Mechanical properties and thermal stability of nanostructured ODS RAF steels

Mechanics of Materials 67 (2013) 15–24

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Mechanical properties and thermal stability of nanostructured ODS RAF steels Z. Oksiuta a,⇑, M. Lewandowska b, K.J. Kurzydłowski b a b

Bialystok Technical University, Faculty of Mechanical Engineering, Wiejska 45c, 13-351 Bialystok, Poland Warsaw University of Technology, Faculty of Materials Science and Engineering, Wołoska 141, 02-504 Warsaw, Poland

a r t i c l e

i n f o

Article history: Received 23 July 2012 Received in revised form 8 July 2013 Available online 7 August 2013 Keywords: ODS RAF steel HIPping HSHE High temperature-long term annealing Tensile testing Activation energy

a b s t r a c t The main goal of the research reported in this paper was to evaluate the microstructure and mechanical properties of an oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel after a long-term and high temperature annealing. The mechanically alloyed ODS RAF steel powder with a composition of Fe–14Cr–2W–0.3Ti–0.3Y2O3 was consolidated by hot isostatic pressing at 1150 °C for 3 h followed by a high speed hydrostatic extrusion at 900 °C. The samples were subsequently exposed to various heat treatments, such as annealing at 850 °C–1350 °C for 1 h in argon and to a long-term isothermal annealing at 750 °C up to 10 000 h in air. The results indicate that due to the Zener pinning of the nanoparticles at the grain boundaries, the ODS RAF steel is thermally stable up to high annealing temperature of 1250 °C. However, annealing at 1050 °C applied after the hydrostatic extrusion of the steel significantly improves its ductility. Further increase in the temperature up to 1350 °C resulted in nanoparticles coarsening (size 8.0 nm, number density 2.5  023 m 3) accompanied by an increase in the volume fraction of the coarse grains and a reduction of microhardness from 460 to 330 HV0.1. Long-term annealing at 750 °C up to 10 000 h brought about a systematic loss of both strength and ductility. However, the microhardness increases up to 500 HV0.1. TEM observations revealed that the grain size after ageing remains almost unchanged. On the other hand, the number of dislocation free larger grains and of larger precipitates with a size of 25 nm located at grain boundaries significantly increase. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The oxide dispersion strengthened (ODS) alloys are fabricated by a powder metallurgy route (Gessinger and Bomford, 1974; Gessinger, 1984; Benjamin and Volin, 1974; Benjamin, 1992). Two the most successful types of these alloys are nickel- and iron-based. The ODS nickel-based superalloys have very high temperature (creep) resistance and they found their application in aerospace industry. The ODS ferritic superalloys have improved high temperature thermal properties and strength as well as irradiation ⇑ Corresponding author. Tel.: +48 85 746 9254; fax: +48 85 746 9248. E-mail addresses: [email protected] (Z. Oksiuta), [email protected] (M. Lewandowska), [email protected] (K.J. Kurzydłowski). 0167-6636/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mechmat.2013.07.006

resistance (Abd El-Azim 1997; Klueh and Alexander 1996). The ODS reduced activation ferritic (RAF) steels and alloys are composed of so called low activated elements, such as: Fe, Cr, W, V, Si and Ti. Such alloying elements as Al, Ni, Co, Mn, Mo and others, commonly used in manufacturing of the iron-based superalloys are not allowed to be used in the future fusion reactor application due to their long-term radioactive decay. The ODS RAF steels are next generation oxide-strengthened superalloys to be used as a structural material in future fusion reactor with a temperature operating window up to 800 °C. The very small oxide nanoparticles in these alloys not only ensure higher strength and thermal stability (creep) of ODS RAF steels but also act as the sinks of the He bubbles formed during neutron irradiation, providing a

