韧损伤容限型钛合金,研究了固溶温度对Ti-62A合金30 mm厚板材的显微组织、拉伸性能以及断裂韧性的影响规律。研究结果表明:Ti-62A合金Φ720 mm铸锭经单相区和两相区多道次大变形轧制后所得的30 mm厚板材组织为典型的片层组织,由片层状的α相和β转变组织构成,组织均匀,片层状α相平均宽度约为2.5μm,长度在40～65μm之间。两相区固溶+时效处理后,合金的组织类型为片层状组织,即片层状的初生α相(αp)相与β转变组织,随固溶温度升高,合金中的初生α相(αp)相含量显著减少,β转变组织逐渐增多,次生α相(αs)片层宽度增大,同时合金的强度下降,塑性上升,当接近相变点时这种趋势变缓。单相区固溶+时效处理获得魏氏组织,晶粒粗大,晶界平直而清晰,其拉伸强度高于920℃和940℃固溶时的片层组织,但塑性显著降低;与900℃固溶时相比强度和塑性均降低。合金的断裂韧性随固溶温度的上升而逐渐升高,单相区固溶并时效后的魏氏组织的断裂韧性明显优于两相区固溶并时效后的片层组织。     

Microstructure and mechanical property correlations in AA 6061 aluminium alloy friction stir welds

The influence of friction stir welding on the microstructure development and its role on residual stress distribution in the weldment and mechanical properties has been investigated. The study also focused on the impact of post weld heat treatment on the microstructure and mechanical properties as well as on residual stress distribution. The weld nugget region contained fine equiaxed grains as a result of thermo-mechanical working. Hardness survey showed that nugget region is soft due to precipitates dissolution. Weld joint exhibited lower strength as compared to the parent metal. Post weld Solution Treatment and Aging (STA) of longitudinal welds resulted in strength and ductility equivalent to that of parent metal while transverse weld tensile strength and ductility were lower than that of parent metal even after post weld STA. Residual stress distribution profiles across the weld region are asymmetric with respect to weld centerline, with the largest residual; stress gradients occurring on the advancing side of the weld. Within the region inside the shoulder diameter, residual stress is entirely compressive. Welds exhibited tensile residual stresses in post weld STA condition     

锻造工艺对Ti60合金棒材组织和性能的影响

研究了常规锻造,近β锻造和β锻造对Ti60合金棒材显微组织和力学性能的影响规律。结果发现：Ti60合金棒材经常规锻造和近β锻造后,得到等轴组织,近β锻造的等轴α相较少,β锻造组织为典型的网篮组织,局部有大块α相出现。力学性能结果分析表明,近β锻造具有最好的强度与塑性的匹配。采用SEM观察了拉伸试样的断口形貌,结果表明,常规锻造和近β锻造的试样为韧窝型断口,而β锻造的试样为韧窝＋准解理的混合型断口。     

Influence of heat treatment conditions on the mechanical properties of Ti–6Al–4V alloy

In the current work, several heat treatments were carried out below and above the beta-transition temperature of the Ti–6Al–4V alloy followed by aging at 550 °C for 6 hours. The resultant microstructures and their effects on the mechanical properties of Ti–6Al–4V alloy were investigated. The results showed that solution treatment of Ti–6Al–4V samples followed by water quenching from β and α/β fields raised the alloy hardness from 380 to 575 and 656?HV, respectively, while no remarkable changes were observed after aging. The hot tensile strength of the as-forged sample increased from 671 to 756?MPa after water quenching from the ß- or α/ß- field, while the air cooling from β-phase field decreased the tensile strength to 644 MPa. The fracture mode of the tensile samples was more ductile in case of the solution-treated samples compared to the as-forged samples. A subsurface layer was formed due to the diffusion of oxygen into the surface at high temperatures. This layer which is known as ‘oxygen diffusion layer’ masked the differences of wear behaviour of the specimens.     

The effect of matrix strength on void nucleation and growth in a widmanstätten alpha-beta titanium alloy,CORONA-5

This paper presents the results of a study of the effect of matrix yield strength, at a constant Widmanstätten α microstructure, on the nucleation and growth of voids in an α-β titanium alloy, CORONA-5, Ti-5Al-4.5Mo-l.5Cr. Four microstructures with a retained β matrix and involving coarse or fine Widmanstätten α particles in coarse or fine β grains were used in different heat treatment conditions resulting in yield strengths from 765 to 1018 MPa. Void nucleation occurred atα/α boundaries, grain boundary α/matrix interfaces, α twin/matrix interfaces, and α twin/untwin boundaries. The void nucleation strain varied from 0 to 0.26 and was a function of both microstructure and yield strength. Void growth rates increased with yield strength for all microstructures except for those containing coarse α which decreased with yield strength. Intense shear was observed in colonies of fine α structures and was considered to be the cause of the observed rapid void growth rates in the fine β+ fine α microstructures. Surface cracking occurred in several aged conditions during straining. This cracking was attributed to strain concentration in α.     

