Superplasticity in the Aluminium–Zinc Eutectoid

Abstract

Superplastic extension of the aluminium-zinc eutectoid results·primarily from grain-boundary sliding and grain rotation. The strain rate (e:), flow stress (0), grain size (L), and temperature (T) are related empirically:.where K is a constant, k is Boltzmann's constant, and U correlates with the activation energy for grainboundary diffusion.The following proposed mechanism is quantitatively in agreement with our observations on superplasticity:(i) Certain grains that obstruct the easy relative motion of groups of grains by grain-boundary sliding yield under the resulting stress concentration; (ii) under superplastic conditions, dislocations traverse such yielded grains and pile up at grain boundaries until their back stress prevents the grain-boundary sliding; (iii) the high stress at the head of the pile-up causes accelerated diffusion and dislocations rapidly escape by climb into and along grain boundaries. The replacement of these dislocations makes possible further boundary sliding by the obstructed group of grains.The superplastic behaviour of many alloys1 is of considerable interest because of the extremely large deformations that occur at quite high rates of strain, but neither the mode nor the mechanism of superplastic flow has been established. Superplasticity is most evident in eutectic or eutectoid alloys. However, the marked superplasticity of the quenched aluminium-zinc eutectoid suggests that the structural metastability of such alloys is not the cause of superplasticity; this alloy decomposes spontaneously at room temperature 2 ,3 and separation of the phases would normally occur before testing. The high strain-rate-sensitivity of the flow stress during superplastic deformation led to the proposal 4 that the flow occurred by a combination of Herring-Nabarro creep and dislocation motion. At high temperatures and low strain rates the Herring-Nabarro n1echanism would be dominant and m would approach unity in the equation cr = CXgm, where cr is the stress, g the strain rate, and cx is a constant. However, the maximum observed values of m, at temperatures just below the eutectic or eutectoid temperatures, consistently approach 0.5. 5 -7 In addition, calculations 6 ,8 show that the maximum possible strain rate by pure HerringPaper No. MS 103. Manuscript Nabarro creep (nl = 1),either through bulk or grain-boundary diffusion, is far below those experimentally recorded.The observation 9 that grains of lead-tin and lead-bismuth alloys remained equiaxed during extensions of ,...., 2000% led to the conclusion that grain-boundary sliding was the important mode of deformation. This view is supported by recent metallographic investigations of superplasticity.5,10 Despite this evidence, Chaudhari ll has,recent1y proposed that deformation proceeds solely by dislocation movement within the grains; his mechanism does not explain how the grains can remain equiaxed, and his proposal consequently appears unreasonable.The present work attempts to correlate the microstructure, as observed by electron microscopy: with th...

Failure mechanisms in superplastic AA5083 materials

“…[3] The deformation mechanism generally associated with superplasticity in fine-grained AA5083, such as commercially available superplastic-grade AA5083 materials, is grain-boundary-sliding (GBS) creep. [4][5][6][7][8][9][10][11][12][13][14][15][16] For the QPF process, however, temperatures are lower and strain rates are faster than in traditional SPF operations. It has been shown that the dominant deformation mechanism shifts toward solute-drag (SD) creep under these conditions, [12,[17][18][19] as is observed in other commercial 5000-series alloys at elevated temperatures and slow strain rates.…”

Crecimiento de grano y comportamiento mecánico a alta temperatura de circona itriada (YTZP 4 MOL %)

“…En particular, hemos comprobado que todas las distribuciones de tamaño de grano se ajustan a una distribución de tipo log-normal, : [1] En este caso, los parámetros µ y ξ están relacionados con el valor del tamaño medio de grano, , y su desviación estándar, σ d , a través de las siguientes ecuaciones [5,6]: [2] Las propiedades mecánicas a alta temperatura de las muestras han sido estudiadas mediante ensayos a velocidad de compresión constante realizados en una máquina INSTRON modelo 1065, a temperaturas comprendidas entre 1200 ºC y 1500 ºC, para distintos rangos de tensiones. Las velocidades de compresión impuestas al sistema están comprendidas entre 5 y 80 µm/min.…”

“…Pese a esta evidencia experimental, esta dependencia cuadrática despierta incertidumbres. En efecto, el único modelo existente en la literatura [2] que justifica esta dependencia en el régimen de deslizamiento de fronteras de grano requiere, como mecanismo de acomodación, el concurso de las dislocaciones originadas en el interior de los granos durante la solicitación mecánica. En el caso de la circona dopada con itria, hay sólo un trabajo [3] que, en consonancia con esta imagen, encuentra evidencias de la presencia de dislocaciones en el seno de los granos de muestras de YTZP deformadas.…”

Effect of Microstructure on Cavitation during Hot Deformation of a Fine-Grained Aluminum-Magnesium Alloy as Revealed through Three-Dimensional Characterization

“…[5][6][7] For SPF, grain-boundary-sliding (GBS) creep is widely recognized to be the dominant deformation mechanism in fine-grained AA5083 sheet. [5,[8][9][10][11][12][13] Under GBS creep, the dominant failure mechanism of AA5083 is cavitation. [6] For QPF, solutedrag (SD) creep begins to dominate deformation as temperature decreases and strain rate increases.…”