Αp Phase Research Articles

Study of the Microstructure and Properties of the Butt Joint of Laser-Welded Titanium Alloy with Flux-Cored Wire

A self-developed Ti-Al-V-Mo system titanium alloy flux-cored wire was used to join the Ti64 titanium alloy plate via laser welding with filler wire. The microstructure and properties of the obtained welded joint were investigated. The results showed the WM (welded metal) of the welded joint consisted mainly of acicular α’ martensite, and the HAZ (heat affected zone) was comprised of a primary αp phase, Widmanstatten, and a few α’ martensite and a residual β phase. The strength and elongation of the welded joint after breaking are equivalent to that of the BM (base metal). The tensile fracture, presenting as a microvoid coalescence ductile fracture, was encompassed by massive shear lips with deep and uniform dimples. The overall microhardness of the welded joint was sequenced as WM > HAZ > BM. In the WM, large-angle grain boundaries with intragranular misorientation greater than 15° accounted for about 84%. By XRD, it was discovered the welded joint was mainly composed of the α’ martensite, with a modest amount of extremely weak multi-angle α phase diffraction peak. The test results showed the designed welding method of titanium flux-cored wire and laser wire filling is suitable for high-quality welding of titanium alloy plate.

Development of a Low-Density and High-Strength Titanium Alloy

Weight reduction is often considered the primary goal in the development of structure materials. A new Ti-8Al-1Cr-1V-0.5Fe-0.1Si (wt.%) alloy with a low-density and a high-strength was developed in this work. The microstructures and mechanical properties of the alloy were investigated in hot-rolled and solution-aging (STA) treatment conditions. The microstructures of the alloy in both states consist of the spherical αp phase, acicular αs phase, and a small amount of β phase. Attributed to such heterogeneous microstructures, the hot-rolled alloy exhibits an outstanding tensile strength of 1046.1 MPa, a total elongation of ~8.3%, and an impressive low density of 4.23 g/cm3. After heat treatment, the alloy also exhibits a similar strength and ductility. A detailed analysis of the deformation modes shows that the numerous dislocations slippages and deformation twinning in the αp phase are the main reasons for the high ductility, and the acicular αs phase provides the alloy with high strength after heat treatment. This newly developed alloy is a potential material for various structural applications.

Microstructure evolution, mechanical properties and strengthening mechanism of titanium matrix composite sheets

The microstructure evolution and mechanical properties of high-Zr containing titanium matrix composite (TMC) sheets with different deformation amounts were systematically studied. The results showed that the degree of αp phase refinement and the content of silicide precipitation of TMC sheets increase with the increase in deformation amount during hot rolling, which is sensitive to the preheating and reheating during rolling. The refined αp phase can impede the effective slip distance of dislocations and the silicide precipitated in the inner α phase can also strengthen αp phase, while the nano-αs phase can strengthen the βt domain. Due to the different hardness values of αp and βt domains, the αp/βt boundaries can induce back stress to βt domain and forward stress to αp, leading to heterogeneous deformation induced strengthening effect. The dislocation motion in αp phase was analyzed by transmission electron microscopy (TEM), which showed that the large deformation causes the dislocations to be tangled and unable to be distinguished. This indicates that dislocation strengthening is an indispensable strengthening mechanism until high-temperature tensile deformation. The ultimate tensile strength (UTS) and yield strength (YS) of the TMC sheets reach 726 MPa and 634 MPa at 650 °C, respectively, while the UTS at room temperature reaches the maximum of 1445 MPa, giving the TMC sheets a great application prospect in aerospace fields. Due to the strengthening of βt domain and αp phase, dislocations are blocked at the αp/βt boundaries. As a result, the failure of TMC sheet can be attributed to cracks distributed at αp/βt boundaries. The cracks around reinforcements also cannot be ignored due to the load transfer from matrix to reinforcements.

High-Cycle Fatigue Behavior and Corresponding Microscale Deformation Mechanisms of Metastable Ti55511 Alloy with A Basket-Weave Microstructure.

