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Ti-6Al-4V alloy has limitations in terms of globularization when using additive manufacturing (AM), which requires a final forging step. To optimize this approach, this study investigated the effect of processing variables on the dynamic/static globularization of AM-processed Ti-6Al-4V alloy. Eight double-cone specimens were prepared with different processing variables such as effective strain, solution treatment, hot-forging temperature, and subsequent annealing. Microstructural evolutions were quantitatively characterized to interpret the kinetics of globularization based on the aforementioned variables. In particular, the combination of solution treatment, low-temperature (1073 K) forging, and subsequent annealing significantly accelerated the overall globularization. Such an accelerating effect stemmed from the reduced path for boundary splitting in fine α′ martensitic laths, which was induced via solution treatment. This accelerating effect disappeared at a high temperature (1223 K), which implies the necessity of optimizing the thermomechanical route to exploit martensitic laths.

The present work investigates the thermal stability and mechanical properties of a Co–20Cr–15W–10Ni (wt%) alloy with a bimodal grain (BG) structure. The BG structure consisting of fine grains (FGs) and coarse grains (CGs) is thermally stable under high-temperature exposure treatments of 760 °C for 100 h and 870 °C for 100–1000 h. The size of both FGs and CGs remains no significant changes after thermal exposure treatments. The microstructural stability is associated with the slow kinetics of grain growth and the pinning of carbides. The thermal stability enables to maintain the BG structures, leading to the same mechanical properties as the sample without thermal exposure treatment. In particular, the BG alloy samples after thermal exposure treatment exhibit superior mechanical properties of both high strength and high ductility compared to the unimodal grain (UG) structured ones. The BG structure of the alloy samples after thermal exposure is capable of avoiding severe loss of ductility and retaining high strength. More specifically, the ductility of the BG alloy samples after thermal exposure treatments of 870 °C for 500–1000 h is ten times higher (44.6% vs. 3.5% and 52.6% vs. 5.0%) than that of the UG ones. The finding in the present work may give new insights into high-temperature applications of the Co–20Cr–15W–10Ni alloy and other metallic materials with a BG structure.

Ultrafine-grained (UFG) structure is beneficial for overcoming the strength-ductility trade-off and enhancing the superplasticity of two-phase Ti alloys. Recently, it has been demonstrated that com...

We used selective laser melting (SLM) to produce a three-dimensional specimen of pure Ti and found that an equiaxed grain structure and weak crystallographic texture could be achieved simultaneously via subsequent heat treatment. These traits have never been attained simultaneously in pure Ti prepared by traditional methods such as casting and rolling. This remarkable achievement was possible because recrystallization occurred during heat treatment without plastic deformation; such deformation introduces stored energy that drives recrystallization but inevitably causes a strong texture to develop. The occurrence of recrystallization was attributed to unique features of the SLM process including a very high cooling rate and repetitive layer stacking. These features generated considerable stored energy by affecting solidification and the β → α phase transformation. Steep in-grain orientation gradients and a fine lath structure also contributed to the activation of recrystallization by facilitating recrystallization nucleation. The heat-treated specimen showed tensile properties with significantly reduced anisotropy. This finding will provide new strategies for developing isotropic metallic materials and may introduce new applications of SLM.

A nearly fully dense β stabilized γ-TiAl alloy was additively manufactured by electron beam melting. The as-fabricated specimen exhibited a fine structure consisting of α2/γ colonies ( 50%) in the sample at 800 °C. Annealing and subsequent stabilization treatments resulted in α2/γ colonies (

In this study, the deformation mechanism of a metastable Ti-4Al-4Fe-0.25Si-0.1O alloy was investigated via ex-situ electron backscattering diffraction (EBSD) and field-emission transmission electron microscopy (FE-TEM). A stress-induced α′-martensitic transformation occurred from the retained β phase. New variants of α′-martensite were nucleated and grew in the transformed α′-martensite under increasing levels of applied stress. Microstructural observations revealed that the orthorhombic nanodomains were sheared in the prior α′-martensite; these sheared nanodomains contributed to the formation of different variants of α′-martensite. This additional martensitic transformation resulted in an excellent combination of strength (ultimate tensile strength; UTS: 1340 MPa) and ductility (30%).

Ti–6Al–4V alloy is a typical 3D printing metal, and its application has been expanded to various fields owing to its excellent characteristics such as high specific strength, high corrosion resistance, and biocompatibility. In particular, direct energy deposition (DED) has been actively explored in the fields of deposition and the repair of large titanium parts. However, owing to the complicated thermal history of the DED process, the microstructures of the fusion zone (FZ), heat-affected zone (HAZ), and base metal (BM) are different, which results in variations of their mechanical characteristics. Therefore, the process reliability needs to be optimized. In this study, the microstructure and hardness of each region were investigated with respect to various DED process parameters. An artificial neural network (ANN) model was used to correlate the measured characteristics of the FZ, HAZ, and BM of Ti–6Al–4V components with the process parameters. The variation in the mechanical characteristics between the FZ, HAZ, and BM was minimized through post-heat treatment. Heat treatment carried out at 950 °C for 1 h revealed that the microstructure and hardness values throughout the component were homogeneous.

