Activation Energy For Superplastic Flow Research Articles

Superplasticity in Mg–Li–Zn Alloys Processed by High Ratio Extrusion

Superplastic behavior of Mg-8.5Li-1Zn and Mg-8.5Li-3Zn alloys has been investigated. Dynamic recrystallization occurs in the Mg–Li–Zn alloys during extruding with high ratio after stir casting. Mg-8.5Li-1Zn alloy exhibits an elongation of about 400% at the high strain rate of 1.1 × 10−2 S−1 at 623 K, while the Mg-8.5Li-3Zn alloy shows maximum elongation of more than 540%. The apparent activation energy for superplastic flow of both Mg-8.5Li-1Zn and Mg-8.5Li-3Zn alloys at 573–673 K is about 86 kJ/mol and 79 kJ/mol, respectively, so that the dominant deformation mechanism in Mg–Li–Zn alloys is grain boundary sliding (GBS) controlled by grain boundary diffusion.

On mechanical properties and superplasticity of Mg–15Al–1Zn alloys processed by reciprocating extrusion

A superior combination of specific strength and low-temperature high-strain-rate superplasticity of two-phase Mg–15Al–1Zn alloys could be achieved with reciprocating extrusion directly from as-cast billets. In the as-extruded state, the yield strength and ultimate tensile strength were 306 and 376 MPa, respectively. The optimum elongation of at least 1610% in company with a high m value of 0.7 was obtained at the strain rate of 1 × 10 −2 s −1 when tested at 325 °C. The pronounced strengthening mechanism was attributed to the high volume fraction of fine-grained hard Mg 17Al 12 phase in the refined α-Mg matrix. The excellent superplasticity at high strain rates was also mainly contributed by the high volume fraction of Mg 17Al 12 phase which displays easier grain boundary sliding than α-Mg phase. The apparent activation energy for superplastic flow is thus smaller than that of the boundary diffusion in Mg. In addition, the mechanisms for the filament formation and cooperative grain boundary sliding are discussed.

Activation Energy for Superplastic Flow Above Critical Temperature of Die Steels

Some commercial cold working die steels GCr15 and CrWMn with ultra-fine grain size were chosen as tested materials to research the activation energy for superplastic flow at different temperatures and strain rates above critical temperature. Based on the Arrhenius equation, the activation energy for superplastic flow is evaluated. The activation energy at constant strain rate is estimated by the logσt vs 1/T relationship. The results show that the activation energy is usually small under the conditions of optimal flow. The characteristics of superplastic deformation of steels above the critical temperature were also analyzed.

ECAP加工を施したMg-8 mass%Li 2相合金の超塑性

Significant phase refinement and dispersion were achieved in a cast Mg-8 mass%Li alloy through processing by equal-channel angular pressing (ECAP) using a die having an internal channel angle of 110° and a pressing temperature of 473 K. Following extrusion and subsequent ECAP through 2 passes, the alloy exhibited superplasticity over the temperature range from 423 to 473 K. A maximum fracture elongation of >950% was attained at 473 K with an initial strain rate of 1.0×10-4 s-1. The measured strain rate sensitivity was of the order of 0.6. The maximum elongation is high in comparison with other Mg alloys tested in tension at the same temperature and strain rate. The activation energy for superplastic flow was essentially equal to the value for grain boundary diffusion of Mg. The use of ECAP gives a significant improvement in strength and ductility at room temperature.

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 έ ≈ 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.

Experimental prediction of deformation mechanism after continuous dynamic recrystallization in superplastic P/M7475

The deformation mechanism in high-strain-rate superplastic P/M7475 before and after continuous dynamic recrystallization (CDRX) was investigated. The recrystallization process in P/M7475 differed from that in conventional superplastic material, I/M7475. In I/M7475, the fine-grained microstructure was obtained by static recrystallization before deformation. On the other hand, the substructure in P/M7475 evolved into fine grains during deformation by CDRX. The percentage of high-angle and random boundaries was low at an initial stage of deformation. However, it increased with strain in P/M7475. The microstructural change in P/M7475 influenced a deformation mechanism and affected grain boundary sliding (GBS). The ratio of contribution of GBS to total elongation was low at an early stage of deformation in P/M7475. However, it increased with deformation progressed. It is suggested that the deformation behavior in P/M7475 changed from dislocation creep to superplasticity as the dominant deformation mechanism changed to GBS. The activation energy for superplastic flow in P/M7475 was close to that for lattice self-diffusion in pure aluminum. It is therefore concluded that the dominant deformation mechanism after CDRX in P/M7475 is GBS accommodated by dislocation movement controlled by lattice self-diffusion, similar to that in I/M7475.

