実験報告書様式（一般利用課題・成果公開利用）（ ※本報告書は英語で記述してください。ただし、産業利用課題として採択されている方は日本語で記述していただいても結構です。 ）MLF Experimental Report

提出日 Date of Report

課題番号 Project No.

装置責任者 Name of responsible person

2017A0136

Kazuya Aizawa

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

In-situ neutron diffraction analysis for transformation-induced

BL19

plasticity effect of ultrafine-grained metastable austenitic steel

実施日 Date of Experiment

実験責任者名 Name of principal investigator

6/26-27

Akinobu Shibata
所属 Affiliation
Kyoto University
試料、実験方法、利用の結果得られた主なデータ、考察、結論等を、記述して下さい。（適宜、図表添付のこと）
Please report your samples, experimental method and results, discussion and conclusions. Please add figures andtables for better explanation.
1. 試料 Name of sample(s) and chemical formula, or compositions including physical form.
Fe-24Ni-0.3C (mass%) alloy

2. 実験方法及び結果 （実験がうまくいかなかった場合、その理由を記述してください。）
Experimental method and results.

If you failed to conduct experiment as planned, please describe reasons.

In general, structural materials, particularly steels, are desired to manage both high strength andlarge ductility. Transformation-induced plasticity (TRIP) effect is one of the promising ways to achieve
simultaneously high strength and large ductility by utilizing deformation-induced  (fcc phase)  ’ (bccor bct phase) martensitic transformation.

Deformation-induced martensitic transformation is aphenomenon that martensitic transformation occurs during deformation. Although TRIP steels exhibit
good mechanical properties and have been applied to automobile steel sheets, it is an important
technical challenge in the steel society to further improve mechanical properties of TRIP steels from therecent environmental and economic points of view.
The present study investigated TRIP effect of metastable austenitic steels with grain sizes of 1 m, 4m, and 34 m by in-situ neutron diffraction. The results of the specimen with grain size of 34 m was
obtained in our previous beamtime (2016E0003). Figure 1 shows nominal stress-strain curves of thespecimens with various grain sizes. With decreasing the grain size from 35 m to 1 m, the yieldstrength increases from 180 MPa to 420 MPa and the total elongation also increases from 83 % to104 %, while the ultimate tensile strength slightly decreases from 1100 MPa to 950 MPa.

The

ultrafine- and fine-grained specimens (d = 1 m and 4 m) exhibit a better combination of strength andductility than that of the coarse-grained specimen (d = 35 m). Work-hardening rates and volume
fractions of martensite are presented in Figure 2(a) and (b), respectively, as a function of the true1

2. 実験方法及び結果（つづき） Experimental method and results (continued)
strain.

For all the three specimens, thework-hardening rates firstly decrease, thenstart to increase at certain strains, which
depend on grain size.

We find that theincrease of the strain hardening rate is
attributed

tothedeformation-induced

martensitic transformation, because the strain,
at which the martensitic transformation starts,
corresponds very well with the strain where thestrain hardening increases.

However, theenhancement of the strain hardening rate
decreases with decreasing the grain size ofaustenite.

This is because

stability of

Figure 1 Nominal stress-strain curves of thespecimen with different grain sizes.

austenite increases by grain refinement.
Figure 3 shows changes of lattice strain of(111) austenite plane and (110) martensite
plane of the specimens with different grain
sizes as a function of the applied stress.
Beforetheonsetofdeformation-induced

martensitic transformation, the lattice strains of(111) austenite plane have a linear relationshipwith the applied stress. After the occurrence
ofdeformation-induced

martensitic

transformation, the lattice strains of (111)austenite plane become lower than the linear
relationship due to the stress partitioning ofmartensite.

On the other hand, the latticestrains of (011) martensite plane quickly
increasewithincreasingofthestrain,suggesting that the phase stress of martensite

Figure 2 (a) Work-hardening rate curves and (b)changes in martensite volume fractions of thespecimens with different grain sizes, plotted as afunction of true stressincreases.
The phase stress was roughly estimated by the following equation: phase = Ehkl hkl, where phase isphase stress, hkl is lattice strain of (hkl) plane, and Ehkl is diffraction elastic constant of (h k l) plane.
The total stress of specimen was calculated by rule of mixture: total = mart fmart + aust (1 - fmart). Theresults are summarized in Figure 4, which reveal that the flow stress of the specimen could beformulated well by the rule of mixture. For all the three specimens, the phase stress of martensite ismuch larger than that of austenite. As shown in Figures 3 and 4, with decreasing the grain size, thephase stress of austenite at a given strain increases due to the grain refinement strengthening. The
phase stresses of martensite in the specimens with grain sizes of 35 m and 4 m are almost thesame. However, when the grain size decreases to 1 m, the lattice strain of martensite significantly22. 実験方法及び結果（つづき） Experimental method and results (continued)
same. However, when the grain size decreases to 1 μm, the lattice strain of martensite significantly
increases with increasing of the strain. This result suggests that the ultrafine-grained specimen exhibit
large work hardening due to the large phase stress of martensite, even though deformation-induced
martensitic transformation is suppressed to some extent by grain refinement.

Figure 3 Changes of lattice strains of (a) (111) austenite plane and (b) (110) martensite plane as afunction of true stress.

Figure 4 Changes of phase stress of austenite, phase stress of martensite, and total stress of thespecimens with different grain sizes; (a) 1 m; (b) 2 m; (c) 35 m.

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