High Strain Rate Superplasticity Research Articles

The mechanical properties of nanostructured materials

In this paper, the state-of-the-art progress in research on novel mechanical properties of nanocrystalline materials and carbon nanotubes is reviewed. There is evidence that the relation between the strength of nanocrystalline materials and grain size does not observe the classic Hall-Petch plot. Lowtemperature and high-strain rate superplasticity have been found in some nanocrystalline materials. Theoretical prediction and experimental data indicate that carbon nanotubes are materials with high stiffness, high strength, great toughness, and low density. There are already some application examples for novel mechanical properties of nanocrystalline materials and carbon nanotubes.

Processing commercial aluminum alloys for high strain rate superplasticity

The superplastic forming of sheets, although used on a small, specialized scale, has not yet been adapted for high-volume purposes. In processing for superplastic properties, the key microstructural features are grain size and grain-boundary misorientation. Current research trends are two-fold: to evaluate bulk processing of ultrafine-grained materials, and to understand the superplastic behavior of ultrafine-grained materials. Work is continuing on understanding the relationship between grain size and superplastic strain rate and superplastic temperatures. In this paper, strategies are reviewed for producing microstructures with enhanced superplastic characteristics.

An experimental investigation of a superplastic constitutive equation in Al–Mg–Si alloy composites reinforced with Si3N4 whiskers

Superplastic properties at 818 K were investigated by tensile tests for Al–Mg–Si alloy composites reinforced with Si3N4 whiskers whose volume fractions were 0–30%. The 20 and 30 vol.% composites exhibited large elongation of 615 and 285% at a high strain rate of 2×10−1 s−1, respectively. High strain rate superplasticity of the composites is attributed to the very small grain size of less than 3 μm. The stress–strain rate relation for the composites was almost the same as that of the alloy, taking into consideration the influences of threshold stress and grain size, and the relation was independent of the volume fraction of whisker. This is probably because grain boundary sliding was not hampered by the whiskers due to the presence of liquid phase for the composites.

Microstructure and high strain rate superplasticity of in situ composite synthesised from aluminium and nano­ZrO2 particles by powder metallurgy

Pure aluminium matrix composites reinforced mainly with a fine needle­like Al3Zr phase were synthesised by the reaction between nanocrystalline ZrO2 particles and pure aluminium using powder metallurgy followed by hot extrusion and hot rolling. After extrusion, the composite fabricated with 5 vol.-%ZrO2 had a strain rate sensitivity (m value) of 0.31 and a total elongation of 156% at 650°C and 1.3×10-1 s-1. The high strain rate superplasticity was improved by hot rolling. After hot rolling, the m value of the composite fabricated with 10%ZrO2 particles was 0.40 and the total elongation was 204% at 640°C and 1.3×10-1 s-1. The superplastic deformation mechanism for the present composites involves grain boundary sliding and interfacial sliding. From this mechanism, the following experimental results can be easily understood: the total elongation increased with temperature, and the strain rate at which maximum total elongation was attained increased with increasing deformation temperature.

High Strain Rate Superplasticity in Al-16Si-5Fe Based Alloys with and without SiC Particulates

Superplasticity at a high strain rate of 1.4×10 -1 s -1 was exhibited at 783K in the powder metallurgy processed Al-16Si-5Fe-1Cu-0.5Mg-0.9Zr (wt%) alloys with and without 5 wt% addition of SiC particulates. The maximum elongations of 350% and 270% were obtained for the monolithic and composite material, respectively, where high m values were found. The high strain rate superplasticity in these materials was attributed to the presence of a thermally stable fine microstructure and the operation of grain boundary sliding. The lower superplastic elongation of the composite specimen compared to that of the monolithic specimen was suggested to the result of an increased density of cavities owing to the SiC decohesion at the SiC/matrix interface.

Friction Stir Processing: A New Grain Refinement Technique to Achieve High Strain Rate Superplasticity in Commercial Alloys

Friction stir processing is a new thermo-mechanical processing technique that leads to a microstructure amenable for high strain rate superplasticity in commercial aluminum alloys. Friction stirring produces a combination of very fine grain size and high grain boundary misorientation angles. Preliminary results on a 7075 Al demonstrate high strain rate superplasticity in the temperature range of 430-510 °C. For example, an elongation of >1000 % was observed at 490 °C and 1 × 10 -2 s -1 . This demonstrates a new possibility to economically obtain a superplastic microstructure in commercial aluminum alloys. Based on these results, a three-step manufacturing process to fabricate complex shaped components can be envisaged: cast sheet or hot-pressed powder metallurgy sheet + friction stir processing + superplastic forging or forming.

