Concurrent Grain Research Articles

Microstructural evolution and superplastic deformation behavior of fine grain 5083Al

The microstructural evolution during superplastic deformation of a fine grain Al-4.7 pct Mg alloy (5083Al) has been studied quantitatively. Starting from an average grain size of 7 µm, grain growth was monitored in this alloy both under static annealing and with concurrent superplastic deformation at a high test temperature of 550°C. Grain size was averaged from measurements taken in longitudinal, transverse, and thickness directions and was found to grow faster during concurrent superplastic deformation than for static annealing. A grain growth law based on an additive nature between time-based and strain-based growth behavior was used to quantify the dynamics of concurrent grain growth. The extent of void formation during deformation was quantified as the area fraction of voids on L-S planes. This void fraction, referred to as the cavity area percent, was recorded at several levels of strain for specimens deformed at two different strain rates. A constitutive equation incorporating this grain growth data into the stress-strain rate data, determined during the early part of deformation, was generated and utilized to model the superplastic tensile behavior. This model was used in an effort to predict the stress-strain curves in uniaxial tension under constant and variable strain rate conditions. Particular attention was paid to the effects of a rapid prestrain rate on the overall superplastic response and hardening characteristics of this alloy.

Quantitative studies of irradiation-induced segregation and grain boundary migration in Fe [sbnd]Cr [sbnd]Ni alloy

An irradiation-induced phenomenon was studied on account of solute segregation and concurrent grain boundary migration in a model Fe Cr Ni alloy during electron irradiation with a high voltage electron microscope (1000 kV) and also by computer simulation. The calculation was conducted by solving the coupled rate equations for solute and defect concentrations, which involve the Kirkendall effects at a grain boundary sink, so that the solute redistribution profiles experimentally obtained were quantitatively explained. We also extensively studied the effects of the probe size in the EDS analysis on composition profiles, the Gibbsian segregation in the vicinity of a grain boundary and the discriminant of segregation under irradiation, as well as other important factors that influence the phenomenon.

A theoretical upper limit to Coble creep strain resulting from concurrent grain growth

The rate of diffusional creep varies with grain size x, either as 1/x 2 or 1/x 3, depending on whether lattice or grain boundary diffusion is dominating. Since the rate of grain growth is proportional to 1/x p, where p⩾1, the creep and grain growth relationships can be combined to predict the transient creep that results from the two processes operating concurrently. An important result is obtained for grain boundary diffusion creep (Coble creep), where two regimes of behaviour are predicted depending on the value of p. For normal grain growth (p=1) and up to a critical value p=2, the transient gives rise to an upper limit to the grain boundary diffusional creep strain. For p>2, no limiting strain is predicted. The role of the limiting strain is discussed in the context of the various experimental attempts that have been made to verify the Coble mechanism.

Diffusional Creep and Cavitational Strains in High‐Purity Alumina under Tension and Subsequent Hydrostatic Compression

Diffusional creep and cavitation in pure alumina prepared with three different fabrication processes are compared under tension and subsequent hydrostatic compression. The deformation rates are separated into a volume‐conserving creep rate and cavitational rate by measuring the longitudinal and transverse strains intermittently during deformation. Concurrent grain growth causes the volume‐conserving strain rate to decrease in a manner consistent with Nabarro‐Herring creep. The creep stress index of n= 1.3 and the average activation energy of Q= 480 kJ/mol are also consistent with Nabarro‐Herring creep controlled by aluminum lattice diffusion. Anelastic loading and unloading transients are also identified and separated from the creep strains. High‐voltage electron microscopy indicates that cavities nucleate at grain edges early and continuously in the creep process. The surfaces of these cavities tend after some growth to exhibit negligible curvature and various dihedral angles. The activation energy of Q= 450 kJ/mol and stress dependence of the cavitation rate of n= 1.3 are consistent with a grain boundary diffusional growth mechanism. The loading mode is found to have no significant effect on the cavitation rate during tensile creep and the subsequent decavitation rate during hydrostatic compression. The cavitation and decavitation rates are in good agreement with the model proposed by Speight and Beere when the effects of grain growth on cavity accumulation on grain boundaries are included. Exaggerated grain growth in high‐density specimens can lead to early cavity coalescence and failure.

Microstructural aspects of superplastic tensile deformation and cavitation failure in a fine-grained yttria stabilized tetragonal zirconia

A fine-grained yttria stabilized tetragonal zirconia exhibits an optimum superplastic elongation to failure of ∼ 700% at 1823 K and a strain rate of 8.3 × 10 −5s −1. A detailed microstructural investigation of the superplastically deformed specimens reveals the occurrence of extensive concurrent grain growth and internal cavitation. An expression is developed to characterize the extent of deformation enhanced concurrent grain growth, as influenced by experimental factors such as true strain, strain rate and temperature. The variation in the level of concurrent cavitation with strain rate conforms closely to the variation in elongation to failure with strain rate. It is demonstrated that the tendency towards cavity interlinkage in a direction perpendicular to the tensile axis is an important factor influencing the total elongation to failure obtained in superplastic materials.

