irradiation-induced hardening of 316 austenitic...

Секция 2. Радиационные эффекты в твердом теле Section 2. Radiation effects in solids

13-я Международная конференция «Взаимодействие излучений с твердым телом», 30 сентября - 3 октября 2019 г., Минск, Беларусь 13th International Conference “Interaction of Radiation with Solids”, September 30 - October 3, 2019, Minsk, Belarus

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IRRADIATION-INDUCED HARDENING OF STRUCTURAL MATERIALS AFTER LOW TEMPERATURE IRRADIATION

S.A. Karpov, G.D. Tolstolutskaya

National Science Center “Kharkov Institute of Physics and Technology”, 1 Academicheskaya Str., 61108 Kharkov, Ukraine, [email protected], [email protected]

Irradiation-induced hardening has been investigated in relation to austenitic SS316 steel, ferritic-martensitic T91 steel and 20Cr-

40Fe-20Mn-20Ni high-entropy alloy (HEA). Specimens were irradiated with 1.4 MeV/Ar ions to doses of 0.5 - 10 displacements per atom (dpa) at room temperature. Hardening of the irradiated layer was examined with nanoindentation technique. The behavior of the hardness-depth curves was analyzed with respect to the ion irradiation dose. Regression analysis performed for high dose regime of irradiation by using a power-law function of the form ∆Н ∝ (dpa)n gives good agreement with the experimental data at n ~ 0.1-0.16 for structural alloys having different composition and different structural state.

Keywords: irradiation; nanoindentation; hardness; steels; high-entropy alloy.

Introduction Metals exposed to irradiation are known to harden

due to the generation of Frenkel pair defect clusters that act as obstacles to dislocation motion under an applied stress. This hardening increases the yield strength, σy, of the material but reduces the ductility and causes embrittlement. Irradiation hardening in metallic materials is strong after irradiation at low temperatures (usually below 300 °C) because significant quantities of radiation-induced defect clusters are retained, and they impede the generation and glide of dislocations during deformation [1].

To simulate neutron irradiation damage of the structural materials, heavy ion irradiation experiments have been used because of the simplicity of use, easier control of irradiation parameters, reduction of cost, rapid damage production, the absence of induced radioactivity, and the occurrence of the co-implantation of helium/hydrogen. The solution of problem ion irradiation – shallow depth of damage layer that making it difficult to investigate the mechanical properties – is possible by using nanoindentation method. However, for the successful implementation of this methodology, it is necessary to resolve such issues as the correlation the change in strength with the plastic deformation, the dose dependent of defect-cluster accumulation.

The purpose of the present work is the investigation of the irradiation hardening behavior in different types of structural materials with irradiation dose.

Main part

The specimens of SS316 and T91 steels and high entropy alloy having the dimensions of 10×7×0.3 mm were used for investigations. The specimens of SS316 steel before experiments were annealed at 1340 K for one hour in a vacuum ~10-4 Pa. The T91 ferritic-martensitic steel of composition 9Cr–1Mo with minor alloying elements of Ni, Nb, V, and C was supplied by INDUSTELL, Belgium (melting: 504/3, heat: 82566-4). The material was delivered as hot rolled and heat treated plates with a thickness of 40 mm. The heat treatment consisted of a normalization treatment at 1040 °C for 30 min followed by air cooling and then tempered at 730 °C for 60 min followed by air cooling to room temperature.

High entropy alloy with the compositions (in at.%) of 20Cr-40Fe-20Mn-20Ni were produced by arc

melting in a high-purity argon in a water-cooled copper mould. The purities of the alloying elements were above 99.9%. To ensure chemical homogeneity, the ingots were flipped over and re-melted a least 5 times. The produced ingots had dimensions of about 6×15×60 mm. The alloy was studied after homoge-nization annealing. Homogenization was carried out at 1050 °C and lasted for 24 hours. Prior to homoge-nization samples were sealed in vacuumed (10-2 Torr) quartz tubes filled with titanium chips to prevent oxidation.

TEM observation of unirradiated samples (Fig. 1) showed single-phase FCC crystal lattice for SS316 and 20Cr-40Fe-20Mn-20Ni high-entropy alloy and dual phase morphology containing ferrite and martensite phases for T91 steel.

a

b

Ferrite Martensite

c

Fig. 1. The initial microstructure of SS316 steel (a), 20Cr-40Fe-20Mn-20Ni high-entropy alloy (b) and T91 steel (c)



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Samples were irradiated with 1.4 MeV argon ions in a dose range of 0.5-10 dpa. All irradiations were carried out with accelerating-measuring system “ESU-2” [2] at room temperature (RT). The depth distribution of gas atoms concentration and damage for ion irradiation with argon ions shown in Fig. 2.

