Photon Factory Activity Report 2017 #35 (2018)

BL-9C/2017G034

Local thermal expansions and lattice strains in Invar, Elinvar and stainless steel alloys
1Toshihiko YOKOYAMA1,*Institute for Molecular Science, Myodaiji-cho, Okazaki, Aichi 444-8585, Japan

1 Introduction
Local structures and thermal expansions of alloys areoften different from those from crystallographic expectations. Even when the x-ray diffraction patterns from thealloys are definitely observed, there should exist strains
depending on the atom pairs and the local environments.
In the present works, I will show element-specific local
structures and thermal expansions of Invar (Fe65Ni35) [1],Elinvar (Ni Span C, Fe49.66Ni42.38Cr5.49 Ti2.47), and SUS304(AISI304, Fe71.98Ni9.07Cr18.09Mn0.86) [2] by the analysis oftemperature dependent EXAFS concerning corresponding
metal K edges and also by the Monte-Carlo (MC)simulations based on the path-integral effective classicalpotential (PIECP) method that includes the vibrational
quantum effect approximately.
2 Experiment
The alloy foils were purchased from Sugiyama Metals
Co., Ltd. in Osaka, Japan. The thicknesses of Invar,Elinvar, and SUS304 are 10, 15.0, 10.5 µm, respectively.
The Cr, Fe, and Ni K-edge EXAFS were recorded withthe transmission mode using Si(111) double crystal
monochromator. The measurement temperatures were 10300 K. The path-integral effective potential simulations
for several kinds of alloy supercells of 500 atoms withperiodic boundary restrictions were performed based on
the NPT (constant number of particles, pressure, andtemperature) conditions.
3 Results and Discussion
The results of the Cr, Fe, and Ni K-edge EXAFS
analysis concerning the first nearest-neighbor (NN) shells
are depicted in Fig. 1. The local thermal expansion
around Fe is found to be considerably smaller (1.33×10-5Å/K) than the ones around Ni (2.17×10-5 Å/K) and Cr(2.32×10-5 Å/K). This observation can be understood
simply because Fe in the Elinvar alloy exhibit anincomplete Invar effect. Here, the Invar effect indicatesthat the system consists of high-spin more stable Fe withlarger atomic radius and low-spin more unstable Fe withsmaller atomic radius and that as temperature increases
enhancement of the low-spin Fe concentration yields
thermal contraction, leading to compensation of thermal
expansion due to anharmonic vibration. The SUS304
alloy exhibits normal thermal expansion as 2.85×10-5,2.39×10-5, and 2.40×10-5 Å/K around Cr, Fe, and Ni.
Moreover, it should be noted that in both the Elinvar andSUS304 alloys, the local thermal expansions and thelattice strains around Cr are larger than those around Feand Ni.
From the PIECP MC simulations of both the alloys,
the first-nearest neighbor Cr-Fe pair shows extraordinary-Fig. 1: Temperature dependence of the first-NN
distances around Cr, Fe and Ni in the Elinvar andSUS304 alloys.
ly large thermal expansion, while the Cr-Cr pair exhibits
quite small or even negative thermal expansion (figures
not shown due to limited space). These findings
consequently indicate that the lattice strains in both theElinvar and SUS304 alloys are concentrated predominantly on the Cr atoms. Although the role of Cr in
stainless steel has been known to inhibit corrosion by theformation of stable and tight surface chromium oxide, thepresent investigation may interestingly suggest that the Cratoms in the bulk play a hidden new role of absorbing
inevitable lattice strains in the alloys.
References[1] T. Yokoyama and K. Eguchi, Phys. Rev. Lett. 107,065901 (2011).
[2] T. Yokoyama, A. Koide, and Y. Uemura, Phys. Rev.
Materials 2, 023601 (2018).
* yokoyama@ims.ac.jp