High-cycle fatigue (HCF) is a critical property of metastable β Ti alloys in aerospace applications. In this work, the HCF behavior and corresponding microscale deformation mechanisms of a metastable Ti-5Al-5Mo-5V-1Cr-1Fe (Ti55511) alloy with a basket-weave structure were investigated. HCF and its deformation mechanisms of a Ti55511 alloy were systematically studied in the deformed condition by using a scanning electron microscope (SEM), a transmission electron microscope (TEM), and electron backscatter diffraction (EBSD). It was found that the Ti55511 alloy exhibited an excellent HCF strength (107 cycles, Kt = 1, R = 0.06) of 738 MPa. The fractographic investigation demonstrated that fatigue striations and secondary cracks were the main features in the crack initiation zone. Dislocation analyses indicated that the HCF deformation of the basket-weave microstructure was mainly affected by the dislocation slipping of the primary α (αp) phase. In addition, the dislocation pile-up at the αp/βtrans interface led to crack initiation. EBSD analyses indicated that the prismatic <a> type slip system of the αp phase was preferentially activated during the HCF deformation process of the Ti55511 alloy, followed by the basal <a> type and pyramid <a> type systems.

High-strength and low-dwell-sensitivity titanium alloy showing high tolerance to microcracking under dwell fatigue condition

Dwell fatigue failure of titanium alloy components has been one of the major threats to aircraft safety for the last 50 years. Numerous studies have focused on the identification of critical microstructural configurations, such as microtextured regions, rogue grain pairs and (0001) twist grain boundaries. In this work, a microstructure with a low primary α phase (αp) content was designed to avoid the known weaknesses in the Ti-5Al-7.5 V alloy to achieve high-strength and low-sensitivity to 2-min dwell loading. Conventional and dwell-fatigue tests confirmed the efficiency of such an approach. The deformation occurred primarily by planar slip in the αp grains, resulting in possible cracking along preexisting slip bands in the cycling process. Fatigue and dwell-fatigue failure was dominated by surface crack initiation at facet matching basal planes. Moreover, numerous internal microcracks were formed along basal slip bands and occasionally along basal twist grain boundaries. Load holds were found to facilitate cracking along prismatic planes, which highlights a mechanism switch between basal-dominant cracking mode and collective basal-prismatic mode. The mechanism switch suggests that the prismatic dislocation activity considerably increases due to the low temperature creep induced by load holding. Thanks to the low αp phase microstructure, the alloy shows remarkable tolerance to microcrack growth under dwell condition. Limited growth of the internal microcracks across transformed β regions was identified as a key feature of high dwell-fatigue resistance of the investigated material.

High cycle and very high cycle fatigue properties and microscopic crack growth modeling of Ti‐6.5Al‐3.5Mo‐1.5Zr‐0.3Si titanium alloy at elevated temperatures

AbstractPulsating tension fatigue tests were conducted at 25°C, 150°C, and 250°C to investigate the effect of temperature on high cycle and very high cycle fatigue behavior of titanium alloy. The fatigue strength decreases with the increase of temperature, and the surface failure is more sensitive to temperature than interior failure. Afterwards, the fatigue fracture morphology, microstructure, and texture of titanium alloy were characterized. The results show that the fracture of larger αp phase with the maximum shear stress and higher strain concentration is responsible for the facet formation. The formation of facets leads to the interior failure at different temperatures. Finally, combining with the definition of interior crack growth rate, an assessment approach was proposed to predict the crack growth rate within the inhomogeneous microstructure area (IMA), which takes crack growth acceleration caused by the increase of temperature into consideration.

The interrupted hot compression of an as-forged titanium matrix composite: flow softening behavior and deformation mechanism

