Effect of alloying element modification on superplastic deformation properties of Ti–4Mo–4Al–2Sn–0·5Si (IMI550)

Abstract

The superplastic deformation properties of a commercial titanium alloy, IMI550 (Ti–4Mo–4Al–2Sn–0·5Si, wt-%) are compared with a specially melted ‘modified’ alloy in which 1 wt-%Mo (slow diffusing element) is replaced with 1 wt-%Fe (fast diffusing element) on the basis that the diffusional properties of the β phase, to which these alloying elements segregate, is a significant factor in determining the overall flow properties. It is shown that the superplastic deformation properties of both alloys in the temperature range 800–850°C develop with strain as the initial heterogeneous α + β structure develops into uniform duplex equiaxed microstructures. Thereafter, the modified alloy exhibits enhanced superplastic properties (reduced flow stresses in the strain rate range 5 × 10^-5–5 × 10^-3 s^-1). The results are analysed quantitatively on the basis of the Ashby and Verrall model applied to the α and β phases combined according to the isostress model for two phase deformation which accounts for the interaction of the α and β phases.     

Post-forming tensile properties of superplastically bulge formed high strength α/β Ti–Al–Mo–Sn–Si alloy (IMI 550)

Abstract

The effect of biaxial superplastic deformation at 900°C on ambient temperature tensile properties and texture of high strength α/β titanium alloy sheet of nominal composition Ti–4Al–4Mo–2Sn–0·5Si (IMI 550) has been examined. Superplastic straining led to significant decreases in both proof stress and tensile strength values. Heat cycling studies on as received sheet material showed that the decreases in strength were in part due to the influence of temperature, but this had little effect on elongation. The further losses in strength after superplastic forming were attributable to grain growth. The decreases in elongation after superplastic deformation were believed to be due mainly to changes of tensile specimen geometry, while the more isotropic tensile behaviour which was observed was due to the gradual removal of the relatively low level of texture in the as received material. Application of the standard heat treatment to the as received IMI 550 sheet material led to increases in proof stress and tensile strength values of ~70 and ~170 MN m^?2, respectively, and to a slight decrease in elongation. Heat treatment of heat cycled and superplastically bulge formed specimens increased the proof stress and tensile strength values almost to the levels attained in the as received material.

MST/684     

Mechanical anisotropy and stress–strain rate characteristics in superplastic deformation of titanium alloys

Abstract

Uniaxial tension and compression tests have been carried out on two titanium based alloys, Ti–6Al–4V in the form of extruded tubes and forged plates and Ti–3Al–10V–2Fe sheets, to study anisotropic behaviour during superplastic deformation. The following were observed: (i) originally round cross-section became elliptical after deformation; (ii) the flow stresses and strain rates were dependent on the orientation of the specimens; and (iii) the strain anisotropy became less severe as the strain rate increased. These characteristics of anisotropy were related to the original microstructure (e.g. the mechanical fibring of the α grains) and the microstructural evolution during superplastic deformation. New constitutive equations for describing anisotropic superplastic deformation have been proposed to explain the effect of strain rate or stress on anisotropy.     

Superplasticity and production of mechanically milled Ti–6Al–4V–0.5B

Abstract

Superplastic forming of conventional titanium alloy sheet is limited commercially by the relatively long cycle times imposed by the high temperatures and slow strain rates required. In order to minimise cycle times material with a fine grain size is required to allow either, an increase in the forming rate or a reduction in the deformation temperature. This study details the manufacture of Ti–6Al–4V–0.5B powder with a nanocrystalline grain size, which was produced by mechanical milling. The material was consolidated by hot isostatic pressing at a range of temperatures during which ～2.5 vol.-%TiB was formed by an in situ reaction between the titanium and boron. The TiB particles limited the growth of the grain size in the titanium from the nanocrystalline structure in the powder to sizes in the range 600 nm–4 µm after consolidation. The consolidated material was hot tensile tested at a range of temperatures and strain rates. A superplastic elongation of 310%was achieved when testing at 900°C at a strain rate of 6×10^-2 s^-1 compared with 220% for conventional Ti–6Al–4V sheet. However, extensive cavitation, induced by the presence of argon, occurred during high temperature deformation and limited the superplastic extensions achieved.     