The mechanism of void formation,void growth,and tensile fracture in an alloy consisting of two ductile phases

An investigation has shown that it is possible to relate void formation, void growth, and tensile ductility to microstructural features in an α-β titanium alloy, Ti-5.25A1-5.5V-0.9Fe-0.5Cu, heat treated to a constant yield strength. Equations relating tensile void growth rates to microstructure for both equiaxed,E, and Widmanstätten plus grain boundaryα, W + ITG. B.,in aged β morphologies have been derived. A mechanism for void formation at α-β interfaces is presented which accounts for the observed fact that voids do not form at Widmanstätten α platelets. Tensile fracture is shown to be intergranular in nature and occurs when a critical crack length-stress relationship is satisfied. The amount of ductility achievable in a specimen depends upon the rate of void growth. If the rate is large, the void reaches a critical size for fracture at a lower applied stress and strain and hence the ductility is less.     

High-Temperature Tensile Behaviors of Base Metal and Electron Beam-Welded Joints of Ni-20Cr-9Mo-4Nb Superalloy

Electron beam welding of Ni-20Cr-9Mo-4Nb alloy sheets was carried out, and high-temperature tensile behaviors of base metal and weldments were studied. Tensile properties were evaluated at ambient temperature, at elevated temperatures of 625 °C to 1025 °C, and at strain rates of 0.1 to 0.001 s^?1. Microstructure of the weld consisted of columnar dendritic structure and revealed epitaxial mode of solidification. Weld efficiency of ~?90 pct in terms of strength (UTS) was observed at ambient temperature and up to an elevated temperature of 850 °C. Reduction in strength continued with further increase of test temperature (up to 1025 °C); however, a significant improvement in pct elongation is found up to 775 °C, which was sustained even at higher test temperatures. The tensile behaviors of base metal and weldments were similar at the elevated temperatures at the respective strain rates. Strain hardening exponent ‘n’ of the base metal and weldment was ~?0.519. Activation energy ‘Q’ of base metal and EB weldments were 420 to 535 kJ mol^?1 determined through isothermal tensile tests and 625 to 662 kJ mol^?1 through jump-temperature tensile tests. Strain rate sensitivity ‘m’ was low (<?0.119) for the base metal and (<?0.164) for the weldment. The δ phase was revealed in specimens annealed at 700 °C, whereas, twins and fully recrystallized grains were observed in specimens annealed at 1025 °C. Low-angle misorientation and strain localization in the welds and the HAZ during tensile testing at higher temperature and strain rates indicates subgrain formation and recrystallization. Higher elongation in the weldment (at Test temperature >?775 °C) is attributed to the presence of recrystallized grains. Up to 700 °C, the deformation is through slip, where strain hardening is predominant and effect of strain rate is minimal. Between 775 °C to 850 °C, strain hardening is counterbalanced by flow softening, where cavitation limits the deformation (predominantly at lower strain rate). Above 925 °C, flow softening is predominant resulting in a significant reduction in strength. Presence of precipitates/accumulated strain at high strain rate results in high strength, but when the precipitates were coarsened at lower strain rates or precipitates were dissolved at a higher temperature, the result was a reduction in strength. Further, the accumulated strain assisted in recrystallization, which also resulted in a reduction in strength.     

Influence of the second phase on the room-temperature tensile and creep deformation mechanisms of α-β titanium alloys, part II: Creep deformation

This work focuses on the effect of the second phase on the ambient temperature creep deformation mechanisms of titanium alloys, using Ti-6.0 wt pct Mn and Ti-8.1 wt pct V with Widmanstätten microstructures as the model systems. In Part I it was observed that the presence of a second phase can affect the tensile deformation behavior. Likewise, the creep deformation mechanisms of the two-phase alloys differ from the mechanisms of single-phase alloys. These α-β deformation mechanisms include twinning in fine grains of the α phase and stress-induced hexagonal martensite in the β phase of Ti-8.1 V. This is the first time that twinning in the α phase and stress-induced martensite in the β phase are reported as creep deformation mechanisms in an α-β titanium alloy. Several factors, including elastic interaction effects, shear stress due to deformation products in adjacent phases, and the stability of the β phase, affect the creep deformation mechanisms in these alloys. Models for the time-dependent growth of martensite are suggested. In addition, the difference between tensile and creep deformation in regard to accumulation of stresses to reach the critical stresses is described.     

