Experimental studies on the superplastic forming of square shaped components from sheets of Ti-6Al-4V alloy

Superplasticity is the ability of a polycrystalline material to exhibit, in a relatively isotropic manner, large elongations when deformed in tension. This property is exploited during superplastic forming in the fabrication of complex shaped components which are otherwise technically difficult or economically costly to form by conventional methods. The ability of some titanium alloys to undergo superplastic deformation coupled with their diffusion bonding capability (SPF/DB) provides excellent opportunities to fabricate intricate parts in a single operation resulting in significant cost and weight savings, particularly in the manufacture of aerospace structures. In the present work, experimental studies to characterize the superplastic behaviour of an as-received titanium Ti-6Al-4V alloy sheet commonly used in aerospace structural applications are reported. Tensile test coupons prepared from the alloy sheet were subjected to high temperature tensile tests in the temperature range of 1123 K (850°C) to 1223 K (950°C) and strain rate range of 10^?4 s^?1 to 10^?2 s^?1 in order to characterize the superplastic deformation behaviour. Suitable dies, for superplastic forming of 80 mm × 80 mm square components to depths of 43 and 50 mm, were designed and fabricated. Components were superplastically formed at a temperature of 1200 K (927°C) and 0.7 MPa constant argon pressure. The components were characterized for their thickness distribution, mechanical and metallurgical properties and the results are presented.     

Dislocation creep controlled superplasticity in the high carbon steel 140NiCr16-6

Superplasticity in the alloyed high carbon-steel 140NiCr16-6 with phosphorus additions and a fine grained microdupiex structure – containing cementite in volume fractions of 22 % (Fe,Cr,Ni)₃C, particle size of about 1 μm and with a medium ferrite grain size of about 2 μm – has been investigated in the temperature regime of 550 to 675°C and in the strain rate range of 10^?5 to 5 · 10^?2 s^?1. Maximum strain rate exponents of m = 0,45 at 675°C with strain rates of the order of 10^?4 s^?1 have been determined. Maximum superplastic elongations of about 700 % were detected. At higher strain rates of 10^?3 s^?1 superplastic elongations of about 570 % were achieved. At relatively low test temperatures of 550°C elongations up to 230 % were recorded. The activation analysis in the temperature regime of 550 to 650°C show an activation energy for superplastic flow of 250 ± 20 kJ/mol. This is in agreement with the activation energy for lattice self diffusion of iron in α-iron. Above 650°C the activation energy decreases to 70 kJ/mol. This is due to a stress induced decrease in the eutectoid α-γ-transformation temperature from 685°C to somewhat lower temperatures during superplastic deformation. The superplastic deformability (m > 0.3) of this steel in a wide strain rate range at relatively low temperatures above 550°C allows near net shape forming of complex parts applying low flow stresses.     

Dynamic Strain Aging of Nickel-Base Alloys 800H and 690

The objective of the current investigation is to characterize the dynamic strain aging (DSA) behavior in alloys 800H and 690. Constant extension rate tests were conducted at strain rates in the range of 10^?4 s^?1 to 10^?7?s^?1and temperatures between 295?K and 673?K (22?°C and 400?°C), in an argon atmosphere. Maps for the occurrence of serrated flow as a function of strain rate and temperature were built for both alloys. The enthalpy of serrated flow appearance of alloy 800H was found to be 1.07?±?0.30?eV.     

Development of Nanolayer Hydroxyapatite (HA) on Titanium Alloy via Superplastic Deformation Method

In this work, hydroxyapatite (HA) is successfully embedded onto titanium alloy using the superplastic deformation method. An embedded layer of approximately 249?nm is obtained at a temperature of 1200?K (927 °C), strain rate of 1?×?10^?4?s^?1, and process duration of 90?minutes. X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) analyses indicate that HA is retained after the embedment process, and a significant amount of titanium (Ti) is diffused into the HA, forming a dense HA/Ti composite layer. Wear tests under a simulated body fluids (SBF) condition show that the adherent strength of HA and the interfacial strength between the HA layer and substrate are superior compared with the nonsuperplastic sample.     

