Characterization of hot deformation behavior of as-forged TiAl alloy

The deformation behavior of as-forged Ti–43Al–9V–Y alloy was investigated by hot compression tests in the temperature range of 1100–1225 °C and strain rate range of 0.01–0.5 s⁻¹. The results show that the alloy exhibits negative temperature sensitivity and positive strain rate sensitivity. The stress exponent (n = 3.02) and the apparent activation energy (Q = 342.27 kJ/mol) of the present alloy are lower than that of previous reported TiAl alloys, which suggests that the as-forged Ti–43Al–9V–Y alloy exhibits better deformability at low temperatures and high strain rates. A processing map for hot working was developed on the basis of a dynamic material model. The deformation mechanisms were analyzed by the processing map. The optimum processing condition at the strain of 0.6 is 1180–1210 °C/0.01–0.05 s⁻¹. A crack-free Ti–43Al–9V–Y sheet was prepared by hot rolling at these optimized parameters. EBSD results show that dynamic recrystallization is more likely to occur for γ phase.     

High temperature deformation of Ti–Al–Nb–Cr–Mo alloy with ultrafine microstructure

Ti–Al–Nb-based alloys have shown promise for high temperature applications, however limited research has been conducted in γ(TiAl) + σ(Nb₂Al) alloys that are aimed towards increasing strength and operating temperatures through microstructural control. Alloys with less than 30% σ-phase have been investigated, exploring relationships between microstructure and deformation mechanisms. An alloy with composition Ti–45Al–14Nb–5Cr–1Mo has been produced, characterized, and tested at high temperature under compression at strain rates ranging from 10⁻³ to 10⁻⁵ s⁻¹. The flow behavior of this alloy shows strain rate and temperature dependence with constant strain rate sensitivity at each temperature in the testing regime. Microstructural analysis reveals the γ-phase is primarily responsible for the alloy's ability to accommodate deformation to large strains under compression.     

High temperature deformation and processing map of a NiTi intermetallic alloy

The deformation behavior of a 49.8 Ni-50.2 Ti (at pct) alloy was investigated using the hot compression test in the temperature range of 700 °C–1100 °C, and strain rate of 0.001 s^?1 to 1 s^?1. The hot tensile test of the alloy was also considered to assist explaining the related deformation mechanism within the same temperature range and the strain rate of 0.1 s^?1. The processing map of the alloy was developed to evaluate the efficiency of hot deformation and to identify the instability regions of the flow. The peak efficiency of 24–28% was achieved at temperature range of 900 °C–1000 °C, and strain rates higher than 0.01 s^?1 in the processing map. The hot ductility and the deformation efficiency of the alloy exhibit almost similar variation with temperature, showing maximum at temperature range of 900 °C–1000 °C and minimum at 700 °C and 1100 °C. Besides, the minimum hot ductility lies in the instability regions of the processing map. The peak efficiency of 28% and microstructural analysis suggests that dynamic recovery (DRV) can occur during hot working of the alloy. At strain rates higher than 0.1 s^?1, the peak efficiency domain shifts from the temperature range of 850 °C–1000 °C to lower temperature range of 800 °C–950 °C which is desirable for hot working of the NiTi alloy. The regions of flow instability have been observed at high Z values and at low temperature of 700 °C and low strain rate of 0.001 s^?1. Further instability region has been found at temperature of 1000 °C and strain rates higher than 1 s^?1 and at temperature of 1100 °C and all range of strain rates.     

High strain rate superplasticity in a commercial Al–Mg–Sc alloy

It was shown that an Al–5.7%Mg–0.32%Sc–0.3%Mn alloy subjected to severe plastic deformation through equal-channel angular extrusion exhibits superior superplastic properties in the temperature range of 250–500 °C at strain rates ranging from 1.4 × 10⁻⁵ to 1.4 s⁻¹ with a maximum elongation-to-failure of 2000% recorded at 450 °C and an initial strain rate of 5.6 × 10⁻² s⁻¹.     

