Strain-Rate and Temperature Dependences of Deformation Behavior of AZ61Mg Alloy Processed by Multi-directional Forging Under Decreasing Temperature Conditions

金沢大学理工研究域機械工学系Strain-rate and temperature dependences of deformation behavior of ultrafine-grained (UFGed) AZ61Mg alloy were examined. For this purpose, AZ61Mg alloy specimens were subjected to multi-directional forging (MDFing) under decreasing temperature conditions to have various grain sizes. The average grain sizes attained by MDFing to 1, 3, 6 and 9 passes were approximately 8, 2, 0.5 and 0.3 μm, respectively. A superior balance of the mechanical properties of strength and ductility at room temperature was achieved by MDFing to three passes and over. The strain-rate sensitivity of mechanical properties of the MDFed specimens increased with decreasing grain size. The specimens with grain sizes of 0.5 and 0.3 μm (0.5 or 0.3 specimen) exhibited stronger strain-rate and temperature dependences of total elongation than those with grain sizes of 8 and 2 μm (8 or 2 specimen). This can be partially ascribed to grain-boundary sliding, since an AFM observation revealed the occurrence of room-temperature grain-boundary sliding in the 0.3 specimen. The activation volume V* for the 8, 2 and 0.5 specimen increased with increasing temperature, while the 0.3 specimen exhibited an inverse temperature dependence of V*. This suggests a change in deformation mechanism with decreasing grain size as well as the occurrence of grain-boundary sliding. © 2017 The Minerals, Metals & Materials Society and ASM InternationalEmbargo Period 12 monthsThe final publication is available at www.springerlink.com/article/10.1007/s11661-017-4303-

Yield-Point Phenomenon and Plastic Bands in Ferrite–Pearlite Steels

Lüders deformation is one type of discontinuous yielding in ferrite–pearlite steel. The yield-point phenomenon and localized plastic bands are two features of the Lüders phenomenon. It is believed that the yield-point phenomenon is related to the formation of plastic bands, but the correlation between them is unclear. In this study, this correlation was investigated by examining the global and local deformation behaviors in the tension processes of four ferrite–pearlite steels (carbon content, 0.05–0.3%; pearlite fraction, 1.2–32%) via an extensometer and digital image correlation (DIC) technique. The main obtained results are as follows: (1) the degree of yield drop decreased with an increase in the pearlite fraction (the magnitude of the yield stress drop was 8.6–0 MPa), and (2) a plastic band was formed at a certain stress level smaller than the upper yield stress; when the stress level was larger than 92% of the upper yield stress, the upper yield point disappeared

In-Situ Observation of Lüders Band Formation in Hot-Rolled Steel via Digital Image Correlation

Although the Lüders yield phenomenon has been investigated for more than 150 years, some understanding of Lüders band formation lack substantial support from experimental evidence. In-situ observation of Lüders band formation in hot-rolled steel experimentally clarified the following facts: (i) When stress reaches the true upper yield stress, the Lüders band begins to nucleate. True upper yield stress is greater than nominal upper yield stress. (ii) Gross stress concentration promotes the Lüders band formation, and the size of the gross stress concentration region determines the initial width of the Lüders band. (iii) The Lüders band nucleates far ahead of the gross yield point

Suppression of Inhomogeneous Plastic Deformation in Medium-Carbon Tempered Martensite Steel

The Lüders phenomenon is one type of inhomogeneous plastic deformation occurring in the elastic-to-plastic transition region, and it is an undesirable plastic deformation behavior. Although conventional measures based on the chemical composition design, plasticity processing principle, or utilization of composited microstructures are used to suppress this phenomenon in engineering, demerits are present, such as high cost and low fracture behavior. The Lüders phenomenon begins with the formation of plastic bands (inhomogeneous yielding) at one or several local sites. If yielding simultaneously occurs everywhere rather than at several local sites, the formation of local plastic bands will be inhibited; as a result, the Lüders deformation will be suppressed. Based on this idea, a new approach was proposed in which the number of local yield sites was increased by heat treatments. A medium-carbon tempered martensite steel (Fe-0.3C-1.5Mn, in wt%) was used to verify the validity of the new approach, and the optimum heat-treatment conditions for the balance of mechanical property and deformation behavior were determined

Grain-to-Grain Interaction Effect in Polycrystalline Plain Low-Carbon Steel within Elastic Deformation Region

A grain is surrounded by grains with different crystal orientations in polycrystalline plain low-carbon steel. The grain is constrained by its adjacent grains in the tension process. The interaction of the grain with the adjacent grains was investigated within the elastic deformation region. The following results have been obtained: (1) the Young’s modulus of a grain without consideration of grain-to-grain interaction is denoted as the inherent Young’s modulus; when the inherent Young’s modulus of a grain is equal to the Young’s modulus of the bulk material, there is almost no interaction between the grain and its adjacent grains; when a grain has a great difference between its inherent Young’s modulus and the Young’s modulus of the bulk material, its grain-to-grain interactions increase significantly; (2) the grain-to-grain interaction is mainly caused by the difference in the inherent Young’s modulus between the grain and its adjacent grains; the misorientation angle between the grain and its adjacent grains has almost no effect on the grain-to-grain interaction