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good resistance to irradiation-induced swelling (Hayashi et al., Acta Materialia 2008). However, these alloys are still in the development and understanding of their mechanical properties is of key importance. In particular, a comprehensive knowledge is needed with regard to dislocation climbing, dislocation–interstitial and dislocation–dislocation interactions under the different stresses and temperatures. The deformation mechanism can be predicted using Arrhenius equation which relates the plastic strain rate and the Gibbs activation free energy (DG) needed to overcome a localized barrier to dislocation motion (Spätig et al., 1999, 1993). The activation energy is closely related to the activation volume (Va) defined as the volume which a dislocation passes during thermal activation process. Since yttria nanoparticles in the ODS RAF steels play an important role by pinning dislocations, thereby have a strong impact on the strength and high temperature thermal stability of the steels. Their role have been therefore studied here by annealing at the temperatures between 850–1350 °C and isothermal ageing at 750 °C for 10 000 h. Thermal activation results, determined by differential strain rate tensile experiments, are also presented and compared with other ODS ferritic alloys.

Table 1 Chemical composition of the ODS RAF steels after HSHE. Element Cr wt.%

W

Ti

Mn C

Al

P

Y

O

H Fe (ppm)

13.8 1.92 0.33 0.38 0.056 0.02 0.006 0.26 0.17 27.4

balance

3. Results and discussion 3.1. High temperature thermal stability of the ODS steels The effect of an annealing temperature on the grain size and hardness of the ODS RAF steel are shown in Fig. 1. These results indicate that tested material is stable up to the about 1250 °C and follows identical trend as an ODS steel with the same chemical composition manufactured by hot extrusion and hot rolling process, reported by Olier et al. (2009). At 1350 °C the hardness decreases from 460 HV0.1 up to 330 HV0.1. This suggests recovery and/or partial recrystalization at this temperature, as confirmed by microstructure observations presented in Fig. 2.

(a) 480 Microhardness, HV 0.1

Samples of an ODS RAF steel were produced from a prealloyed argon atomized Fe–14Cr–2W–0.3Ti (in wt.%) powder by mechanically alloyed with 0.3%Y2O3 nanoparticles under high purity hydrogen atmosphere and consolidated by hot isostatic pressing (HIP) at 1150 °C under a pressure of 200 MPa for 3 h. After HIPping 1.0 kg billet was annealed at 900 °C for 1 h and subjected to the high speed hydrostatic extrusion (HSHE) under the pressure of 990 MPa with a reduction ratio of 4.1. It should be emphasise that application of HSHE after HIPping is a novel approach to manufacturing the ODS RAF steel. A relatively low extrusion ratio was used to avoid cracking of hard ODS RAF material. More information about the manufacturing process is described by Oksiuta et al. (2011). Samples processed by HSHE have been subjected to heat treatment (HT) at temperatures ranging from 850 up to 1350 °C, for 1 h, in argon and to a long-term isothermal ageing at 750 °C up to 10 000 h, in air. The microstructure of the specimens was observed using JEOL 2010 transmission electron microscope (TEM) and Hitachi HD2700 scanning transmission electron microscope (STEM). The tensile tests were carried out at the temperature ranging between RT and 750 °C with a strain rate of 1.0  10 4 s 1 in an argon atmosphere. Flat tensile specimens, with the dimensions 0.5  1.5  25 mm3 and gauge length of 8 mm were machined parallel to the extrusion axis. Microhardness measurements at the polished surface of the specimens were carried out by a Vickers diamond pyramid applying a load of 0.98 N for 10 s. The chemical composition of the ODS RAF steel measured by gas spectroscopy analysis using LECO TC-436 and LECO IR-412 analyzers is given in Table 1.

460 440 420 400

HIP+HSHE (EPFL-UW) HE+Hot Rolling (CEA)

380 360 340 320 0

200

400

600

800

1000

1200

1400

o

Temperature, C

(b)

1.6

Average grain size

1.4 1.2

Grain size, µm

2. Experimental

1.0 0.8 0.6 0.4 0.2 0.0 As-HSHE

850°C

1050°C

1150°C

1250°C 1350°C

Annealing temperature, o C Fig. 1. Effect of annealing for 1 h in argon on: (a) microhardness and (b) average grain size of the ODS RAF steel as a function of annealing temperature up to 1350 °C.

