Void nucleation behavior of single-crystal high-purity iron specimens subjected to tensile deformation

Significance 

A good comprehension of the fracture mechanics is crucial for determining reasons for failure in metals. The most common metal failures are either brittle or ductile fractures depending on the extent to which the metal can undergo large plastic deformation before fracture. For ductile metals, the tensile test for the fracture process is grouped into either uniform deformation or local deformation. During local deformation, voids nucleate and grow due to increase in the stress tri-axiality and simultaneous increase in the dislocation density. Additionally, when both strain and void volume fraction reach the critical values ductile fracture occurs. The void nucleation mechanisms are classified into two: homogeneous and heterogeneous nucleation. For the former to occur there must be an increase in the dislocation density and vacancy migration where the latter is affected by material defects, such as the presence of inclusions, precipitates among other discontinuous structures.

Intense works have been undertaken regarding heterogeneous void nucleation. Such studies have helped reveal that voids nucleate at grain boundaries between the varying phases. More so, similar works have suggested that void nucleation depends on stress triaxiality and the microstructure, including the grain boundaries and particles. However, there exists no published tentative studies of the void nucleation behavior in metal specimens that contain neither particles nor grain boundaries so far, despite the crucial role that void nucleation plays in determining local deformation of ductile materials.

Professor Osamu Furukimi and colleagues at Kyushu University in Japan examined the void nucleation behavior of single-crystal iron in tensile stress. Their main aim was to explore the mechanism of void nucleation during tensile deformation for single-crystal iron specimens of small size containing no microstructural defects, such as grain boundaries or precipitated particles. Their work is now published in the research journal, Materials Science & Engineering A.

The researchers initiated their experiment by selecting two micrometer-size single crystals having a different type of slip extracted from electro-deposited pure iron for use to investigate the fracture mechanics during tensile deformation. The team then applied high voltage electron microscopy imagery and scanning electron microscopy technique to verify the existence of only one single slip system in the smaller specimen while still checking out for voids.  A similar procedure was then carried out for the larger specimen.

The research team was able to observe that the smaller specimen exhibited straight slip patterns while for the larger specimens, a number of shear bands and wavy slip patterns were observed. The researchers also noticed the nucleation behavior of voids in single-crystal iron specimens. It was observed that voids with diameters of 50–100 nm were present along the slip band in the larger specimen.

The Osamu Furukimi et al study has presented novel insight and fundamental information on void nucleation during tensile testing for single-crystal iron specimens of smaller sizes with no precipitated particles or grain boundaries. The critical information that has been brought forward is that upon deformation, voids nucleate even in single-crystal materials, which naturally have no precipitates or grain. Furthermore, since no slip took place in the smaller sample, it can therefore be concluded that multiple slips are a necessary criterion for void nucleation in single-crystal iron.

Void nucleation behavior of single-crystal high-purity iron specimens subjected to tensile deformation. Advances in Engineering

Void nucleation behavior of single-crystal high-purity iron specimens subjected to tensile deformationVoid nucleation behavior of single-crystal high-purity iron specimens subjected to tensile deformation. Advances in Engineering

Void nucleation behavior of single-crystal high-purity iron specimens subjected to tensile deformationVoid nucleation behavior of single-crystal high-purity iron specimens subjected to tensile deformation. Advances in Engineering

About the author

Chatcharit Kiattisaksri is a PhD student in the Graduate School of Engineering at Kyoto University, Japan. He graduated with a bachelor degree of material science and engineering from the University of Western Australia, in 2014.  After that, he moved to Japan to pursue his master degree in Material Science and Engineering at Kyushu University. During his time in Kyushu University from 2014 to 2016, he researched ductile deformation and void nucleation in single crystal high-purity iron (Fe: 99.98 mass %). In Kyoto University, his current researching interests lie in the area of material engineering, which involves the study of solidification process by using synchrotron radiation X-rays.

About the author

Masaki Tanaka is an Associate Professor in Department of Materials Science and Engineering, Kyushu University, Japan. He studied Materials Science using high-voltage electron microscope to elucidate the dislocation structure at a crack tip, completed PhD at Kyushu University in 2005. He was appointed to the faculty of Engineering at Kyushu university after working as a postdoctoral research fellow at Department of Materials, University of Oxford in UK. His current research interest is mechanical property of crystalline materials such as steels, silicon, titanium alloys and so on, basing on dislocation theory.

About the author

Masatoshi Aramaki is an Assistant Professor in the Department of Materials Science and Engineering at Kyushu University, Japan. He received his Ph.D. from the Faculty of Engineering at Kyushu University, focusing attention on aspects of superplastic phenomenon in steels. He studies a variety of areas pertinent to the strength and ductility of metallic materials. He also conducts researches in the field of plastic working such as automotive parts. Currently he is an editorial board member for the Journal of the Japan Society for Technology of Plasticity.

About the author

Osamu Furukimi is a Professor in the Department of Materials Science and Engineering, Kyushu University, Japan. He graduated from the master course in Graduate School of Engineering, The Tokyo University in 1977, and got a position of research for development of steel plates used for low temperature at Kawasaki Steel Co. (now JFE Steel Co.). He received his Ph.D. from the Faculty of Engineering at Tohoku University, focusing attention on improvement toughness of steel plates for storage tanks of low temperature use in 1989. From 2005, he is a practicing professor in Kyushu University.

He studies mainly a void nucleation, growth and coalescence mechanism of metals during plastic deformation. He also conducts researches in the field of heat treatment. He elucidated the effect of void nucleated during nitriding and quenching process on wear property for commercial grade pure iron (Fe: 99.82 mass%). Currently he is a vice chairman for the Japan society for heat treatment.

About the author

Satoshi Oue is an Assistant Professor in the Department of Materials Science and Engineering, Kyushu University, Japan. He graduated from the master course in Materials Science and Engineering of Kyushu University. His authority is Metal Electrochemistry, especially observation of microstructure of many kind of metal including electrodeposited iron. He also conducts researches in the field of electroplating and electrowinning such as copper, nickel and zinc. He is currently assigned to be secretariat of Kyushu Branch of Japan Society of Corrosion Engineering and The Surface Finishing Society of Japan.

About the author

Shinji Munetoh is an Associate Professor in the Department of Materials Science and Engineering at Kyushu University, Japan. He graduated from the master course in Material Science and Engineering of the Kyushu University in 1998, and got a position of research for development of semiconductor at SUMITOMO Metal Co. (now Nippon Steel & Sumitomo Metal Co.). He received his Ph.D. from the Faculty of Engineering at Kyushu University, focusing on analysis of crystal growth mechanism of silicon. From 2003, he moved to Kyushu University, he studies mainly thermoelectric materials and fracture mechanics of metals.

About the author

Yuji Takeda is an engineer at JFE steel, Japan. He graduated from the master course in Graduate School  of Engineering from the Kyushu University, in 2017. During his time in Kyushu University, he researched void nucleation in single crystal high-purity iron (Fe: 99.98 mass %), and effect of specimen thickness in tensile test on local elongation. At JFE steel, his research in development a new process of smelting of pig iron.

Reference

Osamu Furukimi, Chatcharit Kiattisaksri, Yuji Takeda, Masatoshi Aramaki, Satoshi Oue, Shinji Munetoh, Masaki Tanaka. Void nucleation behavior of single-crystal high-purity iron specimens subjected to tensile deformation. Materials Science & Engineering A volume 701 (2017) pages 221–225.

 

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