Juan Velázquez Aguirre

Bio: Juan Velázquez Aguirre is an academic researcher from Osaka Prefecture University. The author has contributed to research in topic(s): Strain rate & Deformation (engineering). The author has an hindex of 2, co-authored 2 publication(s) receiving 25 citation(s).

Cavitation during grain-boundary-sliding deformation in an AZ61 magnesium alloy

Abstract: Cavitation behavior has been investigated in a relatively coarse-grained AZ61 alloy deformed under two conditions for which grain-boundary sliding (GBS) creep controls plastic flow and which produce the same flow stress of 10 MPa. At a strain rate of 10−5 s−1 and a temperature of 573 K, GBS creep is rate controlled by grain-boundary diffusion, DGB. At a strain rate of 2 × 10−4 s−1 and a temperature of 648 K, GBS creep is rate controlled by lattice diffusion, DL. Tensile elongation is slightly greater when DGB accommodates GBS deformation. Despite accommodation of GBS by different diffusion mechanisms, cavity evolution under both deformation conditions is quite similar. Cavity volume percent increases similarly with strain under both conditions, as does the radius of the largest cavities. Cavity areal number density distributions are similar between the different deformation conditions when strain is a constant. All the features observed for cavitation indicate that cavity growth is plasticity controlled under both deformation conditions. The theory of plasticity-controlled cavity growth is in very good agreement with experimental data produced for this investigation.

Experimental Investigation of Cavitation Behavior in AZ61 Magnesium Alloy

Abstract: The rate of cavitation with superplastic strain was investigated for a superplastic AZ61 magnesium alloy at a strain rate of 2 × 10 - 4 s - 1 and temperature of 648 K, under the conditions of which an elongation of more than 250% has been found. Cavities initiated at grain boundaries. The cavitation showed a growth perpendicular to the applied stress direction after the initial strains. The subsequent growth and coalescence of cavities invariably leads to failure of the material. The experimental growth rates are in good agreement with the rate predicted by the plasticity-controlled growth mechanism.

Mechanical properties and texture of a superplastically deformed AZ31 magnesium alloy

Abstract: Room temperature tensile properties of a superplastic AZ31 magnesium alloy prior to and following high temperature deformation at 673 and 723 K were examined. The material behaved in a superplastic manner at low strain rates, and dislocation creep dominated deformation at high strain rates. The specimens after high temperature deformation exhibited higher ductility and lower strength as the strain rate during high temperature deformation decreased. Pre-existing basal texture changed gradually toward random during high temperature deformation at low strain rates, whereas it was strengthened at high strain rates. Both the basal texture weakening and lower cavity volume fraction brought about the higher ductility following superplastic deformation at lower strain rates.

Accelerated precipitation behavior of cast Mg-Al-Zn alloy by grain refinement

Abstract: This study demonstrates that the precipitation behavior of β-Mg 17 Al 12 phase during aging and the resultant variation in hardness and mechanical properties of cast Mg-Al-Zn alloy are strongly dependent on initial grain size. Grain size reduction accelerates discontinuous precipitation at the early stage of aging treatment by increasing the area fraction of grain boundaries that can act as nucleation sites for discontinuous precipitates (DP), but it does not influence DP growth rate. Grain refinement also prematurely terminates continuous precipitation because the formation of a large number of DP reduces the amount of Al dissolved in the matrix, which is required for the formation of continuous precipitates (CP). This promotion of DP formation and early termination of CP formation significantly decrease the peak-aging time to one-third. The enhanced precipitation behavior also leads to an additional hardness improvement in the aged alloy, along with an increase in hardness owing to grain boundary strengthening by grain refinement. The amount of increase in hardness changes with aging time, which is determined by the variation of three variables with aging time: DP fraction difference between refined and nonrefined alloys, hardness difference between DP and matrix, and matrix hardness difference between the two alloys. Grain refinement improves both tensile strength and ductility of the homogenized alloy owing to grain boundary strengthening and suppression of twinning activation, respectively. However, the loss of ductility after peak-aging treatment is greater in the refined alloy because of the larger amount of DP acting as a crack source in this alloy.

Effect of γ precipitates on the cavitation behavior of wrought AZ31 magnesium alloy

Abstract: In the present work, the influence of γ eutectic phase on the cavitation behavior of wrought AZ31 magnesium alloy has been studied through applying a set of low strain rate hot tensile tests. The tensile tests were executed in two temperature range of 300–425 °C and 450–500 °C; i.e. somewhat below and higher eutectic melting temperature of γ precipitates (∼437 °C). The hot compression characteristics of the experimental alloy were also considered to assist explaining the related deformation mechanisms. The results indicated that a large amount of cavities originates from the γ second phases, specially located on grain boundaries. A sharp transition was realized from higher (>3%) to lower cavity area fraction (

Superplasticity of a dual-phase-dominated Mg-Li-Al-Zn-Sr alloy processed by multidirectional forging and rolling

Abstract: The microstructures, mechanical properties, deformation mechanism and cavitation growth of Mg-10.2Li-2.1Al-2.23Zn-0.2Sr alloy subjected to multidirectional forging and rolling (MDFR) were studied to examine the deformability of the Mg-Li alloy. X-ray diffraction (XRD) results confirm the existence of α (Mg) and β (Li) phases and Mg17Al12, Al4Sr and LiMgAl2 intermetallic compounds. Studies of the microstructures reveal that a thin banded-grained microstructure with a grain size less than 3.75 µm is obtained via MDFR and annealing at 523 K for 1 h. There were significant grain refinements. The maximum elongation to failure was 712.1%, and this was obtained in the current alloy at 623 K with a strain rate of 1.67 × 10−3 s−1. The strain rate sensitivity exponent and the activation energy for deformation were estimated to be 0.884 and 91.9 kJ/mol, respectively. This indicates that the rate-controlling deformation mechanism under the aforementioned condition is grain boundary sliding controlled by lattice diffusion. Experimental cavity observations show the existence of cavity interlinkage or stringers. A new plasticity-controlled cavity growth rate equation considering cavity interlinkage was established, and a cavity growth diagram was constructed. The diagram prediction is consistent with the experimental results. In addition, fractographs show that the intergranular fracture is a ductile fracture mechanism. At room temperature, the ultimate tensile strength of 242 MPa and the elongation of 23.59% were obtained in the present alloy.

Hot deformation behavior of hot-rolled AZ31 and AZ61 magnesium alloys

Abstract: The hot deformation behavior and microstructure of AZ31 and AZ61 alloys processed by the hot-rolled technique were investigated over a strain rate range from 1×10−4 to 1×10−1 s−1 and temperatures ranging from 350 to 500 °C. Alloys were manufactured by melting and casting, and then hot-rolled at two temperatures of 350 and 450 °C. Tensile test revealed elongations of over 300% in the two alloys. The highest elongation value of 321% at a strain rate of 3×10−3 s−1 was achieved in the AZ61 alloy with a hot-rolling temperature of 350 °C. The hot tensile tests showed that there are some differences in ductility between the AZ31 and AZ61 alloys with the same rolling process specifically at low strain rates. A small enhancement in ductility is found using a lower temperature for hot-rolling at 350 °C compared to 450 °C due to improved grain refinement. It is suggested that the dominant deformation mechanism was glide controlled dislocation creep based on the stress and grain size exponent, and activation energy.