Z. Oksiuta et al. / Mechanics of Materials 67 (2013) 15–24

17

Fig. 2. TEM images of the ODS steel: (a) and (b) as-HSHE, and (c) HSHE and HT at 1350 °C, for 1 h, in argon.

The microstructure of the ODS RAF steel after HSHE mostly consists of an equiaxed grains with a mean size of about 0.65 ± 0.25 lm, as measured from TEM images using the linear intercept method. However, some areas with larger and randomly oriented elongated grains were also detected (Fig. 2b), presence of which may indicate nonuniform plastic deformation during HSHE and/or non-uniform distribution of the nanoparticles. However, these larger grains were also stable up to 1250 °C, as inferred from the average grain size being stable up to this temperature. At higher annealing temperatures, locally recrystalization process takes place and the larger grains coarsen. Note, that the large grain shown in Fig. 2c is dislocation free and is surrounded by the small grains with a large number of dislocations. All together, the results indicate that the pinning effect caused by nanoparticles in the ODS RAF steel is effective up to 1350 °C. With regard to the stability of the oxide nanoparticles, TEM studies revealed that annealing up to 1250 °C for 1 h has no significant influence on their size (Fig. 3). However, at the highest annealing temperature the size increases from 2.8 ± 0.8 up to 7.8 ± 1.6 nm (Fig. 3(c)). These results are in a good agreement with the data in Schneibel et al. (2011) who reported recently no changes in nanoparticle population in the 14YWT ferritic steel after annealing at 1200 °C for 24 h. Mechanical properties of the ODS RAF steel were tested by tensile tests before and after annealing at 1050 °C for

1 h in argon. The measured yield stress (YS0.2) and total elongation (e) as a function of testing temperature are summarized in Fig. 4. It can be noted that the annealing at 1050 °C slightly decreases the tensile strength. On the other hand, increase in the ductility of the as-HSHE ODS RAF steel, in a full range of the testing temperatures was observed. The tensile tests also revealed a drop of the yield strength at the temperature above 450 °C and a sharp peak of ductility at about 600 °C. These effects were reported by Steckmeyer et al. (2010) and are probably related to the changes in the temperature dependent strengthening mechanisms. The effect of temperature and grain size on the yield stress of the PM2000 and 14YWT ferritic steels was investigated by Schneibel et al. (2011) using Hazzledine model. It was found out that at RT, in the case of fine-grained 14YWT alloys (size of 0.2–0.5 lm), the Hall–Petch strengthening (DrH–P) prevails over the Orowan strengthening (DrOr) caused by the nanoparticles. At higher testing temperature (500 °C), however, the yield strength decreases due to the thermally activated annihilation of dislocations at grain boundaries, as described by Blum and Zeng (2009). Since similar effect are observed for the ODS RAF steel studied here (Table 2), it can be concluded that the same mechanism governs the tensile properties of ODS materials. It is worth noting in this context, that the Hazzledine model better describes the yield strength of the ODS RAF steel than of the other alloys and that the Oro-

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(a)

(b)

10

(c)

Nano-particles size

Nano-particles size, nm

9 8 7 6 5 4 3 2 1 0

200

400

600

800

1000 1200

1400

Temperature, °C Fig. 3. TEM images of the nanoparticles after: (a) HSHE, (b) HSHE and HT at 1350 °C and (c) the variation of the nanoparticles size vs. annealing temperature.