Step rolling can be used to mass-produce ultrafine-grained (UFG) Ti-6Al-4V sheets. This study clarified the effect of subsequent annealing on the tensile properties of step-rolled Ti-6Al-4V at room temperature (RT) and elevated temperature. The step-rolled alloy retained its UFG structure after subsequent annealing at 500–600 °C. The RT ductility of the step-rolled alloy increased regardless of annealing temperature, but strengthening was only attained by annealing at 500 °C. In contrast, subsequent annealing rarely improved the elevated-temperature tensile properties. The step-rolled Ti-6Al-4V alloy without the annealing showed the highest elongation to failure of 960% at 700 °C and a strain rate of 10−3 s−1. The ambivalent effect of annealing on RT and elevated-temperature tensile properties is a result of microstructural features, such as dislocation tangles, subgrains, phases, and continuous dynamic recrystallization.

From the point of view of designing materials, it is important to study the complex correlational research that involves measuring several variables and assessing the relation among them. Hence, the notion of machine-oriented data modeling is explored. Among various machine-learning tools, artificial neural networks (ANN) have been used as a stimulating tool to solve engineering-related issues. In this study, the ANN model is designed and trained to correlate the complex relations among composition, temperature and mechanical properties of 25Cr-20Ni-0.4C austenitic stainless steel. The developed model was exploited to estimate the composition&ndash, property and temperature&ndash, property correlations. The ANN predictions are well suitable for experimental results. The model was able to correlate the complex nature among input and output variables. The model was used to investigate the effect of service temperature on the mechanical properties of 25Cr-20Ni-0.4C steels over a wide temperature range. The effective response of the alloying elements on the mechanical properties of ambient as well as elevated temperatures was quantitatively estimated with the help of the index of relative importance (IRI) method. Hence, this handy technique is the best tool to overcome the designing complications and to develop the components having remarkable properties.

The γ-TiAl alloys fabricated by additive manufacturing have gathered significant attention in recent years due to their unique microstructural features and properties. However, the oxidation kinetics of additively manufactured γ-TiAl alloys have not been studied thoroughly. Herein, the TNM-B1 alloy was fabricated with the help of electron beam melting (EBM) for the first time and tested for 100 h over a temperature range of 800 °C-1000 °C; this was aimed at investigating its isothermal oxidation kinetics. Subsequently, the exposed samples were systematically analyzed to study the oxide scales, which was achieved with the help of X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Spallation was observed at 900 °C and 1000 °C after an exposure of 30 h and 5 h, respectively. The results showed that the oxide comprises alternative layers of Al2O3, TiO2, and mixture phases. The oxidation resistance of the EBM-processed TNM-B1 alloy was observed to be superior to those of the conventionally processed TNM-B1 and other γ-TiAl alloys. Finally, the significance of a fine fully lamellar microstructure on the oxidation kinetics of TNM-B1 alloy was explained.

A bimodal grain structure consisting of fine grains (d ~ 1–2 µm) and coarse grains (d ~ 10–20 µm) was achieved in the biomedical Co–20Cr–15W–10Ni alloy via medium cold rolling (area reduction of 50%) followed by short-term annealing (15 min) at relatively low temperatures (950–1100 °C). The medium cold rolling induced a heterogeneous microstructure featuring the coexistence of severely deformed regions and weakly deformed domains. With short-term annealing at low temperatures, fine and coarse grains were preferentially recrystallized in severely deformed regions and weakly deformed domains, respectively, resulting a bimodal grain size distribution. During tension, dislocations were more rapidly generated in the fine grains, thus increasing the strain hardening, while they glided longer on the {111} planes in the coarse grains, contributing a high ductility. The combination of these two factors provided well-balanced strength–ductility behavior with an ultimate tensile strength of 1278 MPa, a yield strength of 787 MPa, and an elongation to fracture of 53%, making the alloy suitable for surgical implant and stent applications where the strength and ductility are both important to ensure mechanical reliability in a human body.