Fig. 2. Damage and concentration profiles as a function of depth calculated using SRIM [3] for 1.4 MeV Ar+ irradiation of Fe to a dose of 4·1019 m-2

Fig. 3 shows nanoindentation hardness as a function of indenter displacement of the unirradiated and irradiated 316 steel. The irradiation of SS316 with Ar ions at RT leads to an increase of nanohardness.

0 200 400 600 800 1000

2

3

4

5 0 dpa 0,5 dpa 0,7 dpa 1,0 dpa 2,0 dpa 5,0 dpa 10 dpa

Indentation depth, nm

Hard

ness

, GPa

Irradiation dose

Fig. 3. Nanoindentation hardness vs. indentation depth measured for the unirradiated and irradiated SS316 steel

In all samples, the first 100 nm of displacement shows a considerable increase in the scatter of the data due to tip-rounding artifacts and surface preparation effects. Therefore, for all samples the first 100 nm of data will be ignored for the remainder of the analysis.

Generally, indentation hardness of ion irradiated materials represents the superposition of the bulk hardness, indentation size effect (ISE) and the irradiation induced hardening [4]. The analysis method by nanoindentation measurement is based on the Nix-Gao model [5] that describes the concept of geometrically necessary dislocations required to accommodate the indenter as well as Kasada et al. method [6] that extended model [5] by a film-substrate system based on so-called the soft substrate effect. The ion irradiated materials, according to [6], can be considered as “hardened layer–substrate” systems. By

redrawing the hardness profile in terms of Nix-Gao plot (squared hardness vs. reciprocal depth), the bulk-equivalent hardness of the ion-irradiated region, H0irr, has been evaluated (Fig. 4).

2 4 6 8

5

10

15

20 1 dpa un-irradiated

H0=2.16 GPa

Squa

red

hard

ness

, GPa

2

Reciprocal depth, µm-1

Hirr0 =3.76 GPa

2 4 6 8

5

10

15

20 1 dpa un-irradiated

H0=2.16 GPa

Squa

red

hard

ness

, GPa

2

Reciprocal depth, µm-1

Hirr0 =3.76 GPa

Fig. 4. Nix-Gao plot for unirradiated and argon irradiated (1 dpa) SS316 steel

For the unirradiated SS316, an approximately linear relationship is observed for data with depths greater than 100 nm. The irradiated material represents a bilinear relationship with a shoulder around 160 nm. Such bilinear behavior is associated with the plastic zone extending into the unirradiated region of material, beneath the irradiated layer.

In the same way, the bulk-equivalent hardness of the ion-irradiated region was determined for all irradiation doses of SS316, T91 and HEA.

In this study hardness profiles that have been analyzed for determining the bulk-equivalent hardness of as-received, H0, and irradiated alloys, H0irr, were obtained from load-displacement data using the method of Oliver and Pharr [7]. This method is adopted as the standard method for the analysis of nanoindentation results. One significant limitation of this method is that it does not account for pile-up or sink-in of testing material around the indent. In the case of pile-up, the contact area is greater than predicted by the method that can lead to an overestimation of the indentation hardness.

Thereby, after indentation the hardness impres-sions were imaged using SEM (Fig. 5) to measure contact areas and examine the extent of pile-up, in a similar method to that proposed in [8].

The effect of pile-up was not observed in SS316 and HEA alloy and the effect of sink-in around the in-dents appears to be insignificant for these two mate-rials and not taken into account at processing data on nanoindentation (Fig. 5, a, b, d, e). However, for the T91 steel, the pile-up effect is quite pronounced (Fig. 5 c, f).

In the case of T91, two different contact areas were determined from the SEM images. According to Fig. 5 the pile-up unaffected corner-to-corner area, Acc, represents the area of the triangle defined by the corners of the hardness impression (see Fig. 5 c, f). The second measure of the contact area was the actual contact area, Aact, which includes the extra area contained in the pile-up (Fig. 5 c, f). These specified areas were determined by a digital image processing, and their ratio, Aact/Acc, provided an estimation of the pile-up extent.



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2 µm a b c

d e f

Fig. 5. SEM images showing deformed regions surrounding indents in un-irradiated (a, b, c) and irradiated (d, e, f) regions of HEA alloy (a, d), SS316 (b, e) and T91 steel (c, f). White lines - corner-to-corner (Acc) of contact area (c, f)

The values of correction factor, Aact/Acc, were found in the range of 1.10-1.18 and had a statistical scatter for as-received and argon irradiated specimens. The average value of 1.14 ± 0.04 was accepted as a correction factor for T91 steel.