High-strain-rate Superplastic Flow in 6061 Al Composite Enhanced by Liquid Phase

High-strain-rate superplastic behavior of powder-metallurgy processed 0%, 10%, 20%,and 30% SiC particulate reinforced 6061 Al composites was studied over a range of temperatures from 430 to 610 °C. The strength of the 6061 Al composites was lowerthan that of the 6061 Al matrix alloy in the temperature range where grain boundarysliding is believed to control the plastic flow. The difference in their strength was alsoobserved to be temperature dependent, increasing with increase in temperature.Abnormally high activation energy for superplastic flow was another important featureof the 6061 Al composites. These behaviors in particle weakening and activationenergy have strong resemblance to those noted in the high-strain-rate superplastic 2124Al composites studied previously. The observed particle weakening was attributed toliquid-enhanced superplastic flow and discussed by adopting the concept of effectivediffusivity considering mass flow through liquid phase formed at the solute-segregatedregion near SiC/Al interfaces.

An evaluation of the flow behavior during high strain rate superplasticity in an Al−Mg−Sc alloy

An Al-3 pct Mg-0.2 pct Sc alloy was fabricated by casting and subjected to equal-channel angular pressing to reduce the grain size to ∼0.2 µm. Very high tensile elongations were achieved in this alloy at temperatures over the range from 573 to 723 K, with elongations up to >2000 pct at temperatures of 673 and 723 K and strain rates at and above 10−2 s−1. By contrast, samples of the same alloy subjected to cold rolling (CR) yielded elongations to failure of <400 pct at 673 K. An analysis of the experimental data for the equal-channel angular (ECA)-pressed samples shows consistency with conventional superplasticity including an activation energy for superplastic flow which is within the range anticipated for grain boundary diffusion in pure Al and interdiffusion in Al-Mg solid solution alloys.

Partial melting effect on tensile elongation behavior and activation energy for plastic flow in high-strain-rate superplastic 2124 Al alloy and 2124 Al composites

It is now well recognized that high strain-rate superplasticity (HSRS) can be obtained from powderprocessed aluminum and aluminum matrix composites [1–6]. Though the origin of HSRS is still controversial, it has been proposed that the presence of a liquid phase is important for achievement of HSRS as it serves to help the accommodation process for grain boundary sliding by relaxing the high stress concentration around the matrix/particulate interfaces [7]. Mabuchi and Higashi [8] showed that the optimum superplastic temperature for the MMCs is often close to the onset temperature for partial melting. Mabuchi et al. [8–10] and Koike et al. [11] conducted differential scanning calorimeter (DSC) tests on the 20% Si3N4 2124 Al composites reinforced by particulates or whiskers and discovered that for each material two downward peaks appeared in the DSC trace that were small and sharp, and large and continuous, interpreted to represent the local melting at the reinforcement/matrix interfaces and the melting of the aluminum alloy matrix, respectively. The temperatures for partial melting were 816 K, 784 K and 849 K for the 20% Si3N4 (dp= 0.2μm, where dp is the size of particulate reinforcement), 20% Si3N4 (dp= 1μm) and 20% Si3N4w (whisker) 2124 Al composites. Recently, Zahid et al. [12] reported HSRS in a PM 18 vol% SiCp (dp= 3μm)/2124 Al composite where tensile elongations over 300%, with the maximum value of 450%, were obtained in the temperature range between 743 K and 783 K at 7× 10−2 s−1. Their DSC test, however, did not show the sharp peak for partial melting. The onset temperature for a continuous endothermic curve was 861 K, which is significantly higher than the optimum temperatures where superplasticity was obtained. Kim et al. [13] recently investigated the HSRS in a PM 20 vol% SiCp (dp= 3μm)/2124 Al composite at the temperatures between 773 K and 838 K. Tensile elongations of about 300% could be obtained at temperatures between 793 K and 823 K near 10−1 s−1, with the largest tensile elongation of 400% at 808 K. Kim and Hong [14] also examined the HSRS behavior of a PM 2124 Al alloy where tensile elongations over 300% were achieved at temperatures between 773 K and 823 K at the optimum strain rate of 10−2 s−1, with the tensile elongation as large as 700% at 823 K. The aim of the present study is to look for evidence for partial melting in the HSR superplastic 2124 Al alloy and 20 vol% SiCp/2124 Al composite using a DSC technique, and to compare the DSC results with those reported by other investigators. The tensile ductility and activation energy for superplastic flow are also discussed based on the DSC test results. The chemical composition of the matrix of the composite is 3.5%Cu-1.24%Mg-0.53%Mn-0.14%Fe0.01%Zn-bal Al by weight percent. The 2124 Al alloy and 20% SiC/2124 Al composite were prepared by hot pressing at 843 K and hot extrusion at 723 K. DSC tests were conducted on the as-extruded and the superplastically deformed samples in a ∼50 mg specimen during heating and cooling at a rate of 10 K min−1. Two different DSC schedules were arranged for each material: 1) the sample was heated to 993 K; 2) the sample was heated to 878 K, held there for 30 min, and cooled down to 673 K. Then it was heated again to 993 K. Fig. 1 shows the DSC results for the superplastically deformed 2124 Al alloy (803 K, 10−1 s−1, 280%) and 20% SiCp/2124 Al composite (808 K, 9× 10−2 s−1,