Impact Response and Microstructural Evolution of 316L Stainless Steel under Ambient and Elevated Temperature Conditions

The impact response and microstructural evolution of 316L stainless steel are examined at strain rates ranging from 1?×?10³ to 5?×?10³?s^?1 and temperatures between 298?K and 1073?K (25?°C and 800?°C) using a split Hopkinson pressure bar and transmission electron microscopy (TEM). The results show that the flow behavior, mechanical strength, and work-hardening properties of 316L stainless steel are significantly dependent on the strain rate and temperature. The TEM observations reveal that the dislocation density increases with increasing strain rate but decreases with increasing temperature. Moreover, twinning occurs only in the specimens deformed at 298?K (25?°C), which suggests that the threshold stress for twinning is higher than that for slip under impact loading. Finally, it is found that the volume fraction of transformed ???? martensite increases with increasing strain rate or decreasing temperature. Overall, the results suggest that the increased flow stress observed in 316L stainless steel under higher strain rates and lower temperatures is determined by the combined effects of dislocation multiplication, twin nucleation and growth, and martensite transformation.     

Thermodynamic properties of holmium monogermanide

The heat capacity and enthalpy of HoGe is investigated for the first time over a temperature range between 51.62 and 2096 K. The values of heat capacity, entropy, reduced Gibbs energy (J · mole^?1 × × K^?1), and enthalpy (J · mole^?1) are determined at 298.15 K: C °_P(T) = 49.63 ± 0.20; S °(T) = 89.1 ± 0.7; Φ′(T) = 50.9 ± 0.8; H °(T) ? H °(0 K) = 11391 ± 57. Temperature dependences of enthalpy (J · mole^?1) for holmium monogermanide are determined as follows: H °(T) ? H °(298.15 K) = 8.474 × × 10^?3 · T² + 47.13 · T + 226747 · T^?1 ? 15565 and H °(T) ? H °(298.15 K) = 88.91 · T ? 26507, for 298.15–1765 K and 1951–2096 K, respectively. The enthalpy and entropy of HoGe melting are calculated: T_m = 1765 ± 35 K, ΔH_m = 36.3 ± 2.9 kJ · mole^?1, ΔS_m = 20.5 ± 1.6 J · mole^?1 · K^?1.     

Evaluating the Superplastic Flow of a Magnesium AZ31 Alloy Processed by Equal-Channel Angular Pressing

Experiments show that the magnesium AZ31 (Mg-3 pct Al-1 pct Zn) alloy exhibits excellent superplastic properties at 623 K (350 °C) after processing by equal-channel angular pressing using a die with a channel angle of 135 deg and a range of decreasing processing temperatures from 473 K to 413 K (200 °C to 140 °C). A maximum elongation to failure of ~1200 pct was achieved in this alloy at a tensile strain rate of 1.0 × 10^?4 s^?1. Microstructural inspection showed evidence for cavity formation and grain growth during tensile testing with the grain growth leading to significant strain hardening. An examination of the experimental data shows that grain boundary sliding is dominant during superplastic flow. Furthermore, a comprehensive review of the present results and extensive published data for the AZ31 alloy shows the exponent of the inverse grain size is given by p ≈ 2 which is consistent with grain boundary sliding as the rate-controlling flow mechanism.     

Superplastic behavior and microstructure evolution in a commercial Al-Mg-Sc alloy subjected to intense plastic straining