溶湯撹はん法で作製したＳｉＣｐ／７０７５アルミニウム合金複合材料の高速超塑性

A SiCp/7075 aluminum alloy composite has been fabricated by a vortex method followed by squeeze casting, extrusion and rolling. The composites were hot rolled to the total rolling reduction of about 94% at temperatures between 573 K to 773 K and at a rolling strain per pass of 0.10. Superplastic characteristics such as microstructure and apparent activation energy were compared with these of the composites made by other processes in order to clarify the superplastic deformation mechanism. Fine grain size of about 1 μm was attained in the composite rolled at a temperature of 573 K and at the strain per pass of 0.1. In the case of rolling at 573 K, the composite obtained exhibited high strain rate superplasticity with a maximum elongation of about 230% at strain rate of 7 × 10−1 s−1 and at 798 K. Plots between e1/n and σ showed linear relation when exponent of n=2 was assumed. Threshold stress obtained from the linear relations were largely dependent on the testing temperature. Apparent activation energy determined from relationship between strain rate and testing temperatures was 244 kJ/mol, which was smaller than that for the powder metallurgical and mechanically alloyed aluminum composites. It seems that there is no substantial difference of high strain rate superplastic mechanism between the composite fabricated by a vortex method and powder metallurgical aluminum alloy composites.

High Strain Rate Superplasticity in Mechanically Alloyed Nickel Aluminides

Grain refining for three nickel aluminides such as γ'-Ni 3 Al (25Al) and β-NiAl (49Al) single-phase intermetallics, and (γ'+β) two-phase intermetallic (34Al) has been attained by using a wet mechanical alloying and vacuum hot pressing process. The values of average grain size of the three compacted intermetallics were very close and as small as about 1.0 μm. The superplastic flow behaviors of the fine grain intermetallics were examined mainly by hot compression tests, together with some tensile tests. The compression tests were performed at the temperatures of 1073K-1273K and at an initial strain rate range from 1.4× 10 -4 s -1 to 5.6 x 10 -2 s -1 . The results obtained from the compression and tensile tests revealed that γ single-phase intermetallic (25Al) and (γ'+β) two-phase intermetallic (34Al) can be deformed superplastically at 1273K and at a high strain rate range of about 4 ×10 -3 s -1 ∼ 5.6 × 10 -2 s -1 . However, β (49Al) single-phase intermetallics showed poor ductility in tensile tests, probably due to a cavity formation during the deformation. Grain boundary sliding accommodated by slip was considered to contribute much to the high strain rate superplasticity in the intermetallics including γ' phase.

High Strain Rate Superplasticity in a Zn - 22% Al Alloy after Equal-Channel Angular Pressing

It is well established that superplasticity requires a small grain size and therefore there is an interest in developing processing methods having the capability of reducing the grain size below that generally produced in conventional thermo-mechanical processing. Experiments were conducted on a superplastic Zn-22%Al eutectoid alloy where the material was subjected to equal-channel angular pressing (ECAP) through 4, 8 or 12 passes at a temperature of 373 K. It is shown that this processing procedure leads to additional grain refinement such that the grain size was reduced from an initial value of ∼1.8 μm to values as low as ∼0.6 μm. Tensile testing after ECA pressing revealed a significant enhancement of the superplastic properties including both increases in the total elongations to failure and the occurrence of these high elongations at very rapid strain rates. Elongations were achieved of up to >2380 % at a strain rate of 1 s -1 The results suggest that ECAP may be a very useful procedure for attaining superplasticity at high strain rates in commercial alloys.

Low Temperature and High Strain Rate Superplasticity of Nickel Base Alloys

This paper will describe features of micro-, submicro-, and nano-crystalline structure formation under severe plastic deformation and the influence of structure on superplastic (SP) behavior of high alloyed nickel base alloys with a range of hardened precipitates (γ'; γ+δ, γ'+Y 2 O 3 ). It has been shown that severe plastic deformation over a wide range of homologous temperatures (0.9-0.2T m ) can refine the microstructure to a size of several tens of nanometers. In comparison with the microcrystalline (MC) state, the submicrocrystalline (SMC) structure in dispersion hardened alloys reduces the optimal temperature of SP deformation from 0.9-0.8T m to 0.7-0.6T m . The PDS alloy combines precipitation and dispersion hardening (γ'+Y 2 O 3 ), and, in the SMC state, it can display both low temperature and high strain rate superplasticity. Features of microstructure transformations and failure of samples during SP deformation are considered.