Effect of Nb particles on the flow behavior of TiAl alloy

High temperature compressive deformation behaviors of PM-TiAl alloy containing Nb particles (Ti–45Al–5Nb–0.4W/2Nb (at. %)) were investigated at temperatures ranging from 1050 °C to 1200 °C, and strain rates from 0.001 s⁻¹ to 1 s⁻¹. The flow curves were employed to develop constitutive equations, and the apparent activation energy of deformation Q was determined as 447.35 kJ/mol. A revised processing map was constructed on the basis of the flow stress, which can accurately describe the deformation behaviors and predict the optimum hot forging condition. The addition of 2% Nb particles reduces the peak stress and increases the activation energy of TiAl-based intermetallic, however, it increases the instable domain in the processing map.     

Flow characteristics and constitutive modeling for elevated temperature deformation of a high Nb containing TiAl alloy

The hot deformation behavior of a high Nb containing TiAl alloy with a nominal composition of Ti–42Al–8Nb–(W, B, Y) was investigated at temperatures ranging from 1000 °C to 1150 °C and strain rates from 10⁻³ s⁻¹ to 0.5 s⁻¹ on a Gleeble thermo-simulation machine. The work hardening regime and flow softening behavior of the alloy were analyzed in detail. The results revealed that the onset of dynamic recrystallization (DRX) was quite easy for the present alloy, whereas the dynamic recovery (DRV) was impeded during the hot deformation. The DRX kinetics was studied by Avrami-type equation. The low Avrami exponents of the proposed equation indicate a lower recrystallization rate compared to ordinary metals and alloys. Based on the classical hyperbolic-sine law and the kinematics of DRX, the constitutive equations of the work hardening-recovery period (i.e. flow stress before the peak) and flow softening process (i.e. flow curve after the peak stress) were established for the present alloy, respectively. Comparisons between the experimental and calculated results revealed that except the severely cracked specimens, the stress–strain curves predicted by the established model are in good agreement with experimental results.     

High temperature deformation behavior of near γ-phase high Nb-containing TiAl alloy

High temperature compressive deformation behaviors of a high Nb-containing PM-TiAl alloy (Ti–45Al–7Nb-0.3 W, at.%) were investigated at temperatures ranging from 1050 °C to 1200 °C, and strain rates from 0.001 s⁻¹ to 1 s⁻¹. The microstructure mainly consists of γ phase. The data obtained from the flow curves were employed to develop the constitutive equation, and the apparent activation energy (Q) was determined to be 414 kJ mol⁻¹. The size of the dynamically recrystallized grains (DRX) decreased with the increasing value of Zener–Hollomon (Z) parameter. A processing map was constructed on the basis of the flow stress, and the condition of intermediate Z (1150 °C, 0.1 s⁻¹) was determined to be the optimum hot forging parameter for industrial productions. DRX was observed under all the deformation conditions. At high Z and intermediate Z condition, dislocation climbing and twinning accompanied by DRX can act as the main deformation mechanisms. At low Z condition, DRX becomes the dominant softening mechanism, accompanied by the bending of lamellar colonies as well as the broken of γ grains and α₂ grains.     

Hot working behavior of near alpha titanium alloy analyzed by mechanical testing and processing map

The hot deformation characteristics of the Ti−5.7Al−2.1Sn−3.9Zr−2Mo−0.1Si (Ti-6242S) alloy with an acicular starting microstructure were analyzed using processing map. The uniaxial hot compression tests were performed at temperatures ranging from 850 to 1000 °C and at strain rates of 0.001−1 s⁻¹. The developed processing map was used to determine the safe and unsafe deformation conditions of the alloy in association with the microstructural evolution by SEM and OM. It was recognized that the flow stress revealed differences in flow softening behavior by deformation at 1000 °C compared to the lower deformation temperatures, which was attributed to microstructural changes. The processing map developed for typical strain of 0.7 in two-phase field exhibited high efficiency value of power dissipation of about 55% at 950 °C and 0.001 s⁻¹, basically due to extensive globularization. An increase in strain rate and a decrease in temperature resulted in a decrease in globularization of α lamellae, while α lamellar kinking increased. Eventually, the instability domain of flow behavior was identified in the temperature range of 850−900 °C and at the strain rate higher than 0.01 s⁻¹ reflecting the flow localization and adiabatic shear banding. By considering the power efficiency domains and the microstructural observations, the deformation in the temperature range of 950−1000 °C and strain rate range of 0.001−0.01 s⁻¹ was desirable leading to high efficiencies. It was realized that (950 °C, 0.001 s⁻¹) was the optimum deformation condition for the alloy.     

