Effect of anisotropic microstructure of a 12Cr-ODS steel on the fracture behaviour in the small punch test

Abstract

Oxide dispersed strengthened steels can exhibit a strongly anisotropic microstructure with elongated pancake-like grains in the rolling plane. This gives rise to intergranular fracture and subsequent delamination along large-area grain boundaries. We investigated an oxide dispersed strengthened alloy with 12 mass percent Cr, manufactured by mechanical alloying, hot extrusion and cold rolling by means small punch tests and subsequent fractographic analysis. The fracture behaviour was analysed in dependence of the specimen orientation. The results from small punch tests were contrasted with those from impact tests with sub-sized samples. In both tests, the ductile to brittle transition temperatures as well as the upper shelf energies depend significantly on the orientation of the specimens. However, the delamination affects the fracture of impact and small punch test samples in different ways. Thus, it depends on the load situation whether delamination has a beneficial or a detrimental effect on the fracture behaviour.

Introduction

Oxide dispersion strengthened (ODS) steels are promising candidate materials for fuel claddings of Gen-IV sodium cooled fast reactors as well as for the first wall and blanket structures of fusion reactors [1], [2], [3]. Operation temperatures up to 650 °C are intended. The main focus of mechanical characterisation is put on creep and swelling properties. Nevertheless sufficient fracture mechanical properties and toughness are necessary for safety relevant structural applications in the whole range from room to operation temperature. Lindau et al. [4] investigated mechanical properties of ODS-EUROFER. It was found that yield stress and ultimate tensile stress (UTS) are significantly higher in comparison to non-ODS EUROFER for temperatures up to 750 °C. The creep resistance at 750 °C is also significantly improved. However, the DBTT was found to be significantly higher than that of non-ODS-EUROFER. Chaouadi et al. [5] found a significant crack resistance degradation of Eurofer ODS when the test temperature increases. In particular, at 550 °C and 650 °C, the crack resistance is very low. A decrease of both initiation toughness and tearing resistance was observed. While the strength is relatively high in comparison to non-ODS EUROFER, the crack resistance is significantly lower. It was supposed that the presence of Y₂O₃ particles promotes void nucleation, growth and coalescence and therefore facilitates crack propagation. Nevertheless, Byun et al. [6] have demonstrated that high temperature fracture toughness could be significantly improved by appropriate thermo-mechanical treatments. Under cyclic loading the oxide particles were shown to improve the mechanical stability. Kuběna et al. [7] found that ODS steels are significantly less susceptible to cyclic softening than non-ODS steels with the same chemical composition. They demonstrated that nano-scaled and homogeneously distributed oxide particles prevent strain localisation in persistent slip bands and thus stabilising the microstructure. Up to now, joining is a challenge in the application of ODS steels. Conventional welding cannot be applied because of the demixing and agglomeration of the oxide particles. Friction stir welding is a possible joining method. However, it was suspected that significant residual stresses that are introduced by this process, will cause a decrease of the fatigue strength [8]. The fracture behaviour of ODS materials is closely connected to their microstructural peculiarities. Chao et al. [9] found a grain size anisotropy for a 20Cr-ODS alloy (commercially available as PM 2000) manufactured as a tube with 8 mm wall thickness by hot rolling. The grains were found to be elongated along the rolling direction. KLST impact tests and subsequent EBSD analyses of the crack region revealed intergranular cracks along the elongated grain boundaries which constitute weak interfaces. The term “delamination” originating from laminated composite materials was adopted for this type of intergranular cracking. The delamination phenomenon had earlier been reported for ultrafine grain structure steels by Kimura et al. [10]. Depending on the orientation of the weak planes in the impact specimen, they discriminated a “crack arrester” and a “crack divider” situation (Fig. 1).

A microstructural characterisation for a 14Cr-ODS steel manufactured by hot extrusion of a round bar was reported in [11], [12]. The grains were found to be elongated along the extrusion direction. Tensile tests up to 700 °C revealed a reduced ductility in the direction perpendicular to the direction of extrusion and grain elongation. Again, the delamination effect was observed in KLST impact tests [12].

The small punch (SP) test has widely been used to determine material properties. In particular the ductile to brittle transition temperature (DBTT), the yield stress (YS), the ultimate tensile stress (UTS) and creep strength can be extracted for homogeneous and isotropic metals [13], [14], [15], [16], [17], [18], [19], [20]. The SP test is not intended to replace conventional tests such as tensile tests, Charpy impact tests or fracture mechanical testing, but to be used as screening procedure [21]. The SP test is especially useful in one or more of the following cases: (i) the available amount of material is limited, (ii) the material is highly activated by neutron irradiation, (iii) material properties are non-homogeneous and exhibit significant gradients. Therefore it is generally useful to include this technique in the characterisation of ODS alloys. So far, the effect of the above mentioned anisotropic microstructure on SP test results has only rarely been investigated [22]. A systematic comparison with impact tests is missing.

In the FP7 project GETMAT new Fe–Cr ODS alloys were developed. One of the materials under investigation is a 12Cr ODS alloy provided by KOBELCO (Japan). The material exhibits a anisotropic microstructure with elongated pancake-like grains [23]. We perform mechanical testing for this material in order to reveal the fracture mechanisms governed by the anisotropic structure. We correlate the microstructure with energies and transition temperatures from KLST impact tests and SP tests of different orientations. It will be demonstrated that anisotropy affects the transition temperatures of Charpy tests and of SP test in different ways.