The size effect of SiC particulates on activation energy for superplastic flow in a 2124 Al metal matrix composite

It has been demonstrated that high-strain-rate superplasticity (HSRS) is achievable from the powder-metallurgy (PM) processed metal matrix composites (MMCs) reinforced with discontinuous reinforcement such as SiC or Si{sub 3}N{sub 4}. The present research was undertaken with the main objective to investigate the size effect of SiC particulates on the superplastic flow of a PM20 vol %SiC{sub p}/2124 Al composite (hereafter the subscript p denotes particulate) with a special interest in its effect on the true activation energy for superplastic flow. For this purpose, the plastic-flow characteristics of a 20%SiC{sub p}/composite with an average reinforcement size of 8{micro}m was inspected and then, its result was subject to comparison with the data for the 20%SiC{sub p}/composite with a smaller size of reinforcement (3{micro}m) that was previously shown to be HSR superplastic.

Low temperature superplasticity of AZ91 magnesium alloy with non-equilibrium grain boundaries

The superplastic behavior of a fine-grained AZ91 alloy, processed by equal channel angular extrusion, has been investigated in a low temperature range of 423–523 K. The experimental results showed a stress exponent of 2 and the activation energy for superplastic flow was in agreement with that for grain boundary diffusion of magnesium. The alloy with non-equilibrium grain boundary structures exhibited lower superplastic elongation than the alloy with equilibrium grain boundaries. Furthermore, the strain rate for superplastic flow of the former was lower than that of the latter. These differences probably arise because the accommodation process for grain boundary sliding is hampered by the long-range stresses associated with the non-equilibrium grain boundaries.

Low Temperature Superplasticity in a ZK60 Magnesium Alloy

Low temperature superplastic behavior of a magnesium alloy, ZK60, was investigated at about half the absolute melting point. The grains were equiaxed with an average size of 6.5 μm. The tensile tests revealed that the material exhibited low temperature superplasticity (LTSP) with elongation-to-failure of over 400%. Microstructural observations suggested that grain boundary sliding (GBS) made a substantial contribution to the total strain. The stress exponent, grain size exponent and activation energy for superplastic flow were also characterized. The strain rate was inversely proportional to the cube of the grain size and to the second power of stress. The activation energy was close to that for grain boundary diffusion in magnesium. It is suggested that the deformation mechanism for LTSP is GBS accommodated by dislocation movement controlled by grain boundary diffusion.

Quantitative assessment of superplastic deformation behavior in a commercial 5083 alloy

This paper describes the constitutive equation for the superplastic commercial 5083 (Al–Mg–Mn–Cr) alloy effective in a temperature range of 773–843 K and at a strain-rate range from 10 −5 to 4×10 −3 s −1. The superplastic properties have been investigated using velocity jump tests. The analysis of the experimental results reveals that the true stress exponent is two, and the true activation energy for superplastic flow is similar to the lattice self-diffusion of aluminum (142 kJ mol −1). All mechanical data can be represented by a single constitutive equation for superplasticity according to the Dorn equation.

Superplasticity in Al 2O 3-20vol% spinel (MgO · 1.5Al 2O 3) ceramics

Superplasticity in Al 2O 3-20 vol% spinel (MgO · 1.5Al 2O 3) is investigated by means of tensile testing in the temperature range 1300–1550 °C. The dispersion of spinel phase in Al 2O 3 slightly reduces the flow stress, and highly enhances the high-temperature ductility at the same stress level in comparison with 0.1 wt% MgO-doped single-phase Al 2O 3. A maximum elongation of 396% is obtained at 1550 °C and a strain rate of 2.4 × 10 −4s −1. The flow stress reduction is associated with a slight reduction in activation energy for superplastic flow. The extensive ductility in Al 2O 3-20 vol% spinel cannot be explained only from the stress reduction. The Al 2O 3/spinel boundaries are expected to have much larger resistance to crack extension than Al 2O 3 grain boundaries.