Superplasticity in fine-grained 20% Al 2O 3/YTZ composite

The microstructure and superplastic deformation of fine-grained, 20% Al 2O 3-reinforced, yttria-stabilized, tetragonal zirconia (Al 2O 3/YTZ) has been investigated. Hot identation and tensile tests were carried out at temperatures between 1000° and 1650°C. The material exhibited extensive plasticity at temperatures higher than 1200°C, and a maximum tensile elongation of over 625% was recorded. The microstructure of the Al 2O 3/YTZ was thermally unstable, and concurrent grain growth, and in particular dynamic grain growth, was observed at temperatures higher than 1450°C. Although the apparent strain rate sensitivity exponent was determined to be approximately 0.5, the true strain rate sensitivity, after compensation for grain growth, is about 0.67. The activation energy for superplasticity in Al 2O 3/YTZ was determined to be 257 kJ/mol which is much lower than the activation energy for superplastic Y-TZP. Of particular interest were the grain-boundary structures and the chemical compositions of the grain boundaries. Microstructures were studies using transmission electron microscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. No glassy phase was detected at grain boundaries.

Influence of grain growth on activation energy measurements for high temperature creep and superplastic deformation of metals and ceramics

Grain growth is a very commonly observed phenomenon during the elevated temperature deformation of fine grained metals and structural ceramics. It is demonstrated that the occurrence of grain growth can lead to substantial errors in the measurements of the activation energy for plastic deformation. Experimental procedures and analytical techniques are developed to determine the true activation energy for creep and superplasticity by accounting for the occurrence of concurrent grain growth.MST/1263

α Grain size and β volume fraction aspects of the superplasticity of Ti-6Al-4V

The elevated temperature deformation behaviour of Ti-6Al-4V has been investigated in the temperature range between 775 and 925 °C and for true strain rates 2 × 10−5 to 0.005 s−1. Region III was characterized by m = 0.13 and Q = 602 kJ mol−1. Region II is subdivided into a low temperature region IIa and a high temperature region IIb. Region IIa is characterized by m = 0.77, Q = 216 kJ mol−1 and relatively low ductility while region IIb is characterized by n = 1.3, and identical value of Q and high ductility. Low and high β-phase proportions are responsible for regions IIa and IIb respectively. An apparent region I was accounted for by concurrent grain growth utilizing a grain size sensitivity parameter which is dependent on the β-phase volume fraction. The strain rate-temperature regimes of regions IIa, IIb and III are illustrated in a “region” map.

Recent advances in superplastic ceramics and ceramic composites

AbstractSuperplasticity in ceramics and ceramic composites is reviewed. Deformation characteristics, and in particular the strain rate sensitivities in these materials, are discussed. From the strain rate sensitivity and grain size dependence, deformation mechanisms in these superplastic materials have been addressed. It appears that grain boundary sliding is the predominant deformation mechanism. Constitutive equations governing the superplastic flow in superplastic ceramics and ceramic composites are presented. Microstructural features in these materials, e.g. grain size and morphology, and glassy phases at grain boundaries, are described. Microstructure stability, and particularly, concurrent grain growth during superplastic deformation and its effect on superplastic elongation, is discussed. In this review, the superplastic properties and microstructures of some other advanced materials, e.g. intermetallics, metal matrix composites, and mechanically alloyed alloys, are also briefly reviewed and compar...

Nucleation and Abnormal Grain Growth of Alpha-A12O3 in Gamma-Alumina Matrix

AbstractThe microstructures of reactive sputter-deposited alumina films have been studied by transmission electron microscopy. The as-deposited films contained γ-A12O3 phase in an amorphous alumina matrix. Annealing of the films at 1200° C for 2 h resulted in nucleation and concurrent anomalous grain growth of α-A12O3 in a polycrystaUine γ-Al2O3 matrix which exhibited a layered microstructure and was strongly textured along [001]. The grain sizes of α-A12O3 varied from 3 to 20 μm, while the average grain size of γ-A12O3 was only about 50 nm. It appears that the nucleation kinetics of a-A12O3 was slow. As a result, the abnormal grain growth of α-A12O3 proceeded by consuming surrounding γ-Al2O3 grains. An atomic model is presented to explain the origin of layered structure in γ-A12O3. The nucleation mechanism of a-A12O3 in γ-alumina matrix is suggested. Orientation relationships between γ- and α-A12O3 are reported. The anomalous grain growth of α- A12O3 is discussed in terms of γ/α interface boundary migration.