Commonly used high-temperature near-alpha titanium alloys contain Al, Zr and Si as their alloying elements. Significant losses of mechanical properties and cleavage mode failures are evident due to the presence of Ti3Al and S2 types of zirconium silicides ((TiZr)6Si3) in these alloys. We developed a new alloy (Ti-6.5Al-3.0Sn-4.0Hf-0.2Nb-0.4Mo-0.4Si-0.1B) by replacing Zr with Hf to avoid the formation of zirconium silicides. Aging at 700 °C for five hours successfully eliminated the Ti3Al phase and substantially improved the room-temperature ductility. Tensile tests were carried out at room temperature and at 650 °C. The mechanical properties of the present alloy were significantly improved compared to those of existing high-temperature titanium alloys under testing at ambient and elevated temperatures.

Transformation-induced plasticity (TRIP) Ti alloys are promising structural materials that offer high strength and ductility. However, these alloys often include heavy, expensive, and high-melting-point β-stabilizing elements such as V, Nb, Mo, and W. Herein, an artificial neural network (ANN) was used to develop a Ti–Al–Fe–Mn-based TRIP alloy comprising lighter and/or cheaper elements. The ANN model was trained with 30 experimental tensile datasets for heat-treated (830–920 °C) Ti–4Al–2Fe–xMn (x = 0–4 wt%) alloys, and used to generate 400 tensile datasets with more finely tuned composition and temperature intervals. Based on the predicted data, an 883 °C-heat-treated Ti–4Al–2Fe–1.4Mn alloy was produced (conditions not used in the training datasets), which exhibited ultra-high specific strength (289 MPa·cm3/g) and high elongation (34%). Thus, the ANN approach successfully led to the development of a new alloy while minimizing the number of labor-intensive and time-consuming experiments.

Gum metals feature properties such as ultrahigh strength, ultralow elastic modulus, superelasticity, and superplasticity. They are composed of elements from Groups 4 and 5 of the periodic table and exist when the valance electron concentration (\(\overline{e/a}\)) is 4.24; the bond order (\(\overline{\text{Bo}}\)) is 2.87; and the “d” electron-orbital energy level (\(\overline{\text{Md}}\)) is 2.45 eV. Typical compositions include Ti–23Nb–2Zr–0.7Ta–O and Ti–12Ta–9Nb–6Zr–3 V–O, which contain large amounts of heavy Group-5 elements such as Nb and Ta. In the present study, to improve the specific strength of a multifunctional beta Ti alloy, three alloys (Ti–20Nb–5Zr–1Fe–O, Ti–12Zr–10Mo–4Nb–O, and Ti–24Zr–9Cr–3Mo–O) were designed by satisfying the above three requirements while adding Fe, Mo, and Cr, which are not only lightweight but also have strong hardening effects. Microstructural and mechanical property analyses revealed that Ti–20Nb–5Zr–1Fe–O has a 25% higher specific strength than gum metal while maintaining an ultralow elastic modulus.

Microstructure, mechanical properties as well as phase transformation behavior of newly developed Ni 49 Ti 36 Hf 15-x Ta (x=0, 3, 6, 9, 12) high temperature shape memory alloys has been investigated. Ni 49 Ti 36 Hf 15 alloy consists of (Ti,Hf) 2 Ni phase which is brittle and incoherent with the martensite matrix. However, addition of Ta in place of Hf suppresses the brittle (Ti,Hf) 2 Ni phase formation with white Ta rich phase. More than 70% reduction in area has been observed in Ta added Ni 49 Ti 36 Hf 15 high temperature shape memory alloys. The austenitic transformation temperature is well above 200 °C (473 K) for lower Ta concentrations with up to 6 at. %; however; at higher Ta concentrations (i.e. above 9 at. %) decreases significantly. Overall, the addition of Ta greatly influences the hot deformation behavior and high temperature shape memory properties of Ni 49 Ti 36 Hf 15 alloy. The present paper discusses the detailed investigation on Ni 49 Ti 36 Hf 15-x Ta (x=0, 3, 6, 9, 12) high temperature shape memory alloys.

Direct energy deposited (DED) Ti-5Ni (wt.%) alloys promote significant grain refinement with fine equiaxed grains (~30–50 μm); they have discontinuous grain boundary α, fine eutectoid α laths (~1 μm width), and α + Ti2Ni phases. Post-heat treatment below beta transus for 24 h stimulates the formation of near equiaxed α, and the temperature significantly influenced the microstructural constituents, which included pre-eutectoid α + retained β + Ti2Ni (760°C), pre-eutectoid α + α’ + Ti2Ni (800°C), and fully α’ + Ti2Ni (850°C) phases. The tensile properties of the as-fabricated and the heat-treated alloy remarkably improved compared with the other conventionally processed eutectoid Ti-Ni binary alloys. Thus, the proposed approach opens up new opportunities and possibilities for fabricating other eutectoid-forming alloy systems with improved manufacturing efficiency, which can overcome limitations in the other manufacturing processes.

