Elsevier

Materials Science and Engineering: A

Volume 556, 30 October 2012, Pages 664-670
Materials Science and Engineering: A

Thermal stability of nanocrystalline Fe–Cr alloys with Zr additions

https://doi.org/10.1016/j.msea.2012.07.045Get rights and content

Abstract

The primary objective of this work was to determine the influence of 1–4 at% Zr additions on the thermal stability of mechanically alloyed nanocrystalline Fe–Cr alloys containing 10 and 18 at% Cr. Grain sizes based on XRD, along with microhardness changes, are reported for isochronal annealing treatments up to 1000 °C. Microstructure investigations were done using optical microscopy, channeling contrast FIB imaging, and TEM. Grain size stabilization in the nanaoscale range was maintained up to 900 °C by adding 2 at% Zr. Kinetic pinning by nanoscale intermetallic particles was identified as one source of high temperature grain size stabilization. Intermetallic particles also contribute to strengthening in addition to the Hall–Petch effect. The analysis of microhardness, XRD data, and measured values from the TEM image for Fe-10 at% Cr with 2 at% Zr suggested that both thermodynamic and kinetic mechanisms would contribute to grain size stabilization. There was no significant difference in the results for the 10 and 18 at% Cr alloys, which indicates that the α→γ transformation does not influence the grain size stabilization.

Introduction

Extensive grain growth in pure nanocrystalline metals at low annealing temperatures has been observed in numerous studies [1], [2]. Even metals with high melting points, such as pure Fe, can undergo rapid grain growth at annealing temperatures below 50% of their respective melting points [3]. It is well known that grain boundaries in nanocrystalline microstructures produce a significant increase in the total free energy of the system. The presence of a large driving force for grain growth due to the decrease of grain boundary area is inevitable owing to reduction of the excess free energy [4]. Kinetic stabilization and thermodynamic stabilization are the two basic mechanisms by which stability of a nanoscale grain size can be retained at high temperatures. With kinetic stabilization, the mobility of a grain boundary is reduced by effects such as solute drag, second-phase particle pinning (Zener pinning), chemical ordering, or porosity. All of the kinetic mechanisms are thermally activated and can be overcome at sufficiently high temperatures. Solute drag can be effective at temperatures where the grain boundary mobility M is low. M is generally associated with grain boundary diffusion activation energies and will be less effective at higher temperatures where volume diffusion is significant and intermetallic particle formation and/or grain boundary segregation can occur. Considering this latter temperature regime, the relevant grain-size stabilization mechanisms for the Fe–Cr–Zr alloys are anticipated to be Zener drag and/or thermodynamic stabilization.

With Zener pinning, the grain boundary is subjected to a resisting pinning pressure Pz [5]. The driving force pressure P is due to the expansion of the curved grain boundary [6]. As a consequence, grain growth will be inhibited when the grain size reaches a critical value. This will be discussed in more detail in Section 3. With thermodynamic stabilization, the solution of Gibbs interface equation leads to the relation [7], [8]γ=γ0+ΔGsegΓsγ0 is the non-segregated grain boundary energy and ΔGseg is the free energy change associated with segregation of the solute. Therefore, solute segregation to the grain boundary can decrease the grain boundary energy when ΔGseg<0 and a nanocrystalline alloy can be in metastable thermodynamic equilibrium at γ=0 [7], [9]. Darling et al. [10] reported that 4 at% Zr addition to pure Fe stabilizes a nanocrystalline grain size up to 900 °C. Model calculations suggested that thermodynamic stabilization would be effective for the Fe-4 at% Zr alloy system in this temperature range. Intermetallic formation was observed at higher temperatures in conjunction with grain growth. An issue that is raised for the interpretation of the Fe–Zr results is the possible effect of the α→γ allotropic phase transformation at 913 °C in pure Fe. The lower free energy of the γ phase can drive grain growth and subsequent destabilization of the grain size above the transformation temperature. This effect was in fact recently observed for Fe–Ti alloys using high temperature X-ray diffraction measurements [11]. One important motivation for investigating Zr additions to Fe–Cr alloys, in addition to the possible applications for thermally stable nanocrystalline ferritic alloys, is the fact that the α→γ transformation can be eliminated altogether when the Cr content is sufficiently large. Significant differences in the temperature dependence of the Zr-addition thermal stabilization effect between alloys with 10 and 18 at% Cr would be important for the relevance of an α→γ transformation effect.

Section snippets

Experimental

Non-equilibrium solid solution alloys of Fe–10Cr–Zr and Fe–18Cr–Zr containing 0, 1, 2, and 4 at% Zr were produced by ball milling. Fe–10Cr was selected because it is inside the α→γ “loop” on the Fe–Cr phase diagram at elevated temperatures and it will undergo the α→γ phase transformation. Fe–18Cr was selected because it is outside the α→γ loop at any annealing temperature and this alloy will not undergo the α→γ phase transformation. A hardened steel vial and 440 stainless steel balls were used

Results and discussion

XRD results confirmed the formation of as-milled Fe–Cr–Zr solid solutions with the bcc crystal structure for all of the alloys. However, at 4 at% Zr extra diffraction peaks appear after annealing at elevated temperatures. These extra peaks have been indexed and they correspond to the intermetallic Laves phases C15 and C14/C36 with Zr(FexCr1−x) composition [14], [15]. Fig. 1 shows the XRD patterns of Fe–10Cr+4 at% Zr and Fe–18Cr+4 at% Zr annealed at 900 °C. The extra peaks in Fig. 1 are not present

Summary and conclusions

The results obtained in this investigation show that additions of Zr up to x=4 at% in ball milled Fe–10Cr−xZr and Fe–18Cr−xZr alloys can provide effective grain size stabilization in the nanocrystalline range up to temperatures of 900 °C. Hardness values in the range of 7 GPa are maintained. This provides an opportunity for developing new high strength thermally stable nanocrystalline ferritic alloys. The trends for grain size and hardness as a function of isochronal annealing temperature were

Acknowledgments

This work was supported by the US National Science Foundation, Grant DMR-1005677. The authors thank Dr. Dale Batchelor at the Analytical Instrumentation Facility at North Carolina State University for assistance with the FIB sample preparation.

References (20)

  • T.R. Malow et al.

    Acta Mater.

    (1997)
  • J. Weissmüller

    Nanostruct. Mater.

    (1993)
  • F. Liu et al.

    Scr. Mater.

    (2004)
  • K.A. Darling et al.

    Mater. Sci. Eng. A

    (2010)
  • J.M. Dake et al.

    Scr. Mater.

    (2012)
  • J. Li et al.

    Mater. Charact.

    (2006)
  • S. Scudino et al.

    Intermetallics

    (2009)
  • J.A.H. Coaquira et al.

    J. Alloys Compd.

    (1999)
  • C. Suryanarayana

    Int. Mater. Rev.

    (1995)
  • D.L. Bourell, T.P.M. Committee, F.o.E.M. Societies, M. Minerals, M.S. Meeting, Synthesis and processing of...
There are more references available in the full text version of this article.

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