Impurity (Fe, Cl, and P)-Induced Grain Boundary and Secondary Phases in Commercially Pure Titanium (CP-Ti)
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TRODUCTION
DEPENDING on temperature and pressure, titanium can be stable as a-Ti (closed-packed hexagonal, cph), b-Ti (body-centred-cubic, bcc), or x-Ti (simple hexagonal, h)[1–5]; their crystal structures are shown in Figure 1. The a–b transformation occurs at ~1155 K (882 C) under atmospheric pressure, while the x-Ti phase is usually regarded as a high-pressure (>2 GPa) and/or low-temperature [< 475 K (202 C)] phase in pure Ti.[1–5] The four grades of commercially pure titanium (CP-Ti) are classified according to their oxygen (O) and iron (Fe) contents with specified carbon (C) and nitrogen (N) contents. These impurities affect the mechanical and corrosion behaviors of CP-Ti materials, which, including their modified forms, account for ~25 pct of the current titanium market. Powder-based approaches have received increasing attention in recent years as a cost-effective alternative to the fabrication of Ti products.[6–14] In particular, the cold-compaction-and-sinter powder metallurgy (PM) approach is technically the simplest and economically the most attractive near-net shape PM fabrication method, compatible with nonfatigue critical applications.[9] This is in addition to the constitutional (greater chemical homogeneity) and microstructural (finer grain size) attributes offered by this approach.[9] CP-Ti materials, irrespective of the fabrication route, are generally assumed to be as single-phase a-Ti materials. M. YAN, Queensland Smart Future Fellow (Early Career), S.D. LUO, Postdoctoral Research Fellow, G.B. SCHAFFER, Professor, and M. QIAN, Reader in Materials, are with The University of Queensland, School of Mechanical and Mining Engineering, ARC Centre of Excellence for Design in Light Metals, Brisbane, QLD 4072, Australia. Contact e-mail: [email protected] Manuscript submitted April 19, 2012. Article published online April 10, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
This study presents a detailed TEM analysis of the microstructures of two as-sintered CP-Ti materials containing 40 ppm and 1280 ppm of Fe. We show that the microstructure of a CP-Ti material can be a complex mixture of phases. The implications of the findings are discussed.
II.
EXPERIMENTAL
AEE-Ti powder (prepared by plasma spray and supplied by Atlantic Equipment Engineers; to be denoted as high-Fe CP-Ti powder) and WUYI-Ti powder (hydride–dehydride powder, supplied by Kemit Special Metal Powder; to be denoted as low-Fe CP-Ti powder) were used. Impurities of these two powders were analyzed by various means (Model Leco TC-436 for O, SPECTRO ICP-OES.09 for Fe and METROHM 751 TITRINO for Cl and P). The results are summarized in Table I. The Fe content is lower than the limit (2000 ppm) for CP-Ti ASTM Grade 1. Titanium powder was uniaxially pressed into cylinders of 10 mm in both diameter and height at 800 MPa. The sample preparation details can be found elsewhere.[15] Sintering was conducted at 1623 K (1350 C) for 60 minutes in an alumina-tube vacuum furnace. The vacuum pressure was in the order of 10 2 Pa during isothermal sintering at 1623
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