For this study, several TiAl–Nb–Si-based alloys were designed for a ductility improvement, whereby the high-temperature strength and oxidation resistance were not sacrificed. The environmental properties under the cyclic oxidation behaviors of the TiAl alloys were evaluated at 900 °C for up to 360 cycles. The compositions of the as-cast alloys determined their microstructures, and the cyclic oxidation behavior of the selected alloy was relatively comparable to that of a commercial TiAl alloy that is currently used in automotive engines.

A newly developed Ti–46Al–6Nb-0.5W-0.5Cr-0.3Si-0.1C alloy was oxidized isothermally and cyclically in air, and its high-temperature oxidation behavior was investigated. When the alloy was isothermally oxidized at 700 °C for 2000 h, the weight gain was only 0.15 mg/cm 2 . The parabolic rate constant, k p (mg 2 /cm 4 ·h), measured from isothermal oxidation tests was 0.002 at 900 °C and 0.009 at 1000 °C. Such excellent isothermal oxidation resistance resulted from the formation of the dense, continuous Al 2 O 3 layer between the outer TiO 2 layer and the inner (TiO 2 -rich, Al 2 O 3 -deficient) layer. The alloy also displayed good cyclic oxidation resistance at 900 °C. Some noticeable scale spallation began to occur after 68 h at 1000 °C during the cyclic oxidation test.

A hybrid study of in-situ transmission electron microscopy experiments and finite element analyses was carried out in order to understand the effect of precipitates on crack initiation and propagation in TiNi–(Hf) alloys. Samples of Ti-rich (Ti–46Ni–15Hf) and Ni-rich (Ti–50.3Ni–15Hf) alloys were prepared. The alloys showed differences in room temperature ductility and the ability of rolling at 900 °C. From the in-situ straining transmission electron microscopy experiments, it was revealed that the crack growth behavior is different in the two TiNi–(Hf) alloys. Cracks were easily initiated and propagated along the interface between the matrix and precipitates of Ti–46Ni–15Hf alloy which showed no ductility. However, in the Ti–50.3N–15Hf alloy with some ductility at room temperature, the precipitates were not brittle, and the cracks did not interact with the precipitations instead propagating in a ductile manner with a lot of dislocation activities. The fracture toughness of an interface crack between the matrix and the precipitates was determined by matching numerical crack opening profiles in finite element analyses with the experimental counterpart.

Owing to the high production cost of a high alloying system, the commercialization of β-Ti alloys with high elastic admissible strain (= yield strength/elastic modulus) for biomaterials is challenging. To reduce the alloying elements and maintain a high elastic admissible strain, a new alloy design of the metastable α+β-Ti alloy (Ti–4Al–2Mo–2Fe–2Cr) is proposed in this study. The microstructure of the alloy consists of an α phase with a high yield strength and a metastable β phase with a low elastic modulus. The existence of the hard α phase increases the critical stress for stress-induced martensitic transformation due to stress and strain partitioning. In addition, heat treatment in the α+β regime is an effective method for grain refinement, resulting in high strength without increasing the elastic modulus. This alloy exhibited an excellent combination of strength and ductility with a high elastic admissible strain. Moreover, electron backscattering diffraction and field-emission transmission electron microscopy were used to understand the mechanical behavior of the α+β Ti alloy.

In this study, the low-cycle fatigue behavior of a cryogenic-rolled commercially pure titanium alloy was investigated, and compared with those of undeformed and room-temperature rolled ones. The amounts of deformation twins increased to >69.8% after cryogenic temperature rolling, and to 27.5% after room temperature rolling. Strain-controlled low-cycle fatigue tests were performed at total strain amplitudes 0.4 % ≤ Δ e t 2 ≤ 1.2%. The Coffin-Manson and hysteresis energy-based models confirmed that low-cycle fatigue resistance was remarkably improved with increasing the volume fraction of deformation twins by pre-deformation. As the proportion of deformation twins in the microstructure increased, the hysteresis loop area decreased, the striation spacing decreased, and the severity of crack-deflection behavior increased. At low Δ e t 2 = 0.4%, dislocation recovery was suppressed in the pre-deformed microstructure, so cyclic behavior was stable. However, at Δet/2 ≥ 0.8%, the recovery became the predominant mechanism, so well-defined cell structures formed and cyclic softening occurred. The smaller dislocation cells formed in RTR and CTR were considered to cause more severe crack arrest than in AR.