A commercial Al-6 pct Mg-0.3 pct Sc-0.3 pct Mn alloy subjected to equal-channel angular extrusion (ECAE) at 325 °C to a total strain of about 16 resulted in an average grain size of about 1 μm. Superplastic properties and microstructural evolution of the alloy were studied in tension at strain rates ranging from 1.4 × 10⁻⁵ to 1.4 s⁻¹ in the temperature interval 250 °C to 500 °C. It was shown that this alloy exhibited superior superplastic properties in the wide temperature range 250 °C to 500 °C at strain rates higher than 10⁻² s⁻¹. The highest elongation to failure of 2000 pct was attained at a temperature of 450 °C and an initial strain rate of 5.6 × 10⁻² s⁻¹ with the corresponding strain rate sensitivity coefficient of 0.46. An increase in temperature from 250 °C to 500 °C resulted in a shift of the optimal strain rate for superplasticity, at which highest ductility appeared, to higher strain rates. Superior superplastic properties of the commercial Al-Mg-Sc alloy are attributed to high stability of ultrafine grain structure under static annealing and superplastic deformation at T ≤ 450 °C. Two different fracture mechanisms were revealed. At temperatures higher than 300 °C or strain rates less than 10⁻¹ s⁻¹, failure took place in a brittle manner almost without necking, and cavitation played a major role in the failure. In contrast, at low temperatures or high strain rates, fracture occurred in a ductile manner by localized necking. The results suggest that the development of ultrafine-grained structure in the commercial Al-Mg-Sc alloy enables superplastic deformation at high strain rates and low temperatures, making the process of superplastic forming commercially attractive for the fabrication of high-volume components.     

Effect of friction stir processing on the kinetics of superplastic deformation in an Al-Mg-Zr alloy

The effect of friction stir processing on the superplastic behavior of extruded Al-4Mg-1Zr was examined at 350 °C to 600 °C and at initial strain rates of 1×10⁻³ to 1 s⁻¹. A combination of a fine grain size of 1.5 μm and high-angle grain boundaries in the friction stir-processed (FSP) alloy led to considerably enhanced superplastic ductility, much-reduced flow stress, and a shift to a higher optimum strain rate and lower optimum temperature. The as-extruded alloy exhibited the highest superplastic ductility of 1015 pct at 580 °C and an initial strain rate of 1×10⁻²s⁻¹, whereas a maximum elongation of 1280 pct was obtained at 525 °C and an initial strain rate of 1×10⁻¹s⁻¹ for the FSP alloy. The FSP alloy exhibited enhanced superplastic deformation kinetics compared to that predicted by the constitutive relationship for superplasticity in fine-grained aluminum alloys. A possible origin for enhanced superplastic deformation kinetics in the FSP condition is proposed.     

Structural Superplasticity at High Strain Rates of Super Duplex Stainless Steel Fe‐25Cr‐7Ni‐3Mo‐0.3N

The fine‐grained super duplex stainless steel Fe‐25Cr‐7Ni‐3Mo‐0.3N consisting of two phases (δ‐ferrite/austenite) exhibits structural super‐plasticity at higher strain rates of ? ≈ 10^?2s^?1 in the temperature range between 975 and 1100°C. The equiaxed microstructure with an average grain size of was produced by thermomechanical processing. Maximum strain‐rate‐sensitivity exponents of m ≈ 0.5 and elongations to failure of more than 500% were achieved. From thermal activation analysis an activation energy for superplastic flow of Q = 310 ± 20 kJ/mole was derived. The superplastic behaviour at higher strain rates is quantitatively described by a deformation model where grain or interphase boundary sliding is accommodated by sequential steps of dislocation glide and climb. The high strain‐rate‐sensitivity exponent and the observed dislocation density indicate that dislocation climb in the slightly solid solution strengthened austenite is the rate controlling step for superplastic flow. The deformation mechanism reveals that the investigated super duplex stainless steel exhibits superplastic behaviour that is typical for class II solid solution alloys.     

Influence of Strain Rate and Temperature on Tensile Deformation and Fracture Behavior of Type 316L(N) Austenitic Stainless Steel

Tensile tests were performed at strain rates ranging from 3.16 × 10^?5 to 3.16 × 10^?3 s^?1 over the temperatures ranging from 300 K to 1123 K (27 °C to 850 °C) to examine the effects of temperature and strain rate on tensile deformation and fracture behavior of nitrogen-alloyed low carbon grade type 316L(N) austenitic stainless steel. The variations of flow stress/strength values, work hardening rate, and tensile ductility with respect to temperature exhibited distinct three temperature regimes. The steel exhibited distinct low- and high-temperature serrated flow regimes and anomalous variations in terms of plateaus/peaks in flow stress/strength values and work hardening rate, negative strain rate sensitivity, and ductility minima at intermediate temperatures. The fracture mode remained transgranular. At high temperatures, the dominance of dynamic recovery is reflected in the rapid decrease in flow stress/strength values, work hardening rate, and increase in ductility with the increasing temperature and the decreasing strain rate.     