High Strain Rate Superplasticity in 6061 Alloy with 1% SiO<SUB>2</SUB> Nano-Particles

Four 6061 aluminum systems were prepared in this study, including the DC cast 6061 alloy processed by TMT and ECAP, the PM modified 6061 alloy added with 1 vol% nano-SiO 2 , and the commercial 6061/20%SiC w composite. All materials possessed fine grain sizes ∼0.3-1 μm after processing, but only the latter two could maintain fine grain size upon loading at high temperatures and achieved HSRS over 300% at 570-590°C and 1-5 x 10 -1 s -1 The texture was weaker and the grain boundary misorientation distribution was more random in the 6061/1%nano-SiO 2 alloy as compared with the unmodified 6061 alloys. The 1%nanoSiO 2 modified alloy exhibited low stresses ∼4 MPa, true strain rate sensitivity ∼ 0.5, smooth GBS and a limited amount of liquid phase at high temperatures and strain rates, similar to those observed in HSRS composites. The current results suggest that the addition of a small amount ( 1 vol%) of SiO 2 or Al 2 O 3 nano-particles into commercial Al alloys can effectively suppress grain growth at high temperatures and enhance HSRS.

High Strength and High Strain Rate Superplasticity in Magnesium Alloys

Mechanical properties of fine-grained Mg alloys processed by powder metallurgy (P/M) and ingot metallurgy (I/M) routes have been investigated by tensile tests at room temperature and 573 K. The superplastic strain rate range for the P/M Mg alloys was higher than that for the I/M Mg alloys. Furthermore, the P/M Mg alloys exhibited higher room temperature strength than the I/M Mg alloys. These excellent mechanical properties of the P/M Mg alloys are attributed to a very small grain size of 0.5 ∼ 1 μm.

High Strain Rate Superplasticity and Microstructure Study of a Magnesium Alloy

Superplastic-like behavior was found in a commercial large-grained Mg-3Al-1Zn alloys (AZ31), at high temperature and high strain rate regime. The creep behavior of this material was studied from RT to 550°C and the strain rates of 0.001 s -1 ∼1 s -1 , and the maximum elongation to failure of 170% was obtained at 0.01 s -1 and at 500°C (or 0.84 T m ), which is much higher than the equivalent temperature of most known superplastic materials. Optical microscopy, transmission electron microscopy (TEM) and scanning electron microscopy with electron backscattering diffraction technique (SEM-EBSD) were used to investigate the microstructure and deformation mechanisms involved. It is evident that high temperature and high rate dislocation creep played a critical role at the early stage deformation in breaking down the initial large grains without fracture, and combined GBS and dislocation creep define the steady-state grain size and control the quasi-superplastic behavior.

Grain Refinement of a Commercial Magnesium Alloy for Superplastic Forming

Superplastic forming was performed at a high strain rate of 10 -2 s -1 for a commercial ZK60 magnesium. The relatively high forming temperature of 673 K was decided from the superplastic characteristics owing to its grain size ∼ 5μm. In order to prevent the grain growth during the high temperature superplasticity, the possibility of low temperature superplasticity(LTSP) in magnesium was derived. The required grain size for high strain rate superplasticity(HSRS) at the low homologous temperature was estimated, and the fabrication techniques to develop the fine-grained structure were discussed.

SiCp/6061アルミニウム合金複合材料の超塑性に及ぼす粒子径と粒子体積率の影響

Effects of particle size and volume fraction of SiC particle on superplastic characteristics of SiCp/6061 aluminum alloy composites made by a vortex method before squeeze casting, extrusion and hot rolling were investigated in order to make clear the superplastic deformation mechanism. Flow stress of the SiCp (0.6 μm) /6061 aluminum alloy composite decreased with increasing the volume fraction in the case of the volume fraction of 0.13 and 0.20, and the m value became 0.30∼0.46 in the initial strain rate region of more than 2.4 × 10−1 s−1, and the maximum total elongation more than 200% was obtained at 853 K. And also, SiCp (1.2 μm) /6061 aluminum alloy composite exhibited m value of 0.32 at an initial strain rate of 7 × 10−2 s−1 and maximum total elongation of about 170%. However, 5 and 10 μmSiCp/6061 aluminum alloy composites did not produce superplasticity. It is thought that in SiCp/6061 aluminum alloy composite made by a vortex method, Vf=0.20 is optimum volume fraction for maximum total elongation. The threshold stress of the composites decreased with increasing SiC volume fraction. The stress exponent (n) indicated 3 for the creep deformation in the case of SiCp (5 and 10 μm)/6061 aluminum alloy composites, n showed 2 for the superplastic deformation in the case of SiCp (0.6 and 1.2 μm) /6061 aluminum alloy composites. On and near the fracture surface of the composite after superplastic deformation, filaments and striation bands were formed at sliding grain boundaries and interfacial sliding were observed and it is thought that high strain rate superplasticity of the composite could occur by grain boundary sliding and liquid phase accommodation mechanisms.

Using Severe Plastic Deformation for Grain Refinement and Superplasticity

The grain size of many metals may be reduced to the submicrometer or nanometer level using various procedures incorporating the application of severe plastic deformation. This paper examines the procedure and potential of equal-channel angular pressing (ECAP) in which a material is pressed through a die and a high strain, is imposed without any change in the cross-sectional dimensions of the workpiece. Various factors influence the ultrafine grains attained using ECAP and examples are presented showing the application of ECAP in fabricating materials exhibiting high strain rate superplasticity and rapid forming capabilities.