In this paper, the deformation behavior and microstructure evolution of 2.5vol.% (TiBw + TiCp)-containing high-temperature titanium matrix composite (TMCs) was studied by the interrupted compression tests. The flow stress–strain curves of reloading of TMCs showed the flow softening behavior. The decrease of flow stress was associated with increasing holding time and deformation temperature or decreasing strain rate. The static softening mechanism was associated with the DRV, DRX and consume of dislocations during interrupted holding (SRV) reflected by the Q, n and microstructure. The fraction of αp phase decreased with increased holding time and deformation temperature or decreased strain rate. Due to the transformation of αp phase to β phase, the high-temperature deformation ability of the TMCs will be improved a lot, which is consistent with the decreased flow stress. For the reinforcements and αp phases, they played restricted role in the growth of α/β colonies. However, the decrease of αp phase made the grown up of α/β colonies in low strain rate. The deformation mechanism of the composites was cDRX caused by the transformation of LABs into HABs by progressive lattice rotation of α phase in (α+β) region and dDRX attributed to the nucleation and growth process of β grains in β region. Due to the addition of reinforcements, the weaken microtexture of composite can be obtained. After the interrupted compression, the optimal deformation parameter has been found for the TMCs, which can give guidance for the subsequent hot-working.

Initial Microstructure Effects on Hot Tensile Deformation and Fracture Mechanisms of Ti-5Al-5Mo-5V-1Cr-1Fe Alloy Using In Situ Observation

The hot tensile deformation and fracture mechanisms of a Ti-5Al-5Mo-5V-1Cr-1Fe alloy with bimodal and lamellar microstructures were investigated by in situ tensile tests under scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The results show that the main slip deformation modes are prismatic slip ({11¯00}&lt;112¯0&gt;) and pyramidal slip ({11¯01}&lt;112¯0&gt;) under tension at 350 °C. In the bimodal microstructure, several parallel slip bands (SBs) first form within the primary α (αP) phase. As the strain increases, the number of SBs in the αP phase increases significantly and multislip systems are activated to help further coordinate the increasing deformation. Consequently, the microcracks nucleate and generally propagate along the SBs in the αP phase. The direction of propagation of the cracks deflects significantly when it crosses the αP/β interface, resulting in a tortuous crack path. In the lamellar microstructure, many dislocations pile up at the coarse-lath α (αL) phase near the grain boundaries (GBs) due to the strong fencing effect thereof. As a result, SBs develop first; then, microcracks nucleate at the αL phase boundary. During propagation, the cracks tend to propagate along the GB and thus lead to the intergranular fracture of the lamellar microstructure.

Toward an understanding of dwell fatigue damage mechanism of bimodal Ti-6Al-4V alloys

Dwell fatigue effect is a long-standing problem threatening the long-term service reliability for fan blades and fan disks of an aircraft engine. To understand the basic mechanism of dwell fatigue damage, pure fatigue and 60 s dwell fatigue properties of bimodal Ti-6Al-4V alloys with different volume fractions of the primary α (αp) phase were examined comparatively. The results showed that both pure fatigue and dwell fatigue life decreased with increasing the volume fraction of the αp phase and the dwell fatigue life was lower than the pure fatigue one. The quasi-in-situ test results and the quantitative characterization of damage behaviors of the local microstructure units defined by the αp-secondary α (αs) combination reveal that the αs phase close to the αp phase with extensively slip activities was gradually damaged under dwell fatigue loading, while that under pure fatigue loading was undamaged, demonstrating that the dwell loading induced the damage of the αs phase, and further reduced the fatigue life. A stress relaxation-based model is proposed to describe the physical mechanism on dwell fatigue damage of the bimodal Ti-6Al-4V alloy, i.e. the elastic deformation of the αs phase caused by the strain incompatibility would be gradually transformed into plastic deformation during the dwell stage, and thus promotes fatigue damage. The model provides new insights into the microscopic process of stress/strain transfer between the soft and hard microstructure units under dwell fatigue loading.

Microstructure based cracking behavior and life assessment of titanium alloy under very-high-cycle fatigue with elevated temperatures

High and very high cycle fatigue tests for TC11 titanium alloy were performed to clarify the interior cracking behavior under fully reversed and pulsating tension at three service temperatures. According to the estimation of threshold values for small and long crack growth, the fatigue design curves are established. Combined with electron backscatter diffraction and fracture mechanics analysis, the interior cracking is closely related to grain strength and size, and microtexture. The interior failure is attributed to the facet formation caused by larger αp phase fracture. Based on FIP model, a temperature-based method is proposed to predict the crack nucleation life.

In-Situ Study on Tensile Deformation and Fracture Mechanisms of Metastable β Titanium Alloy with Equiaxed Microstructure.

