Specimen size effects on the weakening of a bulk metastable austenitic alloy

In this work, we are investigating the scaling effects on an austenitic stainless steel with changing grain size using microcompression testing. It is our aim to evaluate at what sample-to-grain size ratio the mechanical properties such as yield stress deviate from the macroscopically determined data. It was found that decreased yield stresses with decreasing specimen size (weakening) occur during the microcompression. The weakening was observed when the diameter-to-grain size ratio (D/d) dropped below a critical value. The effect of the grain size on the critical value, above which the bulk property can be obtained, was systematically investigated utilizing a wide range of D/d values (0.5–30) for specimens with a grain size ranging from 0.3 to 2 μm. It was found that the critical D/d value decreases with increasing grain size. A simple analytical model was developed, which is applicable to both micropillar and tensile tests. Comparison of the model equations with experimental data showed that the reduction in yield stress with sample size can be associated with the weaker near-surface zone, which has a reduced strength compared to the specimen interior zone. Furthermore, this study suggests that the size of the near-surface zone may be related to the dislocation structures in near-surface grains, and therefore the overall behavior is governed by the grain size and stacking fault energy.