On the contribution of concurrent grain growth to strain sensitive flow of a superplastic Al-Cu eutectic alloy

Flow behavior and microstructural evolution in an Al-Cu eutectic alloy of equiaxed grains were investigated over ε ? 2× 10^?6 to 2 × 10^?2 s^?1 andT = 400° to 540 °C. Depending on the test conditions, there appeared either strain hardening or strain softening predominantly in the early part of the σ-ε curves. The microstructural observations showed evidence for grain growth, development of zig-zag boundaries, dislocation interactions, and cavitation. The grain growth adequately accounts for the observed strain hardening at higher temperatures and lower strain rates. However, at lower temperatures the strain hardening can be only partly accounted for by the observed grain growth; under this condition, some dislocation interactions also contribute to the strain hardening. The presence of cavitation causes strain softening predominantly at higher strain rates. Therefore, to develop a proper understanding of the superplastic behavior of the Al-Cu eutectic alloy, it is necessary to take into account the influence of dislocation interactions and cavitation along with that of grain growth.     

Superplastic behavior of spray-deposited 5083 Al

A 5083 Al alloy was synthesized using spray deposition processing with N₂ as the atomization gas. It was noted that the grains that were present in as-spray-deposited 5083 Al were equiaxed with an average size of 15.2 μm. The matrix of the material was supersaturated with Mg and Mn. The asspray-deposited microstructure contained irregular pores with porosity in the range of 0.1 to 5.4 vol pct, depending on spatial location in the preform. The spray-deposited alloy was thermomechanically processed using extrusion and multiple-pass warm rolling to reduce grain size and close porosity. It was observed that spray-deposited 5083 Al exhibited superplasticity following thermomechanical processing by extrusion followed by rolling. Superplasticity was observed in the 500 °C to 550 °C temperature range and 3 × 10⁻⁵ to 3 × 10⁻³ s⁻¹ strain rate range. The corresponding strain-rate-sensitivity factors were in the 0.25 to 0.5 range and increased with decreasing strain rate. A maximum elongation of 465 pct was noted at 550 °C and 3 × 10⁻⁵ s⁻¹. The spray-deposited 5083 Al, thermomechanically processed by direct rolling, exhibited superplasticity in the same temperature and strain rate ranges as those for the extruded and rolled materials. The superplastic elongation of the spray-formed and rolled material was relatively low, being in the range of 250 to 300 pct. The deformation behavior is discussed in light of the presence of porosity in the microstructure.     

Mechanism of Dynamic Strain Aging in a Niobium-Stabilized Austenitic Stainless Steel

Dynamic strain aging (DSA) behavior of a niobium (Nb)-stabilized austenitic stainless steel (TP347H) was studied from room temperature (RT) to 973 K via tensile testing, transmission electron microscopy (TEM), and internal friction (IF) measurements. The DSA effect is nearly negligible from 573 K to 673 K, and it becomes significant at temperatures between 773 K and 873 K with strain rates of 3 × 10^?3 s^?1, 8 × 10^?4 s^?1, and 8 × 10^?5 s^?1, respectively. The results indicate that a dislocation planar slip is dominant in the strong DSA regime. The Snoek-like peak located at 625 K is highly sensitive to the diffusion of free carbon (C) atoms in solid solution. C-Nb octahedrons are formed by C chemical affinity to substitutional Nb solute atoms. Octahedron structure is very stable and captures most free C atoms and inhibits DSA at low tensile test temperatures of 573 K to 673 K. At high test temperatures in the range from 773 K to 873 K, C-Nb octahedrons break up and release free C and Nb atoms, resulting in the stronger Snoek-like peak. The interaction between C atoms and dislocations is responsible for DSA at low temperatures ranging from 573 K to 673 K. At higher temperature of 773 K to 873 K, the Cr and Nb atoms lock the dislocations, and this formation contributes to DSA.     