Superplasticity of Fe3Al(Cr)

Abstract

The superplasticity of an Fe₃Al based intermetallic alloy with 3 at.-% chromium has been investigated in the strain rate range 10^-5-10^-2 s^-1 at test temperatures between 700 and 900°C. The composition of the iron aluminide was Fe–28Al–3Cr (at.-%) with additions of titanium and carbon. After thermomechanical processing the material possessed a coarse grained microstructure with an average grain size of 55 ± 10 μm. Strain rate exponents of 0·33≤m≤0.42 were recorded at strain rates of approximately 10^-5-10^-3 s^-1 in the temperature range 750-900°C. Superplastic elongations of 350% and more were achieved. From thermal activation analysis of superplastic flow, an activation energy of 185 ± 10 kJ mol^-1 was derived. This value is comparable to activation energies of superplastic flow in Fe₃Al(Ti) alloys. However, in unalloyed Fe₃Al the activation energy is higher, ~ 263 kJ mol^-1. Optical microscopy showed grain refinement to ~ 30 ± 5 μm in size in superplastically strained tensile specimens. Transmission electron microscopy gave evidence of the formation of subgrains of 0·3–0·5 μm in size. Superplasticity in this iron aluminide is mainly attributed to viscous dislocation glide, controlled by solute drag in the transformed B2 lattice at the deformation temperatures. During superplastic deformation, subgrain formation and grain refinement in the gauge length were revealed. From this it is concluded that dynamic recrystallisation makes an important contribution to the deformation mechanism of superplastic flow in this material.     

Post-forming tensile properties of superplastic Ti–6Al–4V alloy

Abstract

A study has been made of the influence of uniaxial superplastic deformation on the ambient temperature tensile properties of Ti–6Al–4V sheet. Material was deformed to various strains up to 200% at temperatures from 850 to 970°C at strain rates in the range 1·1?18 × 10^{;amp;#x2212;4}s^?1 (0·7?11% min^?1). Tests were also performed on statically annealed material to separate the effects of high temperature exposure and superplastic deformation. Mechanical property changes were complex and depended on the relative contributions from the strengthening and softening mechanisms occurring during either superplastic deformation or heat cycling. Structural features influencing mechanical properties were phase size and morphology, dislocation density, and crystallographic texture. The strength after superplastic deformation was always less than that of as-received material but a significant reduction in strength was attributable to heat cycling. In some cases, the strength of the superplastically deformed material was greater than that after heat cycling.

MST/593     

Microstructural changes occurring during superplastic deformation of Ti-6AI-6V-2Sn alloy

Abstract

Microstructural changes occurring during superplastic deformation of Ti-6Al-6V-2Sn alloy with an initial microstructure consisting of mixed fine lamellar and equiaxed α grains were investigated. Uniaxial tensile tests with constant strain rate were conducted at temperatures ranging from 775 to 925°C and at strain rates rangingfrom 7 × 10^-5 to 1 × 10^-3 S^-l. To investigate the microstructural changes occurring during deformation, some of the tests were terminated at preprogrammed true strains of 0.5, 0.9, and 1.5 for subsequent metallographic investigation. The effects of high temperature exposure on the microstructural changes and on the superplastic deformation behaviour were also evaluated. It was found that both static and dynamic recrystallisation were initiated under certain test conditions and could be related to the flow stress behaviour during the superplastic deformation tests. For tests at low temperature and high strain rate, the flow stress increased quickly at the very beginning of the deformation without significant microstructural change. After the flow stress reached its maximum value, dynamic recrystallisation occurred at a lamellae accompanied by a decrease of the flow stress, known as strain softening. Raising the test temperature or decreasing the deformation strain rate provided the opportunity for thermal energy to initiate static or semidynamic recrystallisation. Thereafter, the flow stress behaviour at the beginning of the test changed to a slow strain hardening type. There also existed a transition temperature; soaking before tensile testing above this temperature would result in static recrystallisation, and the superplastic deformation characteristics would be affected.     

Mechanism of saturated flow stress during hot tensile deformation of a TA15 Ti alloy

The recently developed high pressure gas forming technique can efficiently form parts of titanium alloys at a lower temperature with a higher strain rate as compared to the superplastic forming (SPF) technique. However, the deformation mechanism of titanium alloys at the temperatures suitable for high pressure gas forming is still not well understood. The deformation mechanism of a TA15 titanium alloy at 750 °C suitable for high pressure gas forming was investigated in the present work. It was found that the flow stress saturated after a true strain of 10% whilst the dislocation density was not saturated and increased continuously with straining. In addition, the Taylor factor was found to be nearly constant during tensile test. As a result, it is concluded that the α value, which represents the interaction between dislocations in the Taylor hardening model, decreases continuously with strain. It is worth noting that the α value in the Taylor hardening model is usually assumed to be a constant during tensile test in literature. The present work is the first one to report the dependence of α value on the strain for titanium alloys deformed at high temperatures of 750 °C.     