湿式ＭＡ法により結晶粒微細化した（γ′＋β）２相ニッケルアルミナイドの高速超塑性特性

Grain refinement of Ni3AI (γ') and NiA1(β) two-phase nickel aluminides (34A1), together with nominally single γ' phase (30A1) or β phase (43A1) nickel aluminides has been attained by a wet mechanical alloying and vacuum hot pressing process. The average grain sizes of the three as-compacted intermetallics were very close and about 1.0 μm. High temperature compression tests were carried out at a temperature range of 1073 K-1273 K and at an initial strain rate range of 1.4×10-4s-1- 5.6×10-2 s-1. Tensile tests were also performed at selected strain rates at 1273 K.The strain rate sensitivity exponent m for 34A1 evaluated by compression test at 1273 K was as high as 0.38 at an initial strain rate range from 1.4×10-4s-1 to 5.6×10-2 s-1. The maximum elongation of about 250% was obtained at a strain rate of 4.2×10-3s-1 for 34A1. Even at high strain rate of 4.2×10-2s-1 and at 1273 K, 188% elongation was obtained for 34A1. These results suggested the possibility of high strain rate superplasticity in the two phase intermetallics. By the microstructure observations using SEM and TEM, grain boundary sliding and dislocation activity within grains were found in the two phase intermetallics deformed at the strain rates higher than 10-2s-1. The deformation behavior and mechanism for the two phase nickel aluminides were discussed in the comparison of the results of γ' and β nominally single phase intermetallics.

ＴｉＣ粒子を複合化した（γ′＋β）２相ニッケルアルミナイドの熱調質処理による高温機械的性質の改善

Ni3AI (γ')-NiAI(β) two-phase nickel aluminides with or without reinforcement of 10vol.% TiC particles (expressed as 10%TiC and 34Al, respectively) can be fabricated by a wet mechanical alloying and vacuum hot pressing process. Recently, we have reported these two-phase nickel aluminides have very fine grain structure and, thus, show a high strain rate superplasticity. In this study, an effect of two types of heat treatment on the microstructure have been investigated in order to improve a mechanical properties at high temperatures. The microstructure control is on concerning of the change from the fine grain structure suitable for superplasticity to a coarse grain structure or a lamella structure for the strengthening at high temperatures after the superplastic deformation.The heat treatment to obtain a coarse grain structure in the two-phase nickel aluminides was carried out at 1623 K for 43.2ks. The grain coarsening to about 14μm from 1.2μm in 34Al and to about 8μm from 0.9μm in 10%TiC were attained in average grain sizes. The heat treatment to obtain a lamella structure was consisting of oil quenching from 1623 K and tempering at 1073 K or 1273 K. After this heat treatment, the network like precipitation of γ' phase at the prior β-grain boundaries was observed in all intermetallics. A fine lamella structure was only formed in the network of 34Al tempered at 1073 K.After both heat treatments, the bending strength and defection to fracture in three point bending test above 873 K, and 0.2% flow stress in compression test above 973 K for 34A1 and 10%TiC were improved, compared with fine grained intermetallics. Especially, the formation of ductile networks of γ' phase surrounding brittle β phase was considered to be effective for improving both high temperature strength and ductility. The heat treatment for the coarse grain structure showed more advantage for strengthening at temperatures above 1173K.

On the Activation Energies Observed in Al-based Materials Deformed at Ultrahigh Temperatures

This study examines the exceptionally high activation energies estimated fi om data obtained from various composites exhibiting high strain rate superplasticity and loaded at ultrahigh temperatures near or slightly above the solidus. It is shown that the activation energies for the 6061 composite and for Al 6061 and 1050 alloys are below 200 kJ/mol at temperatures lower than the incipient melting temperature, T-i, but they become significantly higher at temperatures near or slightly above T-i and this increase is independent of the alloy system, the grain size, whether the material is superplastic and the dominant deformation mechanism at T 610 degreesC and in the Al-1050 alloy at T>650 degreesC. It is concluded that the high values of Q(t) have no physical meaning in terms of the rate-controlling diffusive species.

High strain rate superplasticity in Japan

Recent research into superplasticity has developed substantially, to the extent that there are now important directions for future investigations, one of which is high strain rate superplasticity (i.e., superplastic behaviour at strain rates over 10-2 s-1) in metallic materials. Superplasticity at extremely high strain rates over 100 s-1 has been demonstrated and labelled as positive exponent superplasticity. Recent work on processing to produce ultrafine grained materials, and on deformation mechanisms and applications of high strain rate superplasticity, is reviewed and considered along with previously reported data.