The dose dependence of measured and corrected for a pile-up bulk-equivalent hardness in investigated materials is presented in Fig. 6.

0 2 4 6 8 101.5

2.0

2.5

3.0

3.5

4.0

4.5

SS316 T91 HEA

Har

dnes

s, G

Pa

Damage, dpa Fig. 6. The dose dependence of bulk-equivalent hardness of ion irradiated SS316, T91 and HEA alloy

The plots of hardness vs. dpa data showed two distinctive regimes: a low-dose regime where a rapid hardening occurs and a high-dose regime where the plot shows a considerably reduced slope. Nano-indentation results showed that irradiation hardening in the ion-irradiated SS316 and T91 approaches the quasisaturation mode at doses ≥ 1 dpa. This is not as clear as in the case of HEA alloy due to the limited data points, but the tendency appears to be the same.

In the regression analysis [9], the radiation-induced increase in yield stress, ∆σys (yield strength was calculated from hardness measurement), was expressed in the form of a power law: ∆σys =h·(dpa)n, where h and n are the regression coefficients and dpa is displacements per atom. According to [10], n values were in the range 0.31–0.4 in the low-dose regime for the fcc metals and 0.4–0.55 for bcc metals. In the high-dose regime n values for the fcc metals varied more widely in the range 0.01–0.24.

Considering a simple barrier strengthening model with one obstacle type with the same size and strength factor, a rapid hardening can be explained if the number density of the obstacles increasing linearly with displacement damage. The smaller n values are probably due to effects of saturation at higher doses. Yamamoto et al. [11] also suggested that the saturation can be physically related to the depletion of solutes, in the case of a precipitation hardening mechanism, or an excluded volume type effect in the case of the accumulation of displacement damage-type defects.

Approximation of the irradiation hardening data, ∆H=H0irr–H0, obtained in the present study for high-dose regime by a power function of the form ∆Н ∝ (dpa)n indicates that n values are changing in the range 0.10 – 0.16 for austenitic SS316 steel, ferritic-martensitic T91 steel and 20Cr-40Fe-20Mn-20Ni high-entropy alloy (Fig. 7).

1 10

0.5

1

2

3

4

n=0.16

n=0.10

SS316 T91 HEA n=0.11

∆H, G

Pa

Damage, dpa Fig. 7. Log-log plot for dose dependence of irradiation hardening for SS316, T91 and HEA alloy

Defects that obviously cause hardening at low irradiation temperature are dislocation-type defects. For all investigated here materials, the irradiation-induced microstructure consists predominantly of small (



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The similar microstructural evolution and irradiation hardening behavior between the HEA and SS316 indicate that at room temperature the irradiation damage of fcc alloys is not sensitive to compositional variation and configurational entropy.

Loop density is considerably smaller for T91 steel. It appears that the ferritic-martensitic microstructure has a significant effect in reducing hardening in T91. This can be explained by the fine microstructure which contains carbides along boundaries, so a high density of interfaces act as defect sinks for radiation induced defects. This testifies that the materials containing a number of defects or trapping sites suffer less irradiation hardening or embrittlement.

Conclusions

Nanoindentation results showed that irradiation hardening in the ion-irradiated austenitic SS316 steel, ferritic-martensitic T91 steel and 20Cr-40Fe-20Mn-20Ni high-entropy alloy approach of the quasisaturation mode at doses ≥ 1 dpa.

Regression analysis performed using a power-law function of the form ∆Н ∝ (dpa)n gives good agreement with the experimental data at n ~ 0.10-0.16 for high-dose regime of irradiation.

The similarity of irradiation hardening behavior bet-ween SS316 and HEA as well as their similar microstructural evolution indicate that the irradiation damage of fcc alloys is slightly sensitive to compositional variation and configurational entropy at low temperature irradiation.

The ferritic-martensitic microstructure with high density of interfaces which act as defect sinks for radiation induced defects has a significant effect in reducing hardening in T91. References 1. Voyevodin V.N., Neklyudov I.M. Evolution of the structure

phase state and radiation resistance of structural materials. Кiev: Naukova dumka; 2006. 376 p.

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Problems of atomic science and technology. Series “Physics of Radiation Damages and Effects in Solids” 2010; 1: 135-140.

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irradiation-induced hardening of 316 austenitic...

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