The use of selective laser melting (SLM) has become more common with regard to the fabrication of high-strength Ti-6Al-4V components. However, the as-SLMed Ti-6Al-4V alloy parts typically exhibit low ductility because of the formation of acicular and brittle α′ martensites. It is essential to use post-heat treatment to change their microstructures to achieve superior mechanical properties. In this study, a particular multi-step heat treatment (MSHT) was applied to the Ti-6Al-4V alloy samples fabricated via SLM. Conventional post-heat treatments were conducted for comparison. The microstructures and tensile properties of the as-SLMed and heat-treated samples were investigated. The as-SLMed Ti-6Al-4V sample is dominated by plate and acicular α′ martensites, which exhibit high strength (1280 MPa) and low ductility (9.0%). The conventional solution plus aging or stress-relieving treatment cannot result in a good combination of strength and ductility. A single step annealing process at 700 °C for 2 h changes the martensitic structure to a fine lamellar α+β structure, which provides a strong and ductile Ti-6Al-4V with an ultimate tensile strength of 1108 MPa and a total elongation of 17.6%. In contrast, the MSHT process facilitates the globularization of α and, thereby generates a nearly equiaxed structure, resulting in superior ductility (21.8% of total elongation), while maintaining a moderate ultimate tensile strength of 953 MPa.

An artificial neural network (ANN) model was designed to correlate the complex relations among composition, temperature, and mechanical properties of 18Cr-12Ni-Mo austenitic stainless steels. The developed model was used to estimate the composition-property and temperature-property correlations with 97% and 91% accuracy, for train and unseen test datasets. The ANN predictions are more accurate with experimental results as compared with the calculated properties of the existing model. The effective response of the alloying elements on the mechanical properties at ambient as well as elevated temperatures was quantitatively estimated with the help of the index of relative importance (IRI). The calculated results of the ANN model beneficial for both researchers as well as designers to guide actual experiments. Hence, this proposed technique will be helpful in developing the components of austenitic stainless steel with desired properties.

Electron beam melted Ti–48Al–2Cr–2Nb alloy has undesirable creep resistance due to a near-gamma structure that is not proper for creep. We aimed to control the microstructure of EBM-built Ti–48Al–2Cr–2Nb using two-step heat treatment to improve the high-temperature (750 °C) creep resistance while also maintaining room-temperature ductility. As a result of two-step heat treatment, heat-treated EBM-built Ti–48Al-2r-2Nb alloy has a nearly lamellar microstructure with a thin lamellar structure and fine equiaxed gamma grains. High-temperature creep resistance was significantly improved after two-step heat treatment by inducing a thin lamellar structure. Room temperature ductility was also maintained even while containing a lamellar structure.

The present work investigates the microstructural evolution of a biomedical Co-20Cr-15W-10Ni alloy during multi-pass thermomechanical processing (TMP) and clarifies its effect on the tensile properties. The results indicate that multi-pass TMP consisting of repetitive cold groove-rolling followed by annealing is attractive for fabricating the alloy rod for fine wire and mini-tube making. The multi-pass TMP provided significant grain refinement from 100 to 4 μm via five repetition and, can create bimodal grain structures consisting of fine (≈3 μm) and coarse grains (4−16 μm). The bimodal grain structure provided a superior strength-ductility combination, a yield strength of 593−738 MPa and ultimate strength of 1197−1304 MPa with a uniform elongation of 54.7−61.1%. Changing the annealing condition in the final TMP step enables to further refine the grains to 2 μm and also modify the grain size distribution and, the dispersion and fraction of nano-carbides. Thus, the mechanical properties of the Co-20Cr-15W-10Ni alloy can be further optimized, leading to a yield strength of 795−890 MPa and ultimate strength of 1328−1363 MPa with a uniform elongation of 49.4−64.0%.

In this study, a Ti–3Mo–3Cr–2Fe–2Al alloy (α + β Ti alloy) showing twinning- and transformation-induced plasticity was investigated. The existence of α yielded fine-grained β phases. The β grains, comprising low-angle boundaries, produced a high yield strength of 737 MPa despite the low α volume percentage. Martensite and twin interactions with these boundaries induced local stress concentrations. These led to a high work-hardening rate compared to that in a full β microstructure. The α + β Ti alloy showed an excellent combination of strength (1324 MPa) and ductility (0.37).

Mechanical and oxidation properties of newly-developed TiAl alloys were investigated. The TiAl alloys in this study were manufactured by casting and no further heat-treatment was conducted. In this study, Ti–(40–44)Al–(3,6)Nb–(W,Cr)–Si–C alloys were developed and the possibility of using them for turbine wheels in automobile engines was examined in comparison with commercial TiAl alloys. The new alloys developed in this study showed excellent tensile strength at room temperature and high temperature (900 °C) as well as good oxidation resistance at 900 °C compared to the commercial TiAl alloy. Moreover, the new alloys showed much better castability than the commercial TiAl alloy.

