Comparative study of tensile and high-cycle fatigue properties of extruded AZ91 and AZ91-0.3Ca-0.2Y alloys

doi:10.1016/j.jmst.2021.03.039

Abstract

Abstract:

Mg-Al-Zn-Ca-Y alloys with excellent ignition and corrosion resistances—termed SEN alloys (where the letters “S,” “E,” and “N” stand for stainless, environmentally friendly, and non-flammable, respectively)—have been developed recently. In this study, the microstructure, tensile properties, and high-cycle fatigue properties of an extruded Mg-9.0Al-0.8Zn-0.1Mn-0.3Ca-0.2Y (SEN9) alloy are investigated and compared with those of a commercial Mg-9.0Al-0.8Zn-0.1Mn (AZ91) alloy extruded under the same conditions. Both the extruded SEN9 alloy and the extruded AZ91 alloy have a fully recrystallized structure comprising equiaxed grains, but the former has a smaller average grain size owing to the promoted dynamic recrystallization during extrusion. The extruded AZ91 alloy contains coarse Mg₁₇Al₁₂ discontinuous precipitate (DP) bands parallel to the extrusion direction, which are formed during its cool down after extrusion. In contrast, the extruded SEN9 alloy contains relatively fine undissolved Al₂Ca, Al₈Mn₄Y, and Al₂Y second-phase particles, which are formed during the solidification stage of the casting process. The tensile strength of the extruded SEN9 alloy, which has finer grains and more abundant particles, is slightly higher than that of the extruded AZ91 alloy. However, the difference in their strengths is relatively small because the stronger solid-solution hardening and precipitation hardening effects in the extruded AZ91 alloy offset the stronger grain-boundary hardening and dispersion hardening effects in the extruded SEN9 alloy to some extent. The tensile elongation of the extruded AZ91 alloy is significantly lower than that of the extruded SEN9 alloy because the large cracks formed in the DP bands in the former cause its premature fracture. Although the extruded SEN9 alloy has higher tensile properties than the extruded AZ91 alloy, the high-cycle fatigue life and fatigue strength of the former are shorter and lower, respectively, than those of the latter. The DP bands in the extruded AZ91 alloy do not act as fatigue crack initiation sites, and therefore, fatigue cracks initiate on the specimen surface at all stress amplitude levels. In contrast, in most of the fatigue-fractured specimens of the extruded SEN9 alloy, fatigue cracks initiate on the undissolved Al₂Ca and Al₂Y particles present on the surface or subsurface of the specimens because of the high local stress concentration on the particles during cyclic loading. This particle-initiated fatigue fracture eventually decreases the high-cycle fatigue resistance of the extruded SEN9 alloy.

Key words: Mg-Al-Zn-Ca-Y alloy, Extrusion, Tensile properties, High-cycle fatigue, Crack source

Jin Kim Ye, Min Kim Young, Hong Seong-Gu, Woong Kim Dae, Soo Lee Chong, Hyuk Park Sung. Comparative study of tensile and high-cycle fatigue properties of extruded AZ91 and AZ91-0.3Ca-0.2Y alloys[J]. J. Mater. Sci. Technol., 2021, 93: 41-52.

Figures/Tables 12

Fig. 1. (a) Photographs of extruded AZ91 and SEN9 alloys and (b) dimensions of specimens for tensile and fatigue tests.

Fig. 2. (a, b) Optical and (c, d) SEM micrographs of extruded (a, c) AZ91 and (b, d) SEN9 alloys. d denotes the average grain size.

Fig. 3. (a) Equilibrium phase diagram of Mg-1Zn-xAl (x = 0-20 wt%), as calculated using PANDAT software. (b-d) SEM micrographs showing (b) discontinuous precipitate (DP) band, (c) lamellar structure of the DP, and (d) Mg17Al12 precipitates along grain boundaries of extruded AZ91 alloy. Text. denotes the extrusion temperature (400 °C).

Fig. 4. Analyses of second-phase particles in extruded SEN9 alloy: (a) backscattered electron image and corresponding EPMA scanning maps of Mg, Al, Zn, Mn, Ca, and Y elements; (b) X-ray diffraction pattern; and (c) size distribution and number density of particles.

Fig. 5. Tensile stress-strain curves and tensile properties of extruded AZ91 and SEN9 alloys. TYS, UTS, and EL denote the tensile yield strength, ultimate tensile strength, and tensile elongation, respectively.

Fig. 6. Texture and Schmid factor (SF) analyses of extruded (a, c, e) AZ91 and (b, d, f) SEN9 alloys: (a, b) extrusion direction (ED) inverse pole figures and SF distributions for (c, d) basal slip and (e, f) prismatic slip under deformation along ED. SFbasal and SFprismatic denote the average SF values for basal slip and prismatic slip, respectively.

Fig. 7. Results of high-cycle fatigue tests (S-N curves) of extruded (a) AZ91 and (b) SEN9 alloys. The fatigue crack initiation sites are specified in parentheses.

Fig. 8. SEM micrographs of fatigue-fractured specimen of extruded AZ91 alloy at stress amplitude of 170 MPa.

Fig. 9. Analysis of fatigue fracture surface of extruded SEN9 alloy: SEM micrographs and EDS scanning maps of Mg, Al, Zn, Ca, and Y elements in fatigue-fractured specimens at stress amplitudes of (a) 210 MPa and (b) 180 MPa. The red rectangular regions in the SEM micrographs indicate the fatigue crack initiation sites where EDS scanning is performed.

Fig. 10. (a, b) Optical micrographs and (c, d) SEM fractographs of tensile-fractured specimens of extruded (a, c) AZ91 and (b, d) SEN9 alloys.

Fig. 11. Optical micrographs showing microcracking of extruded SEN9 alloy during tensile deformation: (a) undeformed, (b) 5%-tensioned, (c) 10%-tensioned, and (d) tensile-fractured specimens. The red, blue, and green arrows indicate cracks formed in undissolved particles, deformation twins, and cracks formed along twins, respectively.

Fig. 12. Characteristics of second-phase particles in extruded AZ91 and SEN9 alloys: (a) hardness of matrix and particles; (b, c) SEM micrographs showing deformed particles after hardness measurement (top) and hypothetical structures of particles (bottom) in (b) AZ91 and (c) SEN9 alloys; (d) hypothetical structures of α-Mg matrix (top) and Mg17Al12 phase (bottom) showing the closed-packed plane and direction, as well as their orientation relationship.

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