Deformation of Cu–P alloys at high temperatures

Abstract

The deformation behaviour of Cu–P alloys has been investigated by torsion and tensile testing over a range of strain rates and temperatures. The torsion flow curves are interpreted in terms of dynamic softening processes, and the curves obtained during interrupted testing are used to examine static-restoration behaviour. Constitutive equations relating flow strength to strain rate and temperature are deduced, with allowance made for the effect of deformation heating, and implications of the equation constants are discussed. It is shown from tensile results that a state of superplasticity can be achieved in alloys containing 3·8 and 6·8 wt-%P. Superplasticity can occur only if the small α grain size is stable and if the temperature and strain rate fall within certain limits. The activation energy associated with superplastic flow has been determined.

MST/52     

Influence of prior straining on superplastic behaviour of an Fe–Cr–Ni alloy

Abstract

The superplastic behaviour of a microduplex Fe–Cr–Ni (25·7Cr–6·6Ni) alloy was investigated in the as-worked, annealed, and prestrained conditions. In the early stages of deformation, flow stress depends significantly on strain, and also on the instantaneous microstructural state in the case of as-worked and annealed specimens. Under these conditions, the empirical parameters of the constitutive equation for superplastic deformation were found to depend systematically on strain. At 1000°C, strain hardening predominates, and this could be accounted for by grain growth and by the hardening produced by the noticeable dislocation activity. After suitable prestraining, steady-state deformation conditions may be attained; this may facilitate the collection of σ–ε data, which could then be used to assess the relative importance of the appropriate deformation mechanisms.

MST/125     

Influence of dynamic strain aging on plastic instability behaviour of near alpha titanium alloy Timetal 834

Abstract

The deformation behaviour of near alpha titanium alloy Timetal 834 was investigated by analysing Considére criterion in temperature range between 300 and 500°C at an interval of 25°C. In the dynamic strain aging (DSA) regime (400–475°C) studied in this material, Considére criterion was satisfied partially at peak DSA temperature (450°C). This was attributed to maximum DSA effect at 450°C which eventually resulted in specimen fracture at low ductility levels. Strain hardening exponent (n) values was determined using Ludwik plot, Considére criterion and obtained experimentally from uniform strain. The n values obtained from Ludwik plot and Considére criterion were greater than the uniform strain in DSA regime studied.     

Plastic flow and microstructural evolution in Ti–6Al–2Zr–1Mo–1V alloys during hot deformation

Abstract

The hot deformation behaviour and microstructural evolution in Ti–6Al–2Zr–1Mo–1V alloys have been studied using isothermal hot compression tests. The processing map was developed at a true strain of 0·7 in the temperature range 750–950°C and strain rate range 0·001–10 s^?1. The corresponding microstructures were characterised by means of a metallurgical microscope. Globularisation of lamellae occurring to a greater extent in the range 780–880°C and 0·001–0·01 s^?1 had a peak power dissipation efficiency of 58% at about 850°C and 0·001 s^?1. The specimens deformed in 750–880°C and 0·01–10 s^?1 showed an instability region of processing map, whereas the specimens deformed in 880–950°C and 1–10 s^?1 indicated three kinds of flow instabilities, i.e. macro shear cracks, prior beta boundary cracks and flow localisation bands.     

Grain refinement and superplastic behaviour of a modified 6061 aluminium alloy

Abstract

The superplastic properties and microstructure evolution of a 0.15%Zr and 0.7%Cu modified 6061 aluminium alloy were examined in tension at temperatures ranging from 475 to 600°C and strain rates ranging from 7 × 10^-6 to 2.8 × 10^-2 s^-1. The refined microstructure with an average grain size of about 11 μm was produced in thin sheets by a commercially viable thermomechanical process. It was shown that the modified 6061 alloy exhibits a moderate superplastic elongation of 580% in the entirely solid state at 570°C and ? = 2.8 × 10^-4 s^-1. Superior superplastic properties (elongation to failure of 1300% with a corresponding strain rate sensitivity coefficient m of about 0.65) were found at the same strain rate and a temperature of 590°C, which is higher than the incipient melting point of the 6061 alloy (~575°C). The microstructural evolution during superplastic deformation of the 6061 alloy has been studied quantitatively. The presence of a slight amount of liquid phase greatly promotes the superplastic properties of the 6061 alloy, reducing the cavitation level.     