Effect of grain size on superplastic behavior of Y-TZP

This paper reports that grain size strongly influences the properties of fine-grained superplastic metal alloys. In principle, fine grain sizes enhance grain boundary sliding which is the dominant superplastic deformation mechanism. Recently, some fine-grained ceramics have been shown to be superplastic in tension. A maximum tensile elongation of about 800% has been recorded in a yttria-stabilized tetragonal zirconia polycrystal (Y-TZP). During superplastic deformation, concurrent grain growth, and in particular dynamic grain growth, was observed in this material. It has been demonstrated that this concurrent grain growth strongly affects the superplastic flow stress in the material, and also confuses the accurate determination of the stress dependency of the superplastic strain rate. It is important, therefore, to understand the precise grain size dependence of superplastic flow in ceramics. It has been shown that the superplastic strain rate of Y-TZP is inversely proportional to the grain size raised to a 1.8 power. The effect of concurrent grain growth, however, was not considered in their analyses.

Creep of a low carbon steel at low stresses and intermediate temperatures

The creep of a low carbon steel in the form of helical coils was investigated in the temperature range 698–923 K (0.39 to 0.51 T m), for grain size 6–23 μm and stresses up to 7 MPa. Unlike previous studies on pure iron which showed substantial enhancements in creep rate compared with theoretical predictions, good agreement was obtained for recrystallised material between observed strain rate per unit stress and predictions of diffusional creep theory for the temperature range 698–823 K over which grain growth did not occur. The associated activation energy of 169 ± 12 kJ/mol confirms grain boundary diffusion creep as the dominant creep mechanism under these conditions. At 873 and 923 K, stress independent concurrent grain growth appears to result in enhancement of the creep rate. Transient enhancement at 773 K is inversely proportional to a test duration parameter P 0 = 2D g t d 3 , where w is grain boundary width, D g is grain boundary diffusivity, t is test duration and d is grain size. Creep exhibited a threshold stress with a strong temperature dependence exemplified by an activation energy of 68 ± 14 kJ/mol. This is substantially larger than in previous work on pure iron possibly as a result of interactions between grain boundary dislocations and impurities. An increased enhancement of strain rate per unit stress was observed at 698 K with increase of extent of cold work prior to testing, persisting even after extended creep times. This indicates that increased dislocation density resulting from cold work had a greater effect on creep rate than any creep strengthening arising from the elongated grain shape that also results from prior cold work.

Superplastic Deformation Mechanisms in a Fine-Grained, Yttria-stabilized Tetragonal Zirconia Polycrystal (Y-TZP)

ABSTRACTConcurrent grain growth, and in particular, dynamic grain growth, was observed to take place during superplastic deformation of Y-TZP. As a result of this concurrent grain growth, the measured strain rate sensitivity was found to be lower than that measured under constantstructure conditions. In the present paper, data obtained from the superplastic deformation of YTZP under constant-structure conditions are presented. It is demonstrated that the strain rate sensitivity values are generally higher than 0.5, when measured from the grain size-compensated data; this result suggests a grain boundary sliding mechanism. Microstructures from samples prior to and after superplastic deformation reveal grains which are essentially equiaxed; this observation is also consistent with a grain boundary sliding mechanism. Both high-resolution images of grain boundary triple points using transmission electron microscopy, and fracture surface studies using Auger electron spectroscopy and X-ray photoelectron spectroscopy indicate that there is no evidence for the presence of glassy phases at grain boundaries in Y-TZP.

Effect of cyclic deformation on strain enhanced grain growth during superplastic flow

Generally, superplastic deformation is accompanied by concurrent grain growth. In this paper, the effect of strain reversals on this process is investigated using two materials. In the first, Coronze 638, a precipitate strengthened copper alloy, no effect is found. In the second, Zn–22Al (wt-%) eutectoid, a distinct retardation of grain growth occurs after each strain reversal. A new model based on the ability of grain boundary sliding to overcome the Zener drag process is developed and found to be consistent with the experimental results. MST/731.

Effect of cyclic deformation on strain enhanced grain growth during superplastic flow

Generally, superplastic deformation is accompanied by concurrent grain growth. In this paper, the effect of strain reversals on this process is investigated using two materials. In the first, Coronze 638, a precipitate strengthened copper alloy, no effect is found. In the second, Zn–22Al (wt-%) eutectoid, a distinct retardation of grain growth occurs after each strain reversal. A new model based on the ability of grain boundary sliding to overcome the Zener drag process is developed and found to be consistent with the experimental results.MST/731

