Influence of Inclusions on Fatigue Characteristics of Non-Combustible Mg Alloy

Abstract

It is common knowledge that Mg is very light and high specific strength material. But there is a problem that Mg alloy easily burns during its machining, because the ignition point of Mg is 700K.[1] The ignition point of new Mg alloy to which Ca is added is about 1000K. This new Mg alloy is called non-combustible Mg alloy. As a result, many inclusions exist in Mg alloy. Then, fatigue test was carried out and gave evidence that inclusions have an effect on the fatigue strength. Then relationships between size of inclusions and fatigue limit were drawn. Therefore, the fatigue limit can be predicted by estimating the maximum inclusion size. The method of estimating the maximum inclusion size is “Estimation for particle size distribution in materials“[2] .

Evaluation Method for Mean Stress Effect on Fatigue Limit of Non-Combustible Mg Alloy

“…Three types (smooth, with a small hole and with a small crack) of specimens were prepared. For the non-combustible Mg alloy, if a smooth specimen is used for the fatigue test, the inclusion of various sizes serves to initiate a fatigue crack, the fatigue strength at N = 10 7 varies (Kitahara et al, 2005;Kitahara et al, 2006;Fujii et al, 2008;Masaki et al, 2008), and the influence of the mean stress on the fatigue strength at N = 10 7 cannot be studied in detail. In order to correctly determine the fatigue strength at N = 10 7 , a small hole or a small crack is then introduced in the test specimen.…”

“…No non-propagating crack at N = 10 7 was observed. The fatigue strength at N = 10 7 could be estimated using the size of the non-metallic inclusion at the fracture origin (Kitahara et al, 2005;Kitahara et al, 2006). The fatigue strength of the non-combustible Mg alloy was almost the same as that of the conventional Mg alloy.…”

“…Therefore, it is necessary to consider the influence of inclusions in the case of the fatigue strength characteristics evaluation. The fatigue strength at N = 10 7 for the smooth specimens of the three types of the non-combustible Mg alloys (AMX602B (X=Ca), AZX312D (X=Ca), AZX912D (X=Ca)) using a rotating-bending fatigue testing machine was investigated by the authors (Kitahara et al, 2005;Kitahara et al, 2006;Fujii et al, 2008;Masaki et al, 2008). As a result, it was clarified that the fatigue fracture origin in the noncombustible Mg alloy was due to a non-metallic inclusion as shown in Fig.…”

“…We discuss the equivalence of an artificial defect and an actual defect (inclusion). Figure 1 shows the S-N diagram for the smooth specimens of the non-combustible Mg alloy AMX602B (X=Ca) by the authors (Kitahara et al, 2005;Kitahara et al, 2006;Fujii et al, 2008;Masaki et al, 2008). The relationship between the load stress and the fatigue life of the smooth specimens significantly varies.…”

“…. Non-metallic inclusion observed at fatigue fracture origin of the non-combustible Mg alloy (AMX602B (X=Ca)) smooth specimens (rotating-bending) (Kitahara et al, 2005;Kitahara et al, 2006) …”