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Al-7Si-0.3Mg alloy and its mechanism of low cycle fatigue behavior

Cyclic hardening or softening stage; When the strain amplitude of 0.20%, the strain of a number of cycles curve shows a slight cyclic hardening features, hardened region 500 within the second cycle, and then cycle into the stable phase; when the strain amplitude greater than 0.25%, the cyclic hardening and hardening rate increase with the strain amplitude is more obvious, when the strain amplitude is 0.35%, the significant cyclic hardening, hardening region throughout the fatigue cycle. . . * '* I::. 3sSv020% _015%. . - A l ... \'___i ~ Blit__ of cycles / time Figure 2 under different strain amplitude cyclic hardening under different strain amplitude fatigue life of specimen shown in Figure 3, with the cyclic hardening phenomenon corresponds with the increase of strain amplitude specimen fatigue life is significantly reduced. Application of Manson-Coffin relation to the preparation and _l, the relationship is:. A a ~ (2Nf). + Ef (2Nf). (1) 0.3O0.25 chain 0.2OO. Figure 3 15'00O'O000 fatigue life under cyclic loading in tension and compression strain amplitude fatigue life curve of a 494-style, for the strain amplitude;, e were fatigue strength coefficient and fatigue ductility coefficient Si Lo; E is the elastic modulus ; b. , C. Were fatigue strength and fatigue ductility index index. Can be calculated, a 445MPa, b. One by one 0.13, ef a 0.12, c. 0.68 one by one, the fitting curves are very consistent with real test data, such as aluminum, compared with the 2024,5456 _g], the fatigue strength coefficient and fatigue ductility coefficient values are small, the fatigue strength exponent and fatigue ductility index is not very different, these coefficients and mechanical properties of the material itself is closely related. Analysis shows that, when the strain amplitude of 0.15%, the cycle of material produced only small plastic deformation, deformation to elastic deformation based, then the higher the fatigue life of materials; when the strain amplitude is greater than 0 .20%, the material inside the plastic deformation zone increases gradually began to enter the plastic deformation range, then the fatigue life significantly decreased. It should be noted that the fatigue life of this material fluctuations in the sensitivity of the larger strain amplitude, which is mainly due to A1-7Si 0.3Mg aluminum alloy is a brittle material, the elongation is low, while aluminum in this experiment alloy T6 heat treatment, to obtain a higher tensile strength and ductility decreases, which reduces the plastic deformation zone corresponding to the strain range, showing very close to the tensile strength and yield strength were 197MPa and 170MPa, a result of low cycle fatigue life is sensitive to changes in strain amplitude. Strain amplitude of the increase is caused mainly due to lower fatigue life. 2.3 Observation and mechanism of error in Figure 4 is a 7Si Al 0.3Mg alloy in a different strain amplitude, the fatigue failure of samples dislocation structure. Aluminum alloy for high-level fault energy materials l_l, the applied loads prone to cross slip, so the material dislocation cross-slip direction for the two-dislocation zone (see Figure 4 in the E110] and ~ T01], etc. direction), the higher density of dislocation with the dislocation tangles composed of dislocations with the free movement between the dislocation line. When the strain amplitude of 0.15%, the material that formed the cross with a smaller dislocation, dislocations with a spacing larger,[link widoczny dla zalogowanych], with a small amount of room in the precipitates around the dislocation tangles, dislocation movement larger space (see Figure 4a); when the strain amplitude increases when the (O.25%), cross-band becomes wider dislocations, cross slip characteristics more clearly, take the dislocation density increases, the relative dislocation precipitation pinning effect of apparent dislocation between the band gradually decreased mobility; when the strain amplitude of 0.30%,[link widoczny dla zalogowanych], the whole cell is the basic line of high-density dislocations occupy, and cross-band structure of the dislocation become very obvious (Figure 4c). Figure 4c Figure 4d is a magnified display in the local area,[link widoczny dla zalogowanych], in addition to precipitation relative to the pinning effect of dislocation,[link widoczny dla zalogowanych], the large formation of dislocation loops is caused by dislocation mobility to further reduce the causes. In summary, A1-7Si a 0.3Mg aluminum alloy in low cycle fatigue in the process of dislocation structure on the strain amplitude have a greater dependence. A pressure in the tension under the action of cyclic loading, only when the external force to achieve the most favorable orientation of aluminum in the critical sub-slip system shear stress,[link widoczny dla zalogowanych], the slip systems was only starting, so the increase of strain amplitude in favor of more slippery Move the starting line, and so the dislocation density within the slip systems to increase the rate of acceleration of the interaction between dislocations plus ∞ ∞ ∞ ∞ ∞ Wong \ R more

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