Hot deep drawing of titanium sheet

Abstract

Commercial purity titanium (IMI 125) and a Ti–Cu alloy (IMI 230) in sheet form have been deep drawn at room temperature and at elevated temperatures. Cylindrical cups drawn at room temperature and at temperatures up to about 550°C develop four ear peaks at about 45° to the rolling direction (RD). When drawing is carried out at temperatures above 600°C, two ear peaks are formed at 90 and 270° to RD. The change in anisotropy is attributed to the temperature dependence of crystallographic slip modes, the high temperature behaviour being associated with basal plane slip. Drawing with a temperature gradient (cold punch, heated die) enables high drawing ratios to be achieved.

MST/1352     

Comparison of superplastic forming abilities of as‐cast AZ91 magnesium alloy prepared by twin roll casting and WE43 magnesium alloy

This paper describes and compares the superplastic behaviour and microstructural evolution of twin roll cast AZ91 and WE43 rolled sheet alloys. Tests were carried out in uniaxial tension on both alloys across a range of temperatures (300 °C–525 °C) and strain rates (1?10^‐4 s^‐1–1?10^‐1 s^‐1). In the case of WE43 gas bulge testing was employed at 400 °C and 0.6 MPa to offer a better analogy to superplastic forming than uniaxial tensile testing. Elongations of over 400 % were observed within WE43 when tested at 450 °C and 1?10^‐3 s^‐1 strain rate, and over 200 % within AZ91 when tested at 350 °C and 1?10^‐3 s^‐1 strain rate. A peak cone height of 41 mm was achieved with WE43 at a temperature of 400 °C and pressure of 0.6 MPa. Electron back scattered detection technique was employed to analyse the microstructural evolution of the two alloys during the forming process. Both WE43 and AZ91 were observed to undergo dynamic recrystallization during elevated temperature tensile testing and failed at low strain rates mainly by means of coalescence of cavitation, in the case of AZ91 at high strain rates cracking of Al₁₂Mg₁₇ intermetallic particles was the dominating failure mechanism. Both alloys were seen to achieve good levels of superplastic ductility over 200 % elongation, which would be industrially useful in niche vehicle and aerospace manufacturing.     

Abnormal da/dN – ΔK curves: New failure mode with Ti – alloys

Fatigue crack propagation (FCP) -rates and -threshold values have been determined on the titanium alloys IMI 834, IMI 685 (turbine disk materials) and Ti-6Al-6V-2Sn (plate material). K_max-constant tests were executed in laboratory air at room temperature and run with 50 Hz on C(T) specimens. It was found that FCP-rates in K_max-constant tests followed the well known FCP behavior up to a certain limiting value K_max, denoted as °K_max. Below °K_max, the FCP-rates da/dN decrease with decreasing ΔK down to the threshold value ΔK_T (ΔK for 10^?7 mm/cycle). For K_max-constant tests with K_max > °K_max, the FCP-rates initially decreased with decreasing ΔK, but reached 10^?7 mm/cycle at smaller ΔK. For K_max ≧ ∧K_max > °K_max, FCP-rates of 10^?7 mm/cycle were never reached as ΔK decreased to and below ΔK_T. Instead, as ΔK approaches or gets smaller than ΔK_T, the FCP-rates stay either constant or increase again. The limit value °K_max for this abnormal FCP-behavior had been determined for IMI 834 to be 22 to 28 MPa√m, for IMI 685 to be 46 MPa√m and for Ti-6A1–6V-2Sn to be 26 MPa√m. The important result from a practical stand-point is the large difference in °K_max for comparable Ti alloys, i.e., IMI 834 and IMI 685.     

