Computational modeling of reactive hot pressing of zirconium carbide
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Lingappa Rangaraj Materials Science Division, CSIR-National Aerospace Laboratories, Bangalore 560017, Karnataka, India
Vikram Jayarama) Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, Karnataka, India (Received 19 February 2015; accepted 4 May 2015)
A model of reactive hot pressing of zirconium carbide (ZrCx, 0.5 , x , 1) has been constructed that incorporates four processes that occur in parallel: creep of zirconium (Zr), reaction of Zr and carbon (C), increase in volume fraction of hard phase with progressive reaction that reduces the creep of Zr and, finally, de-densification associated with volume reduction during reaction. The reasonable agreement of the model with experimental results verifies that plastic deformation of Zr is the main factor that is responsible for the low-temperature reactive densification of ZrC and that ZrC may be treated as a rigid inclusion that contributes little to densification. It predicts that densification is impaired by increasing carbon stoichiometry due to the increasing amount of starting hard phase and the greater contraction upon reaction. Additionally, the model predicts that mixtures of Zr and ZrC should show equal or better densification than Zr and C mixtures.
Contributing Editor: Suk-Joong L. Kang a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.147
plastic flow in the metal alone. In summary, these investigations show that zirconium (Zr) metal in the starting powder is the most significant contributor to densification. Further, from the result of the previous work, it could also be seen that the molar volume reduction in the reaction is also an important factor which is responsible for the de-densification observed during RHP.14 The present work seeks to identify the role of reaction and of reaction-derived volume changes during reactive processing of ZrC more closely through modeling of the RHP process. RHP is a complicated phenomenon involving a dynamically evolving phase and pore content that dramatically influences densification. Till now, there has been little modeling effort to understand reactive densification and test its predictive capability. Complexities arise from the multitude of processes operating simultaneously. First, there is densification from HP which itself can occur through a number of pathways ranging from particle rearrangement, timeindependent plastic deformation, creep, both dislocationaided and diffusional. Depending on time, temperature, pressure, or heating rate, the aforementioned mechanisms will be responsible for deformation to different extents. In RHP and especially in systems such as the present Zr and carbon (C) mixtures, there are additional mechanisms. First, as it is a mixture of a soft and a hard phase, i.e., Zr and C, initial densification is severely constrained. As the reaction proceeds, the product phase, also rigid at these low temperatures, develops percolating structures and the
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Ó Materials Research Society 2015
I. INTRODUCTION
Zir
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