Understanding the mechanisms of deformation and fracture of metastable β titanium alloys is of great significance for improving formability and service life. By combining the in-situ tensile test, TEM characterization and EBSD analysis, the tensile deformation behavior, activation of slip systems, crack initiation, and propagation of a high strength metastable β titanium alloy (Ti-5Cr-4Al-4Zr-3Mo-2W-0.8Fe) with equiaxed microstructure are investigated. The equiaxed microstructure is composed of primary α (αp) phase, transformed β (βt) matrix phase, and secondary α (αs) phase. In contrast to the hexagonal αp grain with limited slip systems, the body-centered βt matrix has more slip systems, however the hindering effect of αs phases on dislocation slip leads to the different deformability of the αp phase and βt matrix. The equiaxed αp grains are more prone to deformation and rotation to coordinate the overall deformation. The shear band leads to the formation of sub-grain boundary and even the fragmentation of αp grains. As a result, the microvoids tend to nucleate at the grain boundary, phase interface, slip band, and shear band. The inhomogeneous deformation in the plastic deformation zone around the crack tip is the primary cause of damage. The crack propagation caused by microvoids coalescence advances along the grain boundaries and phase interfaces in the form of intergranular, and along the activated slip systems and shear bands in the form of transgranular. Pinpointing the situation in the equiaxed microstructure and combining that in other typical microstructures will help to summarize the universal deformation and fracture mechanisms of metastable β titanium alloy, and provide a basis for alloy design and microstructure tailoring.

Effect of Solution Temperature on the Microstructure and Properties of TB10 Titanium Alloy Bars

This paper studies the effect of solution temperature on the microstructure and mechanical properties of TB10 titanium alloy bars. The results show that the microstructure is composed of β phase and primary α phase (αp) when solution treatment is below the phase transition temperature. With the increase of the solution temperature, the β phase grain size in the microstructure increases, the thickness of the grain boundary decreases, and the number and size of the αp phase decrease, so that the strength of the alloy decreases and the plasticity increases. When the solution treatment temperature is 800°C, the reticulated grain boundary αp phase causes the plasticity to drop rapidly. When the solution treatment temperature is above the phase transition point, as the solution temperature rises, the β phase re-nucleates and grows, the grain size increases, the number of αp phase decreases. The super-cooled β phase grains induce martensite phenomenon due to stress, which eventually causes the strength and plasticity to decrease.

Deformation Behavior and Tensile Properties of the Semi-Equiaxed Microstructure in Near Alpha Titanium Alloy.

The tensile deformation and fracture behavior of a particular semi-equiaxed microstructure (S-EM) in a near alpha titanium alloy TA19 are investigated by an in situ method. In the S-EM, the thin β lamellae grow through the equiaxed αp phase (αp), and the original αp/βtrans interface in the bimodal microstructure largely disappears, forming a blurry interface between the semi-equiaxed αp phase (equiaxed αp phase that is grew through by the thin β lamellae) and the transformed β microstructure (βtrans). The formation of dense slip bands inside the semi-equiaxed αp phase in the S-EM is inhibited by the thin β lamellae during the tensile deformation. The special characteristics of the S-EM reduce the stress concentration at the interface, and the crack initiation probability in the blurry semi-αp/βtrans interface decreased compared to the distinct αp/βtrans interface in a conventional equiaxed microstructure (EM). Moreover, the ultimate tensile strength of the S-EM is higher than that of the EM with a slight loss of plasticity.

Effect of Heat Treatment on Microstructure and Tensile Property of Ti-6Al-6V-2Sn Alloy

The microstructure and tensile properties of Ti-6Al-6V-2Sn alloy heat-treated at different solution and aging temperatures has been systematically investigated. Specimens were solution-treated at 970 °C (above the β transus) and 920 °C (below the β transus), respectively, followed by water quenching. When the alloy is quenched from 970 °C, α’ (hcp) and α˝ (orthorhombic) martensite phases co-exist in the microstructure. When it is quenched from 920 °C, α´ martensite phase does not form, while equiaxial primary α (αp) phase and α˝ are found in the microstructure. The results also show that the strength of the alloy increases but the ductility deteriorates as the solution temperature becomes higher when the aging treatment is unchanged. This is because the volume fraction of equiaxial αp phase is lower but the volume fraction of the acicular secondary α (αs) phase is higher for higher solution temperature. When the alloy is aged at different temperatures after the solution treatment at 900 °C, the strength of the alloy decreases with the increase of aging temperature and the ductility shows the opposite trend as the size of the acicular αs becomes longer and its volume fraction is lower at higher aging temperature.