The present study describes the influence of β stabilizers (namely Fe and Cr) on the microstructural evolution and mechanical behavior of Ti–6Al–4V alloy fabricated by direct energy deposition. As the β stabilizer content increases from 1 to 4 wt%, the coarse columnar-grained morphology typical of additively manufactured Ti–6Al–4V is significantly refined, with a columnar-to-equiaxed transition. Furthermore, reheating of the deposited layers by the deposition of overlaying layers during fabrication induces an element partitioning effect, resulting in the formation of fine-grained α and isothermal ω. Hence, the heat profile during direct energy deposition resembles conventional solution and aging treatments. In addition, increasing the amount of β stabilizer steadily increases the room temperature strength of the Ti–6Al–4V alloy. The alloy with 4 wt% Fe showed the maximum strength, of ∼1.5 GPa.

Electron beam melting (EBM) has a lower cooling rate than other additive manufacturing processes (selective laser melting, laser metal deposition, etc.), so it is suitable for manufacturing titanium aluminide-based intermetallics. In this study, a gamma Ti-48Al-2Cr-2Nb alloy was manufactured using EBM, and its microstructure and high-temperature creep behaviors were investigated. Conventional Ti-48Al-2Cr-2Nb was also used for comparison and analysis. Initial microstructural observation confirmed that the EBM-built Ti-48Al-2Cr-2Nb alloy had a near-gamma (NG) structure, whereas the conventional Ti-48Al-2Cr-2Nb alloy had a fully lamellar (FL) structure. Room temperature and high temperature compression tests confirmed that the Ti-48Al-2Cr-2Nb alloy with the NG structure had lower strength in all temperature ranges, and the yield-strength anomaly phenomenon occurred in both materials. A 750 °C multi-step creep test confirmed that the EBM-built Ti-48Al-2Cr-2Nb alloy with lower strength had lower creep resistance as well. Microstructural observation after creep deformation confirmed that dislocation movement and mechanical twins were formed dominantly in the creep deformation of the material with the NG structure. For the material with the FL structure, diffusional creep in low-stress regions and large deformation at the colony boundary in high-stress regions dominated the creep deformation. Correlations between the microstructure, strength, and multi-step creep properties were discussed based on these findings.

Unfortunately, the acknowledgements were incomplete in the original version of this article.

Deformation behavior of Ti–6Al–4Fe alloys with high strength and formability was investigated by conducting high temperature tensile test. In tensile test at room temperature, the strength of Ti–6Al–4Fe alloys was higher than that of a commercial Ti–6Al–4V alloy with a similar level of elongation. In particular, the high temperature (700 °C) elongation was dramatically increased in Ti–6Al–4Fe alloys showing ∼800% superplasticity. Grain boundary sliding and suppression of cavity formation (i.e., easier accommodation) during gb sliding due to beta phase were found to be possible mechanisms for the superplasticity of Ti–6Al–4Fe alloys.

The temperature dependence of strengthening mechanisms in the nanocluster-strengthened 14YWT alloy was investigated to elucidate the relative significance of contributing mechanisms in different temperature ranges. This study was also aimed at providing the prediction capability of yield strength for the nanostructured ferritic alloys over a wide range of temperature. The four major strengthening mechanisms: the Peierls stress, grain boundary strengthening, direct nanocluster strengthening, and dislocation forest hardening, were taken into account in the calculation, and their roles and characteristics in different temperature ranges were extensively discussed. The results indicated that the contribution of grain boundary strengthening to total strengthening was the most significant component. Yield strength calculation was made by combining all the strengthening components and the results were compared with the experimental data. Further, the validation of the proposed approach was attempted by applying to the yield strength of other alloys.

The kinetics for the termination migration of a rod-type alpha particle in a two-phase Ti alloy was predicted on the basis of the edge recession theory. The developed model quantified the effect of geometrical dimensions and diffusional factors on the spheroidization rate. Comparison with the experimental results of Ti–6Al–4Fe showed that the model can provide a reasonable prediction of the time to complete the static spheroidization of rod-type particles.

The scope of this investigation is to evaluate the effect of joining parameters on the microstructural features and mechanical properties of dissimilar aluminum alloys, 1mm-thickness fixing AA6K31 at the top position and fixing AA5J32 at the bottom position. The friction stir lap welds were studied under various welding conditions, rotation speed of 1000, 1250, 1500rpm and welding speed of 100, 300, 500, 700mm/min, respectively. Mechanical test has been investigated in terms of tensile shear test and hardness test. The results showed that three type nugget shapes such as onion ring, zigzag type, hooking with the void, have been observed with revolutionary pitch. All welding conditions fractured at the HAZ of top plate, A6K31 and also the strength compare with base metal of lap joints were low efficiency, 52~63%. The thickness of fractured position was decreased with the lower heat input conditions. The relationships were excellent due to linear between the effective thickness of fractured position and peak load. The fractured position was the interface between joint area and not joint area. Also the strength efficiency compared with base metal was lower than decreasing rate of thickness because the hardness was decreased at fractured position due to softened material.