Fatigue Crack Growth Behavior of Ti-4.5Al-3V-2Fe-2Mo Laser Welds

Fatigue crack growth tests of Ti-4.5Al-3V-2Fe-2Mo (SP-700) laser welds after various postweld heat treatments (PWHTs) were investigated. The welds and the mill-annealed base metal had similar fatigue crack growth rates (FCGRs) at a stress ratio (R) of 0.1. After increasing the stress ratio to 0.5, the peak-aged (482 °C) weld exhibited higher FCGRs due to increased notch brittleness of the material. The tough microstructure as well as tortuous crack path of the overaged (704 °C) weld could account for the reduced FCGRs, particularly at a higher R. The fatigue fracture appearance of the welds varied from transgranular to intergranular failures, depending on the stress intensity factor ranges and PWHTs. Experimental results also demonstrated that the 704 °C-aged weld with coarsened α + β structures had better impact toughness than the base metal with banded structures.     

Laser Additive Manufactured Ti–6Al–4V Alloy: Heat Treatment Studies

The effect of heat treatment on microstructure and mechanical behaviours of direct metal laser sintered Ti–6Al–4V samples have been studied. Rectangular parts were built in two different directions; vertical and horizontal and subjected to two different heat treatment cycles: above β transus and below β transus with air cooling. Surface characteristics, microstructural examination and mechanical properties have been investigated. Below β transus treatment creates a modification in the surface morphology with a fine dimple network. Above β transus treatment leads to extensive grain growth at the middle section of the vertically build component thereby increasing its microhardness. Both the selected heat treatment cycles significantly reduces the tensile strength and improves the elongation when compared to as-sintered material. However, below transus temperature treated vertical built specimen results in optimum combination of tensile strength (1124 MPa) and elongation (20%). Higher coefficient of friction has been recorded for specimens after heat treatment.     

Study of MA Effect on Yield Strength and Ductility of X80 Linepipe Steels Weld

Multipass GMAW (Gas Metal Arc Welding) welding was used to join X80 linepipe materials using two weld metals of slightly different compositions. Welding wires with diameters of 0.984 and 0.909 mm were used while applying the same heat input in each pass. The slight difference in the wire diameters resulted in different HAZ microstructures. The microstructures in the doubly reheated HAZ of both welds were found to contain bainite-ferrite. However, etching also revealed a difference in martensite-austenite (MA) fraction in these reheated zones. The MA exhibited twice the hardness of ferrite when measured by nanoindentation. Tensile testing from the reheated zone of both welds revealed a difference in yield strength, tensile strength and elongation of the transverse weld specimens. In the reheated zone of weld A, (produced with a 0.984 mm wire) a higher fraction of MA was observed, which resulted in higher strength but lower elongation compared to weld B. The ductility of weld A was found severely impaired (to nearly half of weld B) due to formation of closely spaced voids around the MA, along with debonding of MA from the matrix, which occurs just above the yield stress.

     

Influence of Surface Mechanical Attrition Treatment Duration on Fatigue Lives of Ti–6Al–4V

This work deals with the influence of surface mechanical attrition treatment (SMAT) duration on fatigue lives of Ti–6Al–4V. The SMAT process was carried out in vacuum with SAE 52100 steel balls of 5 mm diameter for 30 and 60 min at a vibrating frequency of 50 Hz. SMAT treated surface was characterized by electron microscopy. Surface roughness, nano-indentation hardness, residual stress, and tensile properties of the material in both SMAT treated and untreated conditions were determined. SMAT enabled surface nanocrystallization, increased surface roughness, surface hardness, compressive residual stress and tensile strength but reduced ductility. Samples treated for 30 min exhibited superior fatigue lives owing to positive influence of nanostructured surface layer, compressive residual stress and work hardened layer. However, fatigue lives of the samples treated for 60 min were inferior to those of untreated samples due to presence of microdamages or cracks induced by the impacting balls during the treatment.     

Influence of Rotational Speed on Microstructure and Mechanical Properties of Dissimilar Metal AISI 304-AISI 4140 Continuous Drive Friction Welds

 Fundamental investigation of continuous drive friction welding of austenitic stainless steel (AISI 304) and low alloy steel (AISI 4140) is described. The emphasis is made on the influence of rotational speed on the microstructure and mechanical properties such as hardness, tensile strength, notch tensile strength and impact toughness of the dissimilar joints. Hardness profiles across the weld show the interface is harder than the respective parent metals. In general, maximum peak hardness is observed on the stainless steel side, while other peak hardness is on the low alloy steel side. A trough in hardness distribution in between the peaks is located on the low alloy steel side. Peak hardness on the stainless steel and low alloy steel side close to the interface increases with a decrease in rotational speed. All transverse tensile joints fractured on stainless steel side near the interface. Notch tensile strength and impact toughness increase with increase in rotational speed up to 1500 r/min and decrease thereafter. The mechanism of influence of rotational speed for the observed trends is discussed in the torque, displacement characteristics, heat generation, microstructure, fractography and mechanical properties.     

