**1. Introduction**

Iron-chromium-aluminum (FeCrAl) alloys was initially developed by General Electric (GE) Corporation in the 1960s. This material exhibits superior high-temperature oxidation and excellent corrosion resistance, and thus were used in automobile exhaust gas purifying systems and nuclear reactors [1,2]. Single-phase FeCrAl alloys with a body centered cubic (BCC) structure plastically deform through dislocation slips. Predominate slip systems include {110}<111> and {112}<111> at room temperature [3,4]. The development of the constitutive laws for FeCrAl polycrystalline aggregates is in urgent demand for designing and predicting mechanical response of FeCrAl-made structural components. Numerous efforts were made to develop constitutive models based on the knowledge of crystal defects, such as dislocations, twins and grain boundaries [5–12]. Yielding strength of a material is determined by the glide of dislocations, in turn, the glide resistance of dislocations associated with different slip systems is the essential parameter. Strain hardening effect is mainly ascribed to dislocation interactions and/or dislocation-grain boundary interactions.

The glide resistance of dislocations, i.e., critical resolved shear stress (CRSS), can be estimated using different methods. First, CRSS can be estimated by analysis of orientation and slip trace on polycrystal sample after macro mechanical tests. In this method, the orientations of grains are analyzed by using transmission electron microscopy (TEM) [13] or electron backscatter diffraction (EBSD) [14]. The slip system of traces and Schmid factors are calculated with help of orientation information. Then the ratio of CRSS can be calculated by counting the frequency of slip traces. Coupled with in situ observation of deformation process, this method can also provide absolute estimation of CRSS [15]. However, the stress state in each grain may be different from macroscopic stress, which will lead to inaccurate measurement of CRSS. Second, CRSS can be calculated by mechanical tests and data fitting of crystal plasticity modeling. To use this method, the mechanical behavior of material is measured by using mechanical tests on polycrystalline specimens with known texture [16,17] or indentation tests on grains with measured orientation (also known as inverse indentation analysis) [18,19]. Then CRSS and other parameters in the model are fitted to the results of experiments. However, previous study shows that this method yields different CRSS for same material due to the difference in constitutive law [16,17]. The third method is based on in situ far-field high-energy X-ray diffraction microscopy (FF-HEDM) [20,21]. During mechanical tests, FF-HEDM can identify the activated slip system in each grain, measure the corresponding shear stress. This method requires expensive high energy synchrotron X-rays and is not suitable for the measurement of small irradiated region. Another method is directly applying load on single crystal with orientation favorable for a specific slip system. The orientation of grains is characterized by using EBSD. The Schmid factors in each grain can be calculated. In situ micromechanical tests can be performed on the pillars. The activated slip system can be identified by slip traces and the CRSS of slip systems can be directly obtained [22–24].

Dislocations interactions have been studied by different methods, such as dislocation dynamics simulations, molecular dynamics simulations, and experimental testing. For example, to estimate the interaction coefficients of slip systems in BCC iron, which are defined by Franciosi [25] and Devincre [26], the researchers built up the interaction matrix of slip systems by analyzing the crystal structure, and preformed dislocation dynamics simulations [27,28]. Meanwhile, molecular dynamics simulations also were used to study the interactions of dislocations [29,30]. The detailed process of dislocations interaction is closely analyzed in atomic scale. In addition to numerical simulations, dislocations interactions were also investigated by using micro and macro mechanical tests. Based on traditional macro mechanical tests, a lot of effort was made to measure the hardening effect of materials [31–34]. Most easy way is to measure the response of polycrystal materials and fit it with empirical equations, e.g., Ludwik equation [35], Hollomon equation [36], Swift equation [37], and Voce equation [38]. To get more detailed understand on the effect of slip on a specific system on other system, latent hardening tests was employed [25,39,40]. Single crystal sample was loaded in a specific direction to activate a specific primary slip system, and then loaded in another direction to activate another secondary slip system. The CRSS change of secondary slip system can be measured. In recent years, the development of micromechanical test enables us to study mechanical properties of small samples; for example, flow behavior and strain hardening rate by micro-pillar compression testing [41,42].

Micropillar compression/tension testing has been widely used to study mechanical properties of materials with sub-micron and micro-sized microstructural features [43–45], particularly for nuclear materials because limited ion penetration depths, a few microns for typical self-ions (2–10 MeV) and tens of microns for typical light ions (1–3 MeV), make micro-scale mechanical tests a necessity. In situ micromechanical testing is essential for correctly extracting experimental data, because shear instability often happens associated with local stress/strain concentration. In this study, we investigate the orientation dependence of mechanical responses of FeCrAl alloy using micromechanical testing of single-crystal and bi-crystal micropillars in a scanning electron microscopy at room temperature. Single-crystal micropillars were fabricated with designed orientation which favors the activity of single slip system or two slip systems or multiple slip systems in order to study the orientation dependence of mechanical responses of FeCrAl alloy. We evaluated the strain hardening rate by extracting the stress-strain data, i.e., Θ = *d*σ/*d*. By tailoring the orientation of single-crystal micro-pillars, we characterized the strain hardening behavior of micropillars associated with the activity of (a) one slip system, (b) two slip systems, and (c) multiple slip systems. Bi-crystal micropillars with respect to the continuity of slip systems across two high-angle GBs were fabricated to study the effect of grain boundary on slip transmission. These results provide insight into understanding mechanical response of FeCrAl alloy.