Superplastic behavior of a TiAl-based alloy

Superplastic behavior under the conditions of a temperature range from 850 to 1075°C and strain rates varying from 8×10⁻⁵ to 1×10⁻³ s⁻¹ was investigated for Ti–33Al–3Cr–0.5Mo (wt%) alloy with a very fine grain size obtained by the multi-step thermal mechanical treatment. The results show that the TiAl-based alloy with a hot-deformed fine grain size possesses good superplasticity. It exhibits a strain rate sensitivity coefficient of 0.9 at a strain rate of 3×10⁻⁵ s⁻¹ and temperature from 1000 to 1075°C. Moreover, the strain rate sensitivity coefficient is stable during the hot deformation, and a tensile elongation of 517% was obtained at 1075°C and a strain rate of 8×10⁻⁵ s⁻¹. The superplastic behavior of the present fine-grained TiAl-based alloy can be explained by the local strain hardening and high m value during the tensile deformation. Microstructure evolution in the superplastic deformation was also discussed.     

Low temperature superplasticity in a friction-stir-processed ultrafine grained Al–Zn–Mg–Sc alloy

Friction stir processing (FSP) was used to create a microstructure with ultrafine grains (0.68 μm grain size) in an as-cast Al–8.9Zn–2.6Mg–0.09Sc (wt.%) alloy. The ultrafine grained alloy exhibited superplasticity at relatively low temperatures and higher strain rates. Optimum ductility of 1165% at a strain rate of 3 × 10⁻² s⁻¹ and 310 °C was obtained. Enhanced superplasticity was also achieved at a temperature as low as 220 °C. Experimentally observed parametric dependencies and microstructural examinations indicated that the operating deformation mechanism might be the Rachinger grain boundary sliding accommodated by intragranular slip. The FSP microstructure became highly unstable at 390 °C onwards, thus, affecting ductility adversely. In situ transmission electron microscopy heating was used to understand the instability phenomenon, which has been attributed to the drop in particle pinning forces due to the dissolution of metastable precipitates and microstructural heterogeneity.     

Quasi-static and dynamic compression behaviors of metallic glass matrix composites

Compressive tests were conducted on metallic glass matrix composites at a series loading rates. It was found that mechanical properties of the composite, e.g. yielding stress and plasticity, have a week dependence on strain rates of 4.0 × 10⁻⁴ s⁻¹–4.0 × 10⁻¹ s⁻¹. Four composites were tested at a constant strain rate of 2.3 × 10 s⁻¹ to uncover the dynamic deformation behaviors. Compared with the quasi-static case, the yielding strength increased under dynamic loading rate, but the plasticity decreased significantly. On the other hand, the dynamic compressive has closely relation with the dendrite size and volume fraction. The decreasing of the dendrite size and volume fraction leaded to the dynamic yielding strength increased but the plasticity decreased. For a same composite, e.g. T1 alloy, the yielding strengths increased slightly but fracture strain decreased with increasing of dynamic strain rates.     

Deformation behavior and mechanisms of Ti- 1023 alloy

1 Introduction Beta titanium alloys offer a variety of microstructural morphologies and associated mechanical property variations thus giving considerable latitude in microstructure design. They are the most versatile class of titanium alloys and offer th…     

Allotropic phase transformation and high-temperature tensile deformation behaviour of powder metallurgy Ti-5553 alloy

Ti-5Al-5V-5Mo-3Cr metastable beta titanium alloy was prepared by rapid thermomechanical powder consolidation approach from blended elemental powder mixture. Allotropic phase transformation and high-temperature tensile behaviour of the consolidated powder metallurgy Ti-5553 alloy were investigated in this work. The studied alloy has a high β phase transformation temperature of 975 °C±5 °C, which is higher than other conventional ingot metallurgy Ti-5553 alloys. The β grains in the microstructure of the alloy are coarsened significantly with increasing the heating temperature from 890 °C to 1050 °C, however, the grain coarsening tendency is mitigated when the heat treatment temperature reach to the range of 1080 °C–1100 °C. The high-temperature tensile mechanical properties of the alloy are sensitive to both the deformation temperature and strain rate, and superplastic deformation of the alloy was achieved at the condition of 850 °C/0.001 s⁻¹ with the tensile elongation of 103.5%. The microstructural evolution characteristics and the fracture mechanisms of the alloy are varied with changing the deformation variables, which are revealed by the microstructure observation of the fractured specimens from different sampling positions.     

