Photon Factory Activity Report 2018 #36 (2019)

Lattice Strains in Stainless Steel 316
1BL-9C,12C/2017G034

Toshihiko YOKOYAMA1,* and Suwilai CHAVEANGHONG1Institute for Molecular Science, Nishigo-naka 38, Myodaiji-cho, Okazaki 444-8585, Japan

1 Introduction
It is well known that metallic alloys inevitably containlocal lattice strains in the crystal due to different metallic
radius in different elements, even when the average latticeprovides well-defined x-ray diffraction. In the previous
EXAFS study on stainless steel 304 and Elinvar alloys [1],the lattice strain is found to be concentrated around Cr:
noticeably smaller nearest neighbor distances around Cr
than around Fe and Ni. In this work, we have investigatedthe lattice strains in stainless steel 316 by measuring
temperature dependent EXAFS around Mo [2].
2 Experiment
Temperature dependence of Fe, Ni, Cr, and Mo K-edge
EXAFS of stainless steel 316L (Fe67Cr18Ni12Mo2Mn1)
were recorded at BL9C and 12C with the transmission
mode. The measurement temperatures were 20-300 K.
3 Results and Discussion
Figure 1 shows temperature dependence of the first-NNinteratomic distances in the Invar [3], Elinvar [1], SUS304[1], and SUS316. The Invar alloy exhibits almost nothermal expansion around Fe and small expansion aroundNi [−0.03×10−5 (Å/K) for Fe and 1.02×10−5 (Å/K) for Ni].
In the Elinvar alloy, thermal expansion around Fe isnoticeably smaller than those of Ni and Cr [1.33×10−5(Å/K) for Fe, 2.17×10−5 (Å/K) for Ni, and 2.32×10−5 (Å/K)
for Cr], while in SUS304, all the thermal expansions
around Fe, Ni, and Cr are essentially similar and quitenormal [2.39×10−5 (Å/K) for Fe, 2.40×10−5 (Å/K) for Ni,and 2.85×10−5 (Å/K) for Cr]. The local thermal expansion
around Fe, Ni, Cr, and Mo are not extraordinary showing
a normal thermal expansion behavior [2.50×10−5 (Å/K) forFe, 2.32×10−5 (Å/K) for Ni, 2.16×10−5 (Å/K) for Cr, and3.19×10−5 (Å/K) for Mo]. The metallic radii of all theelements in the alloys can be estimated by using theEXAFS results. In SUS304 and SUS316, the Fe atomic
radii are estimated to be 1.264 and 1.266 Å, respectively,
which are slightly larger than that of bcc Fe (1.239 Å) andare quite reasonable because of larger coordination
numbers in fcc alloys. The Ni atomic radii are given as
1.268 Å for SUS304 and 1.265 Å for SUS316, againslightly larger than that of fcc Ni (1.243 Å). It isconsequently remarked that Fe and Ni atoms haveessentially the same interatomic distances for the first-NN
shells and form alloy structures with highcommensurability. In contrast, the environment around Cr
significantly differs from those around Fe and Ni. As
shown in Fig. 1, the first-NN interatomic distances around
Cr in SUS304 are shortened by ∼0.015 Å compared withthose around Fe and Ni, and in the present SUS316, thistrend is more prominently observed. The Cr atomic radii

are estimated to be 1.251 and 1.235 Å for SUS304 andSUS316, respectively, which are similar to that of bcc Cr(1.246 Å) or even shorter. The fact that in the fcc latticewith larger first-NN coordination numbers yields similaror shorter interatomic distances around Cr implies thepresence of large compressive lattice strain around Cr. On
the other hand, the Mo atomic radius in SUS316 is given
as 1.319 Å, being shorter by as much as 0.04 Å than thatof bcc Mo (1.358 Å), indicating more significant
compressive lattice strain in Mo atoms with inherently
larger atomic radii. It is interesting to note that the thermal
fluctuation around Cr is larger than those on the othermetals, which may reasonably coincide with a large
compressive lattice strain on Cr, while the thermal
fluctuation around Mo is smaller with a more significant
compressive lattice strain, probably due to a heavier atomic
weight and a larger cohesive energy. It can be concluded
that although the Invar, Elinvar, SUS304, and SUS316
alloys are recognized to form fcc lattices with completelyrandom atomic distributions, the real interatomic distances
are considerably dependent on the elements.

Fig. 1: The first-nearest neighbor interatomic distances ofInvar, Elinvar, SUS304, and SUS316 (present work) alloys
determined by Fe, Ni, Cr, and Mo K-edge EXAFS (pointswith error bars). Note that the coordinated atoms (scatterer)
are not elementarily distinguished, while the central atoms (xray absorber) are clearly distinguished.
References[1] T. Yokoyama, A. Koide, and Y. Uemura, Phys. Rev.
Mater. 2, 023601 (2018).
[2] T. Yokoyama and K. Eguchi, Phys. Rev. Lett. 107,065901 (2011).
[3] T. Yokoyama and S. Chaveanghong, Phys. Rev. Mater.
3, 033607 (2019).
* yokoyama@ims.ac.jp