Influence of Temperature and Strain Rate on Tensile Deformation and Fracture Behavior of P92 Ferritic Steel

Tensile tests were performed at strain rates ranging from 3.16 × 10^?5 to 1.26 × 10^?3 s^?1 over a temperature range of 300 K to 923 K (27 °C to 650 °C) to examine the effects of temperature and strain rate on tensile deformation and fracture behavior of P92 ferritic steel. The variations of flow stress/strength values, work hardening rate, and tensile ductility with respect to temperature exhibited distinct three temperature regimes. The fracture mode remained transgranular. The steel exhibited serrated flow, an important manifestation of dynamic strain aging, along with anomalous variations in tensile properties in terms of peaks in flow stress/strength and work hardening rate, negative strain rate sensitivity, and ductility minima at intermediate temperatures. At high temperatures, the rapid decrease in flow stress/strength values and work hardening rate, and increase in ductility with increase in temperature and decrease in strain rate, indicated the dominance of dynamic recovery.     

Characterization of superplastic deformation behavior of a fine grain 5083 Al alloy sheet

Superplastic deformation behavior of a fine grain 5083 Al sheet (Al-4.2 pct Mg-0.7 pct Mn, trade name FORMALL 545) has been investigated under uniaxial tension over the temperature range of 500 °C to 565 °C. Strain rate sensitivity values >0.3 were observed over a strain rate range of 3 × 10⁻⁵ s⁻¹ to 1 × 10⁻² s⁻¹, with a maximum value of 0.65 at 5 × 10⁻⁴ s⁻¹ and 565 °C. Tensile elongations at constant strain rate exceeded 400 pct; elongations in the range of 500 to 600 pct were obtained under constant crosshead speed and variable strain rates. A short but rapid prestraining step, prior to a slower superplastic strain rate, provided enhanced tensile elongation at all temperatures. Under the two-step schedule, a maximum tensile elongation of 600 pct was obtained at 550 °C, which was regarded as the optimum superplastic temperature under this condition. Dynamic and static grain growth were examined as functions of time and strain rate. It was observed that the dynamic grain growth rate was appreciably higher than the static growth rate and that the dynamic growth rate based on time was more rapid at the higher strain rate. Cavitation occurred during superplastic flow in this alloy and was a strong function of strain rate and temperature. The degree of cavitation was minimized by superimposition of a 5.5 MPa hydrostatic pressure during deformation, which produced a tensile elongation of 671 pct at 525 °C. R. VERMA, formerly Visiting Scientist, Department of Materials Science and Engineering, University of Michigan     

Microtexture evolution during annealing and superplastic deformation of Al-5 pct Ca-5 pct Zn

The microtexture and grain boundary misorientation distributions (i.e., mesotexture) of the superplastic alloy Al-5 pct Ca-5 pct Zn have been investigated in the as-processed condition, after annealing at 520 °C (for times ranging from 7 minutes to 90 hours) and after tensile straining in the transverse direction (TD). Three different superplastic straining conditions were considered: 550 °C/10⁻² s⁻¹, 550 °C/10⁻¹ s⁻¹, and 400 °C/10⁻² s⁻¹. Microtexture data were obtained by means of computer-aided electron backscatter diffraction analysis methods. The retention of the deformation texture of the as-received material and the development of an increasingly bimodal grain boundary misorientation distribution following static annealing are consistent with the occurrence of recovery and continuous recrystallization. During superplastic straining, deformation texture components are also retained, but with a change in the grain boundary misorientation distribution toward random, indicating that grain switching occurs during grain boundary sliding (GBS). At the midlayer, however, a change from an initial texture component near the Cu-type texture component toward the Brass texture component, {011}〈211〉, was observed even as the misorientation distribution became more random. This change in texture component is associated with the occurrence of single slip during superplastic flow.     