wan stress calculated for ODS RAF steel at room temperature is significantly lower in comparison with 14YWT and PM2000. Also, Kim et al. (2003) reported that the Fe–12%Cr YWT ODS alloy has Orowan stress between 189 and 286 MPa, the values are similar to obtained for the ODS RAF steel. By comparing tensile properties of the ODS RAF steel reported here with the other ferritic alloys available in the literature (Kim et al., 2012; Hoelzer et al., 2007; Miller et al., 2006) it can be concluded that the ODS RAF steel after HIPping, HSHE and HT at 1050 °C exhibits promising compromise between the strength and ductility. 3.2. Long term ageing of the ODS RAF steel As the temperature operating window for ODS RAF steels is to be about 650–850 °C thus, the thermal stability of the ODS alloy after ageing at 750 °C for 10 000 h was also examined, with the resulting microstructural changes shown in Fig. 5 and changes in the hardness, from 460 up to 500 HV0.1, presented in Fig. 6(a). It can be noted that the mean grain size of the ODS RAF alloy after 10 000 h of ageing remains almost unchanged. In contrast, the dislocation density and number density of larger oxides, with an average size of 25 ± 10 nm, located at grain boundaries

(see Fig. 6(c)), increased. Also, ageing process induces changes in the nanoparticle size and number density, with their average diameter increasing from 2.9 to 8.0 nm and number density decreasing from 5 to about 2.5  1023 m 3. These microstructural variation after ageing revealed a systematic loss of tensile properties, both strength and ductility, as it is presented in Figs. 7 and 8. Fracture surface observations of the specimens after ageing, shown in Fig. 9, revealed that a long time ageing results in transition from fully ductile transgranular fracture mode to a quasi-ductile with cleavage facets. It has been found that the number and length of secondary cracks as well as cleavage facets increase with aging time while number of dimples decreases. Also the numbers of microvoids (probably pores) increase with increasing time of ageing. This confirms the role of coarser precipitates and micro-voids on the mechanical properties and brittle fracture of the ODS RAF steel. It is well known that high chromium steels after annealing at the temperature range of 400–550 °C undergo embrittlement accompanied by increase in hardness and yield strength (Lee et al. JNM 2007). On the other hand, the results presented here reveal different behaviour of ODS RAF steel with hardness increase accompanied by the decrease in tensile strength. This inconsistence can

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(a) 1400

HSHE no HT o

HSHE HT 1050 C 1200

YS0.2, MPa

1000

800

600

400

200 0

100

200

300

400

500

600

700

800

o

Temperature, C

(b)

28

HSHE no HT o HSHE HT 1050 C

26

Total elong., %

24 22 20 18 16 14 12 10 0

100

200

300

400

500

600

700

800

o

Temperature, C Fig. 4. High temperature tensile properties of the ODS steel after HSHE and HSHE+HT at 1050 °C for 1 h, in argon: (a) yield strength and (b) total elongation.

Table 2 Calculated and measured values of room temperature yield stress for the 14YWT and RAF ODS steels. Typical value of matrix yield stress is 400 MPa.

a

Material

Measured YS0.2 (MPa)

Calculated YS0.2 (MPa)

Grain size (lm)

Nanoparticle size (nm)

Vol. fraction of particles (%)

DrH–P (MPa)

DrOr (MPa)

14YWTa ODS RAF PM2000a

1469 1280 830

2063 1340 1435

0.5 0.65 1.1

1.8 2.9 16.2

0.03 0.43 0.80

849 740 572

815 200 463

After Schneibel et al. (2011).

be, however, explained in terms of the strengthening effect of oxide nanoparticles. TEM observations revealed that long term heat treatment causes coarsening of nanoparticles combined with reduction in their number density by 50%. Thus, the reduction of strength reflects diminishing contribution of dispersion strengthening of the steel in question. From the image presented in Fig. 6(b) a bimodal character of nanoparticle distribution can be inferred. This suggests that some of the particles resist coarsening and others do not. Although it is difficult to identify by TEM the structure and chemistry of the very small nanoparticles

(size <5 nm), observations of larger, size <10 nm, oxides revealed mostly Ti–Y–Al–O composition. From the literature it is known that the very small yttria nanoparticles are composed of Y–Ti–O. Thus, it can be concluded that the coarsening is caused by Al atoms diffusing faster in the steel matrix then other alloying elements. On the other hand it should be noted that aluminium atoms are most likely doped to the steel during the MA process (as impurity from milling jar and balls). The presence of relatively large Al-enriched oxide particles explains also the hardening effect of the ODS RAF steel as caused by particle induced residual-stresses in the iron

