Effect of Free Surface on the Stability of Individual Retained Austenite Grains in a Duplex Stainless Steel
- PDF / 2,189,490 Bytes
- 7 Pages / 593.972 x 792 pts Page_size
- 20 Downloads / 210 Views
I.
INTRODUCTION
MARTENSITIC transformation (MT) plays a key role not only in shape memory alloys[1] but also in the development of high strength steels, especially transformation-induced plasticity (TRIP) steels,[2] meta-stable austenitic steels[3] and duplex steels.[4] MT in steels is a first order solid state phase transformation in which the meta-stable face-centered cubic (fcc) austenite (c) phase transforms into body-centered cubic (bcc) or tetragonal (bct) martensite (a¢). Externally applied stresses,[5] a lowering of the temperature,[6] application of a magnetic field[7] or a combination of above driving factors can affect MT. During straining of a TRIP steel, the formation of martensite provides a higher work hardening rate and delays the onset of localized plastic deformation (necking),[2] resulting in an improved combination of a high tensile strength and a high uniform elongation.[8] The occurrence of MT is closely related to the stability of the austenite grains, which depends on the chemical composition,[2] morphology,[9] grain size[10] and hardness of the surrounding grains[11] as documented in literature. Different to the above aspects reported in literature, the present work is to explore a new factor which could also affect the stability of retained austenite, i.e., the stability of an individual retained austenite grain could be affected by whether or not it is surrounded by matrix. In other words, if a retained austenite grain is free from the matrix, it may transform to martensite automatically. Focused ion beam (FIB) milling is a suitable technique to create free surfaces in individual grains. Thus, the present work employs FIB milling to create free surface around individual retained austenite grains B.B. HE, Ph.D. Candidate, M.X. HUANG, Assistant Professor, and A.H.W. NGAN, Professor, are with the Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China. Contact e-mail: [email protected] S. VAN DER ZWAAG, Professor, is with the Faculty of Aerospace Engineering, Delft University of Technology, Kluijverweg 1, 2629 HS Delft, The Netherlands. Manuscript submitted January 22, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
and studies the effect of the free surface on their stability.
II.
EXPERIMENTS
The material employed in this study was a duplex stainless steel with a chemical composition of Fe-0.03C6Mn-16Cr-3Ni (wt pct). A cylindrical sample with a diameter of 10 mm and a thickness of 3 mm was cut from the bulk material. The sample surface was mechanically polished down to 1 lm and subsequently electropolished in a solution of 5 pct perchloric acid, 15 pct glacial acetic acid and 80 pct ethanol at room temperature under an applied voltage of 18 V. The austenite and ferrite grains (a) were identified by electron backscattering diffraction (EBSD, Leo 1530). For the EBSD measurement, a step size of 0.1 lm was used. HKL Channel 5 was used to process the EBSD data. After locating the individual austenite grains, the sample was transferred to the FIB (FEI Qua
Data Loading...