Processing,microstructure and property aspects of a spraycast Al–Mg–Li–Zr alloy

This paper describes a microstructural and property investigation of an Al–5.31Mg–1.15Li–0.28Zr alloy produced by spraycasting and downstream processing. Following a dispersoid precipitation treatment of 4 h at 400 °C, samples were hot compressed at strain rates of 2, 1, 0.2 and 0.1 × 10⁻² s⁻¹ at temperatures between 250 and 475 °C. Electron backscattered diffraction showed a strong dependence of recrystallised grain size on deformation temperature. At 250 °C and faster strain rates at 325 °C, a network of fine recrystallised necklace grains formed by progressive lattice rotation. At 325 °C at slow strain rates and at 400 °C and higher, dynamic recrystallisation occurred by discontinuous nucleation and growth at regions of microscopic strain localisation such as grain boundaries and triple points. The microstructures from small-scale hot compression experiments were compared with larger forgings under similar conditions and microstructural evolution was broadly similar. Mechanical properties of larger-scale forgings exceeded the targets for mechanically alloyed Al–Mg–Li alloy AA5091.     

Superplastic deformation behaviour of friction stir processed 7075Al alloy

Commercial 7075Al rolled plates were subjected to friction stir processing (FSP) with different processing parameters, resulting in two fine-grained 7075Al alloys with a grain size of 3.8 and 7.5 μm. Heat treatment at 490 °C for 1 h showed that the fine grain microstructures were stable at high temperatures. Superplastic investigations in the temperature range of 420–530 °C and strain rate range of 1×10⁻³–1×10⁻¹ s⁻¹ demonstrated that a decrease in grain size resulted in significantly enhanced superplasticity and a shift to higher optimum strain rate and lower optimum deformation temperature. For the 3.8 μm 7075Al alloy, superplastic elongations of >1250% were obtained at 480 °C in the strain rate range of 3×10⁻³–3×10⁻² s⁻¹, whereas the 7.5 μm 7075Al alloy exhibited a maximum ductility of 1042% at 500 °C and 3×10⁻³ s⁻¹. The analyses of the superplastic data for the two alloys revealed a stress exponent of 2, an inverse grain size dependence of 2, and an activation energy close to that for grain boundary self-diffusion. This indicates that grain boundary sliding is the main deformation mechanism for the FSP 7075Al. This was verified by SEM examinations on the surfaces of deformed specimens.     

Hot deformation and processing maps of an Fe3Al intermetallic alloy

The deformation behavior of an Fe–28Al–5Cr–0.08Zr–0.04B (at.%) intermetallic alloy under hot compression conditions was characterized in the temperature range of 600–1100 °C and strain rate range of 0.001–100 s⁻¹. Processing maps were calculated to evaluate the efficiency of the hot working and to recognize the instability regions of the flow behavior. The investigated alloy possesses the optimum hot-working conditions at 1100 °C and 0.001 s⁻¹, since the material undergoes dynamic recrystallization to produce a fine-grained structure with a high fraction of high-angle boundaries (∼70%). At lower temperature the material exhibited “large grained superplasticity” with a peak efficiency of ∼60% at 1000 °C and 0.001 s⁻¹. These parameters are the optimum ones for superplastic working of that alloy. The occurrence of large grained superplasticity is attributed to the formation of a subgrain structure within the large original grains and higher strain-rate sensitivity. The material also exhibits flow instabilities due to flow localization at lower temperatures (<700 °C) and higher strain rates (>0.1 s⁻¹).     