Experimental investigation of superplastic behaviour in magnesium alloys

Constant strain rate tensile tests have been conductedfor fine grained Mg-Al-Zn (AZ91) and Mg-Zn-Zr (ZK60 and ZK61) alloys processed by powder metallurgy (PM) and ingot metallurgy (IM) routes. The experimental results revealed that the strain rate was inversely proportional to the cube of the grain size and that the activation energy for superplastic flow was higher than that for grain boundary diffusion. The PM Mg alloys showed superplastic behaviour at higher strain rates than the IM Mg alloys. This is because of smaller grain sizes of the PM Mg alloys. The origin of the high strain rate superplasticity for the PM Mg alloys is unlikely to be associated with the presence of a liquid phase.

Experimental investigation of superplastic behaviour in magnesium alloys

Constant strain rate tensile tests have been conductedfor fine grained Mg-Al-Zn (AZ91) and Mg-Zn-Zr (ZK60 and ZK61) alloys processed by powder metallurgy (PM) and ingot metallurgy (IM) routes. The experimental results revealed that the strain rate was inversely proportional to the cube of the grain size and that the activation energy for superplastic flow was higher than that for grain boundary diffusion. The PM Mg alloys showed superplastic behaviour at higher strain rates than the IM Mg alloys. This is because of smaller grain sizes of the PM Mg alloys. The origin of the high strain rate superplasticity for the PM Mg alloys is unlikely to be associated with the presence of a liquid phase.

Deformation of PM superplastic aluminium alloys : K. Higashi. (University of Osaka Prefecture, Osaka, Japan.) J.Japan Soc. Powder and Powder Metallurgy, vol 42, no 6, 1995, 725–734. (In English.)

The activation energy for superplastic flow in a variety of Si3N4Al composites exhibiting high-strain-rate superplasticity was analyzed. The activation energy in a solid state including no liquid was in agreement with the one for lattice self-diffusion. However, the activation energy was increased by the presence of a liquid phase. The behaviors of the Al-Cu-Mg(2124) alloy composites were different from those of the Al-Mg(5052), AI-Mg-Si(6061) and Al-Zn-Mg(7064) alloy composites. This is probably because partial melting occurred at a temperature lower than the apparent partial melting temperature determined by the DSC investigation, however, the volume of liquid was too small to be measured by the DSC investigations for the Al-Cu-Mg composites.

Activation energy for superplastic flow in aluminum matrix composites exhibiting high-strain-rate superplasticity

The activation energy for superplastic flow in a variety of Si 3N 4 Al composites exhibiting high-strain-rate superplasticity was analyzed. The activation energy in a solid state including no liquid was in agreement with the one for lattice self-diffusion. However, the activation energy was increased by the presence of a liquid phase. The behaviors of the Al-Cu-Mg(2124) alloy composites were different from those of the Al-Mg(5052), AI-Mg-Si(6061) and Al-Zn-Mg(7064) alloy composites. This is probably because partial melting occurred at a temperature lower than the apparent partial melting temperature determined by the DSC investigation, however, the volume of liquid was too small to be measured by the DSC investigations for the Al-Cu-Mg composites.

Dislocation-creep-controlled superplasticity at high strain rates in the ultrafine-grained quasi-eutectoid Ti10Co4Al alloy

The ultrafine-grained two-phase Ti10Co4Al alloy, consisting of α-Ti(Al) solid solution with a fine dispersion of 23 vol.% of the intermetallic Ti 2Co compound, exhibits superior superplastic properties at relatively low deformation temperatures between 650 and 750 °C and high strain rates up to ε ̇ ≈ 5 × 10 −2, s −1 . Maximum m values of about 0.5 and elongations of more than 1000% were achieved. The activation energy of the rate-controlling deformation mechanism at high strain rates ( ε ̇ ⩾ 10 −3, s −1 ) is in good agreement with that reported for lattice self-diffusion of titanium in α-Ti. A grain size exponent of about p = 2 was determined at high strain rates which decreases in the regime of power-law creep. A deformation model is presented to explain the superplastic behaviour of this alloy at high strain rates. This model, proposed by Fukuyo et al., is based on grain boundary sliding, accommodated by sequential steps of dislocation glide and climb. A general flow equation for dislocation-creep-controlled superplasticity involving dislocation glide competing with climb processes is discussed. In the slightly solid-solution-hardened α-Ti(Co, Al) matrix (solid solution class II) dislocation climb becomes the rate-controlling step. The maximum m value, the grain size exponent and the activation energy for superplastic flow of Ti10Co4Al are related to the predictions of solid solution class II alloys in the above model.

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 microduplex structure-containing cementite in volume fractions of 22 % (Fe,Cr,Ni) 3 C, 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 o-'/-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.