Metallurgical characterisation of dissimilar welds between modified 9Cr‐1Mo steel and Alloy 800

Dissimilar metal welds between ferritic low‐alloy and austenitic stainless steels commonly occur in power plant application. In order to overcome some of the problems encountered here, a trimetallic configuration using an intermediate piece (such as Alloy 800) between the austenitic and ferritic steels has been suggested. This paper describes some features of the joints between modified 9Cr‐1Mo steel and Alloy 800, produced with Inconel 82/182 filler material. The joints require heat‐treatment after welding and the results have shown that a treatment at 760 °C for 2 h would be optimal. Although most tensile failures occurred in the weld metal the welds were found to exhibit strength properties that are at least equal to those of Alloy 800, with a tensile elongation lying between those of the two base materials. Similarly, while the weld metals are slightly less tough than the two base materials, the weld metal toughness at 120 J is still quite adequate for the intended application.     

Fracture characteristics of Ti-6Al-4V and Ti-5Al-2.5Fe with refined microstructure using hydrogen

The hydrogenation behavior of Ti-6Al-4V, with the starting microstructures of coarse equiaxed α and coarse Widmanstätten α, respectively, was investigated under a hydrogen pressure of 0.1 MPa at temperatures between 843 and 1123 K. The hydrogen content was determined as a function of hydrogenation time, hydrogenation temperature, and hydrogen flow rate. The phases presented in the alloy of after hydrogenation were determined with X-ray and electron diffraction analysis in order to define the effect of Thermochemical Processing (TCP) on the microstructure of the alloy. Mechanical properties and fracture toughness of Ti-6Al-4V and Ti-5Al-2.5Fe subjected to the various TCP were then investigated. Hydrogenation of Ti-6Al-4V with the starting microstructure of coarse equiaxed α at 1023 K, just below hydrogen saturated β (denoted β″ (H)) transus temperature, produces a microstructure of a, orthohombic martensite (denoted α″ (H)) and β (H). Hydrogenation at 1123 K, above β (H) transus, results in a microstructure of α″ (H) and β (H). Microstructure refinement during TCP results mainly from decomposition of α″ (H) and ;β (H) into a fine mixture of α + β during dehydrogenation. An alternative TCP method is below β (H) transus hydrogenation (BTH), consisting of hydrogenation of the alloy below the hydrogenated β (H) transus temperature, air cooling to room temperature, and dehydrogenation at a lower temperature, which is found to improve mechanical properties significantly over a conventional TCP treatment. Compared with the untreated material, the BTH treatment increases the yield strength and increases the ultimate tensile strength significantly without decreasing the tensile elongation in the starting microstructure of coarse equiaxed α or with a little decrease in the tensile elongation in the starting microstructure of coarse Widmanstätten α, although the conventional TCP treatment results in a large decrease in elongation over the unprocessed material in Ti-6Al-4V. In Ti-5Al-2.5 Fe, both conventional TCP and BTH result in a increase in yield strength, ultimate tensile strength, and elongation; however, the BTH gives the best balance between strength and elongation. The TCP-treated Ti-6Al-4V shows smaller fracture toughness compared with the unprocessed material, while TCP-treated Ti-5Al-2.5Fe shows greater fracture toughness compared with the unprocessed material. The BTH treatment results in a improvement in fatigue strength in both Ti-6Al-4V and Ti-5Al-2.5Fe.     

Microstructure-Based Strength Distribution Across the Welds of Nickel-Based Superalloy Inconel 751 and Austenite Steel 21-4N Joined by Inertia Friction Welding

Welding dissimilar metals is always a challenge for their different physical property and microstructures. In this study, the two dissimilar metals 21-4N and Inconel 751 are welded together by inertia friction welding. Microstructure observation shows that the weld can be divided into three regions in 21-4N: the chemical composition mixture zone, shear zone, and base metal. The width of the chemical composition mixture zone (CMZ) is about 80 μm, with relatively larger grains and lower dislocation density distributed in this zone. Shear banding occurs in the shear zone, and carbides are found to have precipitated strongly along these shear bands noncontinuously. The base metal contains an austenite microstructure with carbides distributed in the matrix. In Inconel 751, only two typical zones can be observed: the CMZ and the base metal. The heat-affected zone is too small to be observed in the whole weld. Finally, a strength model based on microstructural evolution is proposed. The strength distribution along the axial direction of the welds is calculated. The results are in good agreement with the measurements.