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Fig. 5. TEM microstructure of the ODS RAF steel after ageing at 750 °C: (a) for 1000 h, (b) for 5000 h and (c) for 10 000 h.

matrix. Also, the larger precipitates are located at grain boundaries enhancing the grain boundaries decohesion and reducing the tensile strength of the steel. Another factor may influence on the tensile properties of ODS RAF steel after ageing are pores, presences of which can be deduced from the cavities seen on fracture surface of the specimens subjected to ageing (as shown in Fig. 9). It should be noted in this context that residual porosity was observed after HIPping (Oksiuta and Nadine, 2008). It is highly probable that these pores were not entirely healed during HSHE and some nano- and or micro-cracks exist in the tested steel. With prolonging ageing time these flaws may enlarge easing crack initiation and weakening the strength of alloy. There is also a question of the measuring method used which determinate the strengthening mechanism in steel. It is well known that in the material tested here existing oxide particles facilitate the fracture tearing of the material during the tensile test and the tendency of tearing fracture enhances with the increase of volume fraction of larger oxide particles. In a hardness test, however, deformation is localized in regions around the indentation, within which the matrix of the steel experience compressive stresses. As a consequence, particle induced strengthening by indentation is largely supported. Thus the microhardness test tends to show a strengthening effect whereas the tensile test shows decrease of the strength.

3.3. Activation volume and activation energy analyses The activation volume (Va) and activation energy (DG) for particle coarsening and deformation process of the ODS RAF steel were determined from the differential strain rate tensile experiment (strain rate between 10 2– 10 4 s 1). The results of these tests, shown in Fig. 10, were compared with the data for PM2000 and 14YWT alloys reported by Kim et al. (2012). The data collated in Fig. 10 reveals similar values of the activation volume and similar temperature trend for ODS RAF and PM2000 steels. With the increasing the testing temperature, the activation volume increases. A rapid increase in Va at about 650 °C indicates a sudden change in deformation mechanism, which might be related to the transition from dislocation glide (at lower temperature) to dislocation climbing above 650 °C. The increase in Va rate is significantly higher for ODS RAF and PM2000 in comparison with 14YWT alloy. However, Kim et al. (2012) reported the same effect for the 14YWT at temperature higher than 800 °C (not presented here). One should also note, that Va values are much higher than that for 14YWT (Kim et al., 2012). It means that very small nanoparticles effectively prevent grain boundary sliding and impede dynamic recovery of the alloy. The data in Fig. 10 also reveals significant differences in the values of activation energy for three alloys discussed

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(a)

550

Hardness

Microhardness, HV 0.1

525

500

475

450

425

( 400 0

2000

4000

6000

8000

10000

Aging time, h

(b)

(c)

Fig. 6. (a) Microhardness of the ODS steel as a function of ageing time. (b) Nanoparticles and (c) larger precipitations observed after 10 000 h of ageing at 750 °C.

1400

1350

UTS, MPa

1280

1230 1170

1200

1208 1146

1160

YS0.2, MPa 1150

1115

1035

UTS, YS0.2, MPa

997

1000

948

857 805

837

852

812

790

810

800

766

600 400

360 350

340

310

312

297

310

296

300 289

200 0 20°C 450°C 750°C 20°C 450°C 750°C 20°C 450°C 750°C 20°C 450°C 750°C 20°C 450°C 750°C As-extruded

HT 1000 hrs

HT 2500 hrs

HT 5000 hrs

HT 10000 hrs

Ageing time, h Fig. 7. Ultimate tensile strength (UTS) and yield strength (YS0.2) of the ODS steel after ageing at 750 °C up to 10 000 h.