Superplasticity in Al–33Cu eutectic alloy in as extruded condition

AbstractExperiments were carried out to determine the superplastic properties of the Al–33Cu eutectic alloy in an as extruded condition. It is shown that the stress–strain curves do not attain a steady state condition and, except at high strain rates greater than ~10−2 s−1, the curves show strain hardening due to concurrent grain growth. There is a sigmoidal relationship between stress and strain rate, with a maximum strain rate sensitivity of ~ 0·5 at intermediate strain rates in region 2 and a decrease in the strain rate sensitivity to ~ 0·3 at low strain rates in region 1. The maximum elongation to failure in these experiments is ~1400% at an initial strain rate of 6·7 × 10−5 s−1 and there is a decrease in the elongations to failure at both lower and higher strain rates. From detailed experimental measurements of grain growth, it is demonstrated by calculation that there is a genuine region 1 at low strain rates in this alloy in the as extruded condition.MST/911

微細粒７４７５アルミニウム合金の超塑性変形特性

Superplastic deformation behavior of fine grain 7475 aluminum alloy at 788K was characterized up to large plastic strain by means of both tensile tests of constant strain-rate and constant velosity. Significant hardening in constant strain-rate test and softening in constant velosity test were showed in the deformation. The occurrence of deformation in constant strain-rate test induced grain growth which explains nearly all of the hardening were observed in the microscopic studies. The stress-strain behavior deformed in constant velosity test is determined by the strain-rate history. The strain-rate sensitivity, m, is found to keep the initial value and decreases after larger strain as a result of concurrent grain growth. The stress-strain-rate behavior determined initially might not be applicable after large amounts of plastic strain. The strain-rate obtained maximum superplastic elongation is agreement with that where larger m has been kept up to larger strain. The reason why the larger elongation shows at higher strain-rates in constant velosity tests than constant strain-rate tests may be the maintain of large m value during superplastic deformation.

Effect of concurrent grain growth on the stress-strain rate curve of superplastic materials

where K is a constant, a the stress, m (= 1/n) the strain-rate sensitivity index, d the grain size, p the grain size exponent, Q the activation energy, R the gas constant and T the absolute temperature. The log alog ~ plots of superplastic materials are of sigmoidal shape in general. But in the Zn-A1 eutectoid alloy, an increase [1 4] and a decrease [5-9] in m at low strain rates were observed by different investigators. Rai and Grant [10] showed that the cause of sigmoidal shape of the a-~ plot in an A1 Cu alloy is the concurrent grain growth of fine grains at low strain rates. Consequently, the concurrent grain-gro~vth effect was used as an argument to interpret the low m observation in the ZnA1 eutectoid alloy [11]. In a study of the superplastic behaviour of the Sn Pb eutectoid alloy, we observed the opposite effect of an increase in m at low strain rates because of concurrent grain growth in a differential strain-rate test. The purpose of this communication is to present our observations in this regard and to interpret them in terms of the test procedure. The Sn-38wt%Pb eutectic alloy was prepared starting from 99.99% pure metals. The cast ingot of 75 mm diameter was extruded into 9.5mm diameter rods, from which tensile specimens of 6 mm diameter and 25 mm gauge length were obtained. Differential strain-rate tests were performed on an Instron machine making use of an electrically heated silicone oil bath. Three successive cycles of crosshead speed changes were performed on a single specimen in order to obtain the stress-strain-rate behaviour at a given test temperature. The crosshead speeds were varied in increasing order during the first two cycles of the test, whereas the decreasing sequence was followed in the third cycle. The stress-true strain-rate plots thus obtained are shown in Fig. 1. It is seen that the flow stress at low strain rates is lower in the first cycle than in the other two cycles. From a comparison of the slopes of these plots, the rate sensitivity index in the first cycle is higher than that of the other two cycles in the lowest strain-rate range. While the log a-log plots in the second and third cycles are linear over the whole strain-rate range, its slope in the first cycle becomes constant at strain rates higher than 10-Ssec -~. These results suggest that deformationinduced coarsening of the microstructure occurs mostly during the initial stages of the test. Metallographic examination of the specimens before and after these tests was carried out to assess the grain coarsening arising from a total elongation of ~ 50%. The mean intercept length in the starting condition is 5.5 #m + 1.0#m and after the tests at 355 and 423 K, the intercept lengths are 6.1 4- 0.9 and 6.9 + 0.8#m, respectively. The increase in m at low strain rates in this study differs from the trend observed by Rai and Grant [10]. Although the fine grain size of superplastic materials is generally stable because of the second-phase particles or duplex structures, some strain-enhanced grain growth does occur during superplastic deformation [12]. Recently, Sepulveda and Mishra [13] analysed the strain-enhanced grain-growth data of superplastic duplex materials with the grain-coarsening equation

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 e ≃ 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-e 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.

Analysis of Concurrent Grain Growth During Creep of Polycrystalline Alumina

During low-stress creep experiments in a fine-grained alumina, the strain rates decreased continuously with time due to the occurrence of concurrent grain growth. The grain-growth kinetics were independent of the applied stress and depended only on the time of exposure to the elevated test temperature. An analysis of the effect of concurrent grain growth was consistent with experimental observations.