This study investigated the effect of the addition of scandium on the hot extrudability of 7075 alloys. Three kinds of Al-Zn-Mg-(Sc) alloys with up to 0.30 wt.% Sc were prepared, including 7075 aluminum alloy without Sc (AA7075). Differential scanning calorimetry (DSC) analyses and hot compression tests were conducted to examine the incipient melting temperature of the alloys produced. The addition of 0.30 wt.% Sc resulted in significant grain refinement and a fully equiaxed microstructure. Hot compression tests were conducted to determine the critical damage values based on Cockroft-Latham criterion. Then, the surface quality of an extruded bar of each alloy was simulated with a finite element code. The surface condition of an extruded bar was predicted and compared with the experimental results. After hot extrusion, AA7075 without the addition of Sc showed a more moderate surface quality than the alloys modified with Sc. However, AA7075 revealed coarse-grained bands just below the surface area. Extrusion limit diagrams were constructed for the three alloys.

In this study, the modeling of recrystallization and grain growth was investigated to predict the microstructure evolution during hot working of INCONEL 783 superalloy (Alloy 783). The recrystallization model was constructed on the basis of the Avrami formation. A least-square optimization technique was used to determine several important parameters within the model from isothermal heat treatment and hot compression test results. High temperature compression tests were carried out under different temperatures, strain rates and strain conditions. The model for describing the recrystallization and grain growth behaviors of Alloy 783 was implemented onto the user-subroutine of a commercial FE code. In order to demonstrate the reliability of the model, the microstructure evolution of Alloy 783 alloy during the hot compression test was simulated and was accorded well with the experimental results.

Since fastener bolt for airplane require high specific strength and corrosion resistance, Ti-6Al-4V alloy is widely used. However, the Ti-6Al-4V bolt is generally manufactured by cutting and rolling because of their poor workability. The aim of present work is to develop hot forming technology for high strength Ti-6Al-4V. Various heat-treatments were applied to specimen in order to increase hot-workability and prevent galling with die. Multiple forging were simulated with FE code to determine optimum process parameters including specimen temperature, strain rate, local strain, and thermal shrinkage. Forged samples were heat-treated again to increase their mechanical properties.

Hot workability of Ti -6 Al -4 V alloy with different initial microstructures was investigated by considering processing maps and the dynamic material deformation behavior. The emphasis has been focused on the effect of initial microstructure (equiaxed versus bimodal structure). Process maps were generated using the dynamic material model (DMM), unifying the relationships between constitutive deformation behavior, hot workability and microstructures evolution. Also, the flow instability was investigated using the various flow instability criteria and microstructural analysis. To establish the processing maps with different initial microstructures, high temperature compression tests were carried out at various temperatures and strain rates up to a true strain of 0.7. Microstructural changes occurring during the deformation were analyzed in terms of high temperature deformation mechanisms. Finally the useful instability criterion for predicting the forming defects was suggested through the compression test results with different temperatures and strain rates.

The high temperature deformation behavior and flow instabilities of Ni-Fe-Co base superalloy, INCONEL alloy 783 during hot working process were investigated with process maps consisting of a power dissipation of dynamic materials model (DMM) and various flow instability criteria. In order to establish the processing map of INCONEL alloy 783, hot compression tests were carried out under different temperature and strain rate conditions, with true strain up to 0.7. On the basis of the comparison between processing maps and microstructural analysis, the reliability of various flow instability criteria was estimated. Finally the useful instability criterion for predicting the forming defects was suggested through the compression test results and experimental observations of actual ring rolling process of INCONEL alloy 783.

Ingots of Ni-Ti-Hf shape memory alloys were prepared by vacuum arc re-melting. Isothermal hot compression tests were conducted at temperatures ranging from 700 to 1000 degrees C and at strain rates from 10(-2) s(-1) to 1.0 s(-1). A decrease in the Ni content below 50.2 at.% significantly deteriorated the hot workability due to the formation of a brittle second phase. Also, the low Ni content alloy showed poor workability when the temperature exceeded 900 degrees C. Additional compression tests were conducted under various conditions to clarify the effects of the chemical composition, solidification anisotropy, and the strain rate.