Analysis of the Microstructure and Mechanical Properties during Inertia Friction Welding of the Near-\u03b1 TA19 Titanium Alloy

The current research of titanium alloy on friction welding process in the field of aero-engines mainly focuses on the linear friction welding. Compared to the linear friction welding, inertial friction welding of titanium alloy still has important application position in the welding of aero-engine rotating assembly. However, up to now, few reports on inertial friction welding of titanium alloy are found. In this paper, the near-alpha TA19 titanium alloy welded joint was successfully obtained by inertial friction welding (IFW) process. The microstructures and mechanical properties were investigated systematically. Results showed that the refined grains within 15‒20 μm and weak texture were found in the weld zone due to dynamic recrystallization caused by high temperature and plastic deformation. The weld zone consisted of acicular α′ martensite phase, αp phase and metastable β phase. Most lath-shaped αs and β phase in base metal were transformed into acicular martensite α′ phase and metastable β phase in thermo-mechanically affected zone and heat affected zone. As a result, the microhardness of welded joint gradually decreased from the weld zone to the base metal. Tensile specimens in room temperature and high temperature of 480 °C were all fractured in base metal illustrating that the inertia friction welded TA19 titanium alloy joint owned higher tensile strength compared to the base metal.

Effect of heat treatment on the microstructure, microhardness and impact toughness of TC11 and TC17 linear friction welded joint

The service life of whole blisks for the aero-engine compressor is sharply decreased due to the dissimilar linear friction welded (LFWed) joint of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si (TC11) and Ti–4Mo–4Cr–5Al–2Sn–2Zr (TC17) alloys, which have the different microstructure and mechanical properties. To optimize the microstructure and performance of the joint, and improve service life of the whole blisks, the post-weld heat treatment (PWHT) with annealing, solution plus aging were adopted for the welded joint. The results show that the nucleation rate of primary αP phase increases obviously, which are spheres, rods and strips, and orthorhombic martensite αs phases transform into hexagonal martensite αs phases interlaced evenly on the β phase matrix on the WZ of TC17 side; meanwhile, acicular hexagonal martensite αs phases transform into lamellar αs phases on the WZ of TC11 side after solid solution and dual aging. The difference of the microhardness on both sides of the joint is narrowed significantly. The impact toughness of the joint is improved sharply from 9.3 J/cm2 after welding to 27.6 J/cm2 with an increase of 200%. In addition, the dissolution and growth model of primary αp phases were established, which can reasonably predict the nucleation and grow process of α phase. Furthermore, the narrowed discrepancy of the microhardness and increased impact toughness can be attributed to the decreased slice thickness of martensite αs phase, based on the dislocation slip mechanism and distorted discordant model.

Tensile deformation mechanism and micro-void nucleation of Ti-55531 alloy with bimodal microstructure

Tensile fracture of near-β-titanium alloys with bimodal microstructure （BM） is very sensitive to micro-void nucleation. In this study, micro-void nucleation, growth, and propagation during tensile fracture of Ti-55531 alloy are investigated. α/β grain boundary (α/β-GB) and α/β grain boundary shear (α/β-GBSH) micro-voids are the main void sources, accounting for 83% of the total voids. The reason for the formation of α/β-GB voids is the stress concentration caused by the difference in the strength of the primary α (αp) and β transformation (βtrans) microstructure and the low strength of the non-coherent αp/β interface. α/β-GB voids eventually lead to fractures along the αp/β interface. α/β-GBSH voids are caused by the difference in the strength of the αp phase, the grain boundary α phase and the βtrans structure, which will cause the αp phase to tear and fracture along the grain boundary α-phase. The {112¯2} twin is first observed in near-β-titanium alloy, which explains the high plasticity of the alloy.