Hot workability of a NiTiCu shape memory alloy with acicular martensite phase based on processing maps

Hot deformation behavior and processing maps of Ni₄₅Ti₅₀Cu₅ (at.%) shape memory alloy (SMA), which possesses acicular martensite phase at room temperature, were investigated systematically based on isothermal compression tests at the temperatures ranging from 600 to 1000 °C and the strain rates ranging from 0.0005 to 0.5 s⁻¹. The flow stresses of the studied alloy were found to be dependent on the strain rate as well as the temperature. Processing maps at strains of 0.3, 0.6 and 0.9 were constructed on the basis of dynamic material model (DMM) theory by using the flow stresses obtained from compression tests. According to the processing maps, flow instability was found to mainly occur in the region with high strain rate and the scope of instability region was found to increase with the increase in the true strain. Based on the combination of processing map and microstructural observations, microstructural defects were found in the alloy samples deformed in the instability regions and the parameters suitable for hot working of the studied alloy were determined as the temperatures ranging from 750 to 850 °C and the strain rates ranging from 0.0005 to 0.001s⁻¹.     

Impact mechanical response and microstructural evolution of Ti alloy under various temperatures

A compressive split-Hopkinson pressure bar and transmission electron microscope (TEM) are used to investigate the mechanical behaviour and microstructural evolution of a Ti alloy (Ti–1.1Mo–5.2Zr–2.9Al–0.35Fe–0.05N–0.20 O–0.02H in wt.%) deformed at strain rates ranging from 8 × 10² s^?1 to 8 × 10³ s^?1 and temperatures between 25 °C and 900 °C. In general, the results indicate that the mechanical behaviour and microstructural evolution of the alloy are highly sensitive to both the strain rate and the temperature conditions. The flow stress curves are found to include both a work-hardening region and a work-softening region. The strain rate sensitivity parameter, β, increases with increasing strain and strain rate, but decreases with increasing temperature. The activation energy varies inversely with the flow stress, and has a low value at high deformation strain rates or low temperatures. The microstructural observations reveal that the strengthening effect evident in the deformed alloy is a result primarily of dislocations and the formation of α phase. The dislocation density increases with increasing strain rate, but decreases with increasing temperature. Additionally, the square root of the dislocation density varies linearly with the flow stress. Correlating the mechanical properties of the current Ti alloy with the TEM observations, it is concluded that the precipitation of α phase dominates the fracture strain. TEM observations reveal that the amount of α phase increases with increasing temperature below the β transus temperature. The maximum amount of α phase is formed at a temperature of 700 °C and results in the minimum fracture strain under the current loading conditions.     

EBSD investigation on the evolution of microstructure and grain boundaries in coarse-grained Ni–48Al upon large deformation at elevated temperature

The evolution of microstructure and grain boundaries were investigated in coarse-grained Ni–48Al intermetallics during plastic deformation at 1075 °C with the initial strain rate of 1.5 × 10⁻³ s⁻¹ using electron backscatter diffraction (EBSD) technique. Before deformation, most grain boundaries were high-angled (HAGBs), with several particular angles being predominant. During initial deformation, low-angle grain boundaries (LAGBs) with misorientation less than 5° began to evolve. The misorientation of the newly-formed LAGBs increased with the increase of deformation, meanwhile, grain boundaries with misorientations between 6 and 15° were gradually observed, and finally transformed into HAGBs (misorientation angle > 15°). There appeared a steady state transition from the formation of new LAGBs to the transformation into high-angle grain boundaries. As a result, the grain size was refined continuously with the deformation strain.     

Deformation of a multiphase Mo–9.4Si–13.8B alloy at elevated temperatures

A 3-phase silicide alloy, Mo–9.4Si–13.8B (at.%), was prepared via powder metallurgy techniques. The tensile properties of the alloy at elevated temperatures were evaluated in vacuum at temperatures ranging from 1350 to 1550°C and strain rates ranging from 5.0×10⁻⁴ to 1×10⁻³ s⁻¹. The alloy was found to exhibit a stress exponent of about 2.8 and relatively a high activation energy 740 kJ/mol. Also, it displayed unusually large tensile ductility (>100%) at T>1400°C. The deformation mechanism as well as large ductility are discussed in the light of the microstructural observations. The alloy has a very good mechanical strength at elevated temperatures, comparable to some of the most advanced tungsten-based alloys.