Microstructure development and superplasticity in Inconel 718 sheet

Abstract

Within the range of temperatures and strain rates investigated, mechanical testing identified the optimum forming condition for Inconel 718 sheet as being 965°C with a strain rate of 10^-4 s^-1. A detailed investigation of the microstructural behaviour under these deformation conditions was carried out and is reported here. Typically superplastic deformation would generate an enhanced rate of grain growth. This was not the case with the present material; instead it was found that a slight apparent grain size reduction occurred and there was evidence of substantial dislocation activity. It was concluded that the material was deforming on the borderline between 'true' superplasticity and normal slip based deformation.     

Quantitative characterisation of substructural development during warm working of an interstitial free steel

Abstract

A Ti containing interstitial free steel was warm rolled in the temperature range 500–800°C, using wedge shaped slabs to produce a range of strains in a single rolling test. Some plane strain compression tests, under similar conditions, were carried out to obtain accurate stress–strain data. Variations in substructural features including subgrain sizes, subgrain aspect ratios, and misorientations between subgrains were quantitatively measured by TEM. Close correlation was observed between mechanical behaviour and variations in the substructure at different temperatures. At the lower temperature (500°C), the material showed cold worked characteristics, but as the deformation temperature was increased the effects of recovery became more pronounced, and hot working behaviour was obvious in the flow stress as well as in the substructural observations at 800°C.     

Superplasticity in a 7055 aluminum alloy subjected to intense plastic deformation

Abstract

Superplasticity in a 7055 aluminum alloy subjected to intense plastic straining through equal channel angular extrusion (ECAE) was studied in tension over a range of strain rates from 1.4 × 10^-5 to 5.6 × 10^-2 s^-1 in the temperature interval 300 - 450 °C. The alloy had a grain size of ~ 1 μm. A maximum elongation to failure of ~750% occurred at a temperature of 425 °C and an initial strain rate of 5.6 × 10^-4 s^-1, with a strain rate sensitivity coefficient m of about 0.46. The highest m value was ~0.5 at a strain rate of 1.4 × 10^-3 s^-1 and T≥ 425 °C. Moderate superplastic properties with a total elongation of about 435% and m of ~0.4 were recorded in the temperature interval 350 - 400 °C; no cavitation was found. It was shown that the main feature of superplastic behaviour of the ECAE processed 7055 aluminum alloy is a low yield stress and strong strain hardening during the initial stages of superplastic deformation. Comparing the present results with the superplastic behaviour of the 7055 Al subjected to thermomechanical processing (TMP), the highest tensile elongation in the ECAE processed material occurred at lower temperatures because ECAE produces a finer grained structure.     

Superplastic forming behaviour of complex shapes and post-forming mechanical properties of aluminium based SiCp reinforced metal matrix composites

Abstract

From a commercial viewpoint superplastic forming of complex shapes using only a single operation and one surface tool is appealing, especially for metal matrix composites (MMCs) that are hard to form even at elevated temperatures due to low ductility and toughness. Furthermore, secondary machining operations are difficult due to the presence of extremely hard ceramic reinforcements such as SiC. A range of aluminium alloy based MMCs have indeed been shown to exhibit superplastic properties although most of these studies have been concerned with microstructural characterisation using small uniaxial tensile specimens. This paper therefore concentrates on high strain rate biaxial superplastic forming of complex shapes (critical feature) in MMCs where a forming envelope has been defined and post-forming mechanical properties investigated. Particulate reinforced MMCs based on aluminium alloys 7475 and 7178 were superplastically formed in a die with a 45° step at a range of temperatures and pressures. Formed specimens were sectioned to investigate cavitation and cross-sectional thinning. Tensile tests were performed on parent and formed material to investigate the effect of superplastic forming on mechanical properties. The MMCs were successfully formed over the temperature range 450–550°C achieving step angles α of 22–42°. This study has shown that high strain rate superplasticity (～10^-1 s^-1)can be achieved giving a strain of 70% in only 3.5 s without SiC fracture, reinforcement–matrix decohesion or matrix cavitation making this technique economic and very attractive for commercial exploitation. Cross-sectional thinning was found to be uniform and in the order of ～25% which could be accounted for at the design stage. The high strain rate superplasticity was found to be grain size dependent (<3 µm) but greater profile definitions were achieved when forming took place just above the matrix solidus. Superplastic forming above the matrix solidus temperature resulted in the achievement of the highest step angles in the complex shapes but had a detrimental effect on mechanical properties. This is thought to be due to the liquid phase present that aids grain boundary and interfacial sliding but has a similar effect to overheating during solution treatment and brittle phases are formed at the grain boundaries.