Hot Ductility and Compression Deformation Behavior of TRIP980 at Elevated Temperatures

The hot ductility tests of a kind of 980 MPa class Fe-0.31C (wt pct) TRIP steel (TRIP980) with the addition of Ti/V/Nb were conducted on a Gleeble-3500 thermomechanical simulator in the temperatures ranging from 873 K to 1573 K (600 °C to 1300 °C) at a constant strain rate of 0.001 s^?1. It is found that the hot ductility trough ranges from 873 K to 1123 K (600 °C to 850 °C). The recommended straightening temperatures are from 1173 K to 1523 K (900 °C to 1250 °C). The isothermal hot compression deformation behavior was also studied by means of Gleeble-3500 in the temperatures ranging from 1173 K to 1373 K (900 °C to 1100 °C) at strain rates ranging from 0.01 s^?1 to 10 s^?1. The results show that the peak stress decreases with the increasing temperature and the decreasing strain rate. The deformation activation energy of the test steel is 436.7 kJ/mol. The hot deformation equation of the steel has been established, and the processing maps have been developed on the basis of experimental data and the principle of dynamic materials model (DMM). By analyzing the processing maps of strains of 0.5, 0.7, and 0.9, it is found that dynamic recrystallization occurs in the peak power dissipation efficiency domain, which is the optimal area of hot working. Finally, the factors influencing hot ductility and thermal activation energy of the test steel were investigated by means of microscopic analysis. It indicates that the additional microalloying elements play important roles both in the loss of hot ductility and in the enormous increase of deformation activation energy for the TRIP980 steel.     

Cr50Cu50 alloys produced from submicrometre structured powders through hot pressing at different pressures

In this study, two different compositions of submicrometre copper and chromium powders were mixed and the Cr50Cu50 alloys were fabricated via the powder metallurgy technique. The research imposed various hot pressing pressures, while the temperature was maintained at 1050°C for 1?h, respectively. The experimental results showed that the Saint Venant's effect obviously appeared in 1050°C, 48?MPa, 1?h hot pressing Cr50Cu50 alloys, which led to differences in the surface and internal hardness of the specimens. As a result, the optimal parameters of hot pressing of Cr50Cu50 alloys were 1050°C, 36?MPa for 1?h possessing the highest transverse rupture strength value (921.69?MPa). Moreover, the surface hardness increased to HV_0.2 221.78 (HRB 95.4), the relative density reached 97.15% and apparent porosity decreased to 0.10%, respectively. Furthermore, the resistivity decreased to 5.28?×?10^?6?Ω?cm, and ICAS was enhanced to 32.65% after the optimal procedure.     

Kinetics and mechanism of formation of MoO2 by solid state reaction between MoS2 and MoO3

ABSTRACT

The kinetics and mechanism of the solid state reaction between MoS₂ and MoO₃ to form MoO₂ under nitrogen atmosphere between 723 and 973?K were studied using bulk mixed samples, mixed compressed pellets and pure MoS₂ and MoO₃ pellets with one face in contact. The results show that for the bulk samples, the reaction reaches a maximum conversion of 67.3% at 923?K in 75?min, while for compressed samples conversion under similar conditions reaches 96.1% in 75 min, which reflects the effect of the physical conditions of both types of experiments on the reaction kinetics. The calculated activation energy values for both experimental conditions are consistent, with an average value of ?44.2?±?1.9?kJ, which is in the range of diffusion-controlled solid state reactions. For the samples with one face in contact, above 923?K the results appear to indicate that solid state molecular diffusion of both MoS₂ and MoO₃ in opposite directions in the newly formed MoO₂ crystal structure could take place, with estimated diffusion coefficients of MoS₂ in MoO₂ and MoO₃ in MoO₂ at 923?K of 1.08?×?10^?6 and 7.78?×?10^?6cm²?s^?1, and at 973?K of 10^?5 and 1.13?×?10^?5?cm²?s^?1, respectively.