The Critical Stress at the Interface for Cavity Nucleation in Superplastic Aluminum Matrix Composites Reinforced with Si<SUB>3</SUB>N<SUB>4</SUB> Particles

Abstract

The minimum particle size for cavity nucleation was experimentally investigated for superplastic 2124Al and 6061Al composites reinforced with 20 vol% Si 3 N 4 particles. From the minimum particle size for cavity nucleation, the critical stress at the interface for cavity nucleation was estimated to be about 5 MPa. The local stresses at the interface for the particles of average size where large elongation above 200% is attained in a temperature range below the partial melting temperature are lower than the critical stress. It may be therefore suggested that large elongation is attained without the accommodation helper process of liquid under the conditions that the local stress at the interface is lower than the critical stress at the interface for cavity nucleation.

Cavitation Behavior in Superplastically Deformed Zn-22 mass%Al Alloy at Room Temperature

“…Therefore, increasing attention has been devoted to the role of cavitation in superplastic behavior. [14][15][16][17][18][19][20] In Zn-22 mass%Al alloy, some investigations concerned with the cavitation characteristics have been carried out before now. [21][22][23][24] However, almost the reports discussed only the cavitation behavior in Zn-22 mass%Al alloy superplastically deformed at high temperature above 473 K, and the quantitative analysis was also carried out scarcely.…”

Microstructural design for large superplastic elongations in aluminum-base materials containing particles

“…Recently, s ic was investigated within the range of 773 to 823 K in superplastic aluminum-base materials that contain particles reinforced with Si 3 N 4 , resulting in a value of about 5 MPa. [62] A comparison between the derived microstructural features for large superplastic elongations and the microstructural features for superplastic aluminum-base materials that contain particles reinforced with Si 3 N 4 is shown in Figure 9. The open symbols represent the materials attaining large elongations above 200 pct, and the closed symbols represent the materials attaining elongations below 200 pct.…”

“…Cavities are nucleated at particles in aluminum-base materials containing large amounts of particles. [50][51][52][53][54][55]61,62] Mabuchi and Higashi estimated a critical strain rate for large superplastic deformation. [24] When the strain rate for forming is above the critical strain rate, a large stress concentration may occur, and the liquid phase is required for stress relaxation.…”

“…[50][51][52][53][54][55]61,62] It is believed that large elongations under conditions with no liquid phase are due to the inhibition of cavity nucleation; namely, local stress during superplastic deformation is smaller than the critical local stress for cavity nucleation. This concept has advantages over the accommodation helper by the liquid phase, which allows for cavity nucleation, because postsuperplastic forming properties can be maintained since there are fewer cavities.…”

Flat Cavities formed in TZP caused by Superplastic Deformations at High Strain-Rates and Their Effect on Elongation

“…4) However, Primdahl et al 8) showed that such intergranular cracks might be also originated from the volume increase associated with the tetragonal to monoclinic transformation induced in some grains during grinding and polishing processes to a TEM foil. Despite a large number of studies concerning cavitation behaviors in superplastic materials, [3][4][5][6][7][8][10][11][12][13] only a few data -obtained from exactly as-deformed specimens, i.e., without the surface preparation -have been reported so far. It is probable that the surface preparation of specimens before the evaluation of cavitation may give errors in the data.…”