Microstructure and Mechanical Properties of a Novel Ti-6.5Al-2Sn-4Zr-1.5Mo-2Nb-0.25Fe-0.2Si High-Temperature Titanium Alloy

The Ti-6.5Al-2Sn-4Zr-1.5Mo-2Nb-0.25Fe-0.2Si (wt%) alloy is a novel two-phase high temperature alloy for short-term application. The effects of different heat treatments on the microstructure and mechanical properties were investigated through electron probe microanalysis (EPMA), optical microcopy (OM), scanning electron microscope (SEM), electron backscattered diffraction (EBSD) and tensile tests at room temperature and 650°C. Subjected to the annealing treatment at α+β region (1010 °C/2 h, FC to 990 °C+990 °C/2 h, AC), the microstructure was composed of bimodal structure, which consists of equiaxed primary α (αp) phase and lamellar transformed β (βt) structure. As a strong β stabilizer, the content of Fe in α phase is much less than that in β phase. Annealing at β region (1040 °C/2 h, AC) resulted in the formation of widmannstatten structure, consisting of coarse raw β grain and secondary α phase precipitated on the β grain. With respect to the tensile property, different heat-treated alloys obtained similar strength. However, widmannstatten structure was characterized by lower plasticity, with the elongation only half that of bimodal structure. The fracture characteristics at room temperature for the alloy with bimodal structure and widmannstatten structure are dominated by ductile fracture and cleavage fracture, respectively.

Hydrogen-Induced Microstructure Optimization of the Ti-6Al-4V Alloy

To obtain advanced materials through the development of traditional materials without the addition of another alloying element, advanced heat treatment can be used. One such innovative process is a thermo-hydrogen treatment (THT); it facilitates a purposeful adjustment of an improved microstructure using hydrogen as a temporary alloying element within heat treatment. In this paper, the five-step process of homogenization, hydrogenation, solution treatment, dehydrogenation, and aging was used in THT. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), backscattered electron (BSE), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) were utilized to analyze the phases and phase transformations in Ti-6Al-4V. Three different homogeneous microstructures were established for the investigation using different homogenization parameters values. The hydrogenation was carried out for these microstructures via hydrogen gas charging leading to hydrogen concentrations for the formation of hydride (δ TiH2). After the solution treatment at a temperature above β transus temperature (Tβ), the metastable phases of a martensitic structure consisting of a mixture of α ́ (hcp) and α ́ ́ (orthorhombic) was found. Steps 4 and 5 of THT were a vacuum annealing (hydrogen degassing) followed by aging treatment. The aging treatment was applied to complete the martensite phase decomposition and the precipitation of two phases. By means of this THT cycle, very fine equiaxed microstructures could be established. These microstructures consist of the αs phase (secondary α) in the β phase matrix and the α2 phase (Ti3Al) in the αp phase. The precipitation of these phases increases the strength of the Ti-6Al-4V alloy and, consequently, enhances the mechanical properties. No evidence of the δ phase was found.

Evolution of microstructure and texture of a near α titanium alloy during forging bar into disk

Abstract Ti6242S alloy bar with equiaxed microstructure and random textures was selected to investigate evolution of microstructure and texture during the process of forging bar into disk by isothermal constant speed hot compression test. Experimental investigation shows that αp grains experienced bend, rotation under prismatic slip system action and further separated into refined grains by dynamic recovery or dynamic recrystallization with the increase of strain. Sharp macrozones existed in Ti6242S alloy were completely eliminated when strain arrived at 50%. Two main texture components of 2 ¯ 0> and 2 ¯ 3>//compression direction were developed in all deformation strain. The former was mainly caused by the activated prismatic slip system motion of α phase during deformation and the precipitated αs phase neighboring αp phase after β→α transformation which hold the same orientation with αp phase. The latter was the results of αp phase dynamic recrystallization and polycrystalline coordinated deformation of αp/αp or αp/β phases, likewise, the same orientation αs laths with neighboring αp phase preferentially precipitated after β→α. It is also demonstrated that if the strain surpassed 50%, especially for multi-step deformation to 70% dynamic recrystallization lightly softened the texture density.