Effects of solution treatment on the microstructure and mechanical properties in wrought Alloy 718 were investigated. For the improvement of tensile and fatigue properties of wrought Alloy 718, a modified solution heat treatment(MSHT), heating at 1000 for 3 minutes followed by furnace cooling at the rate of 3/minute and holding at 985 for 8 minutes, was proposed. This treatment was performed repeatedly 3 times and the samples were subject to the standard aging treatment. For the purpose of comparison, a standard heat treatment(SHT) was also performed. The microstructures of modified solution heat treated specimens showed that small spherical shaped δ- phases were precipitated without grain growth, and the amount of δ-phases was smaller than that of standard heat treated specimens. However, the δ-phases of the standard heat-treated specimen showed needle-like morphologies. Tensile and low cycle fatigue tests were performed on both modified heat-treated and standard heat-treated specimens at room temperature, 350 and 650. Low cycle fatigue tests on the modified heat treated specimens showed promising results without reduction of strength. However, the tensile properties of modified solution treated specimens was almost the same as those of standard heat treated materials both at room temperature and 650.

Microstructure evolution of Ti-6Al-4V alloy during hot backward extrusion process was simulated with the combined approaches of finite element method (FEM) and microstructure prediction model. From experimental analysis, it can be found that the change of microstructure during hot forming process of titanium alloy has a close relation to α/β phase transformation and grain growth behaviour. A microstructure prediction model was established by considering the change of volume fractions and grain size of both phases varying with process variables and then implemented into the user-defined subroutine of FEM analysis. In order to demonstrate the reliability of the model, the volume fraction and grain size of primary α phase during the hot backward extrusion process of Ti-6Al-4V alloy were simulated. The simulation results were compared with the experimental ones.

In this study, microstructure evolution during ring rolling process of INCONEL alloy 783 (Alloy 783) was investigated with the combined approaches of 3-D FEM (Finite Element Method) simulation and microstructure prediction model. A microstructure prediction model was developed by considering the effects of process variables on recrystallization and grain growth behavior. The model was established by the analysis of hot compression and isothermal heat treatment test results. From the analysis of isothermal heat treatment tests, it can be found that the rate of grain growth dramatically increases after the dissolution of β phase, which acts as an obstacle to grain boundary movement. In order to consider the effect of β phase on grain growth, an additional equation in the transition temperature region was interposed. Microstructure evolution during ring rolling process of Alloy 783 was calculated by de-coupled approach between FEM analysis and microstructure prediction model. The prediction results were compared with the experimental ones. Introduction INCONEL alloy 783 (Alloy 783) is an oxidation-resistant, low expansion, nickel-cobalt-iron alloy with aluminium, chromium, and niobium additions [1]. The mechanical properties of Alloy 783 depend very much on grain size and the strengthening phases, γ′(Ni3Al-type) and β (NiAltype). Especially, grain size control in the hot working processes of this alloy is an important issue in pursing integrity of structure and properties. For instance, a fine grain size structure has high strength and good low-cycle fatigue properties while a larger grain size structure indicates good creep and defect tolerance properties. The grain size change of Alloy 783 during hot forming and heating is mainly controlled by the recrystallizaton and grain growth behaviours. The recrystallization behaviour is influenced by process variables such as the forming temperature, strain rate, initial grain size and amount of deformation [2]. Generally, the kinetics of recrystallization and grain growth processes are complex. In order to control the grain size distribution during high temperature deformation, a basic understanding of the microstructure evolution during complex manufacturing sequences, including heating, soaking and hot working, should be made as an essential prerequisite. In this work, a microstructure prediction model is implemented into a commercial FE code based on the analytical understanding of the microstructural change in Alloy 783 during high temperature deformation. Constitutive model considering dynamic recrystallization and grain growth theories was used for the prediction of grain structure in ring-rolled Alloy 783 ring. Model parameters were determined by the isothermal heat treatment and hot compression tests.

Effects of solution treatment on the microstructure and mechanical properties in wrought Alloy 718 were investigated. For the improvement of tensile and fatigue properties of wrought Alloy 718, a cyclic solution treatment, heating at 1000 for 3 minutes followed by furnace cooling at the rate of 3 /minute and holding at 985 for 8 minutes, was proposed. The cyclic treatment was performed repeatedly 3 times and the samples were air-cooled to room temperature. After the cyclic solution heat treatment, Alloy 718 was subject to the standard aging treatment, holding at 718 for 8 hours followed by cooling at the rate of 55 /hour and holding at 620 for 8 hours, and finally air cooled to room temperature. For the purpose of comparison, a standard heat treatment was also performed by solution treatment at 955 followed by aging. The microstructures of the heat-treated samples were analyzed by optical microscopy and scanning electron microscope (SEM). Cyclic solution treatment resulted in the formation of small spherical shaped δ-phases without grain growth. However, the δ-phases of the standard heattreated specimen showed needle-like morphologies. Tensile and low cycle fatigue tests were performed on both cyclic heat-treated and standard heat-treated specimens at room temperature, 350 and 650 . Low cycle fatigue tests on the cyclic treated specimens showed promising results without reduction of strength.