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25

21 19.4

20

Total elong

Uniform elong

15

HT 1000 hrs

HT 2500 hrs

4.3

HT 5000 hrs

6.7

4.7 2.6

750°C

20°C

450°C

2.5

1.9

750°C

450°C

2.7

2.2

2.2

750°C

450°C

20°C

750°C

450°C

20°C

6.6 6.1

450°C

3.4

2

8.5 4.9

4.6

20°C

5

As-extruded

9.1

5.8

5.9

5.2

0

9.8

8.9

10

12.1

11.4

20°C

11.4 11.2 10

750°C

Elongation, %

16.8

HT 10000 hrs

Aging time, h Fig. 8. Total elongation and uniform elongation of the ODS steel after ageing up to 10 000 h.

Fig. 9. Fracture surface of the ODS steels after ageing at 750 °C for: (a) 1 h, (b) 5000 h and (c) 10 000 h, respectively.

here. The activation energy in the ODS RAF steel gradually increases up to 3.3 eV at 600 °C and above this temperature it remains constant. In the case of the 14YWT steel, the activation energy rapidly increases up to 700 °C and suddenly decreases above this temperature. This is oppo-

site to the trend observed in the case of PM2000 in which up to 650 °C the energy decreases and at higher testing temperatures it rapidly increases. Thus, in each material different dislocation mechanisms are rate controlling at lower temperatures, however, in the temperature range

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(a) 360

(b)

ODS RAF PM2000* 14YWT*

320

ODS RAF PM2000* 14YWT*

4.0

Activation energy. eV

280

3

240

Va. b

4.5

200 160 120

3.5 3.0 2.5 2.0 1.5

80

1.0 40 300

400

500

600

700

800

300

400

0

500

600

700

800

0

Temperature. C

Temperature. C

Fig. 10. Calculated temperature dependence of: (a) activation volume and (b) activation energy of the ODS alloys.⁄PM2000 and 14YWT data after reference (Kim et al. 2012).

of 750–800 °C these mechanisms in all materials converge to nearly the same activation energy. This activation energy is typical of deformation controlled by dislocation climbing over unshearable nanoparticles. It is worth noting that at 600 °C the ODS RAF steel has the highest activation energy in comparison with the other two alloys. This may be explained by the lowest volume fraction of nanoclusters (Vf = 0.031%) in this alloy and as it was reported by Byun et al. (2010) the 14YWT steel unexpectedly, at elevated temperature indicates a weak grain boundary cohesion.

large Al-enriched oxides harden the ODS RAF steel, at the same time reducing the ductility, and enhance the grain boundaries decohesion. 4. The activation volume of the ODS RAF continuously increases up to 600 °C, above this temperature a rapid increase in the activation volume is observed indicating a change in deformation mechanism. 5. The activation energy in the ODS RAF steel gradually increases up to DG  3.3 eV at 600 °C and remains constant above that temperature. This suggests that the dislocation climb is the rate controlling mechanism above 600 °C.

4. Conclusions The results obtained in the study of a long term and high temperature annealing of the ODS RAF steels allow to put forward the following conclusions. 1. Annealing at 1050 °C slightly decreases the strength of ODS RAF steel significantly improving its ductility. The temperature induced changes are in general rather limited up to 1250 °C, while above this temperature, e.g., at 1350 °C, these changes become well visible, with selective grain growth and coarsening of nanoparticles, which are accompanied by a 30% decrease in hardness. 2. Ageing of the ODS RAF steel at 750 °C for 10 000 h causes nanoparticles coarsening compensated by precipitation of new phases, with the overall increase in the hardness and decrease in tensile strength and ductility. Fracture surface of samples exposed to extend ageing revealed presence of cavities which are formed in the sites of pores formed during HIPping. As one could expect high speed hydrostatic extrusion does not heal such pores, changing only their morphology. 3. Decrease in tensile strength and ductility of the ODS RAF steel together with increase in the microhardness after ageing can be explained in terms of nanoparticle coarsening caused by fast diffusing Al atoms. Relatively

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