Microstructure characteristics of the 〈111〉 oriented grains in a Fe-30Ni-Nb model austenitic steel deformed in hot uniaxial compression

Highlights

• The 〈111〉 oriented grains split during straining into deformation bands.

• The grains contain both crystallographic and non-crystallographic microbands (MBs).

• Both MB types display systematically oscillating misorientations.

• The crystallographic MBs are aligned with the highest-stressed {111} slip planes.

Abstract

The work presents a detailed investigation of the microstructure characteristics of the 〈111〉 oriented grains in a Fe-30Ni-Nb austenitic model steel subjected to hot uniaxial compression at 925 °C at a strain rate of 1 s^− 1. The above grains exhibited a tendency to split into deformation bands having alternating orientations and largely separated by transition regions comprising arrays of closely spaced, extended sub-boundaries collectively accommodating large misorientations across very small distances. On a fine scale, the 〈111〉 oriented grains typically contained a mix of “microbands” (MBs) closely aligned with {111} slip planes and those significantly deviated from these planes. The above deformation substructure thus markedly differed from the microstructure type, comprising strictly non-{111} aligned MBs, expected within such grains on the basis of the uniaxial compression experiments performed using aluminium. Both the crystallographic MBs and their non-crystallographic counterparts typically displayed similar misorientations and formed self-screening arrays characterized by systematically alternating misorientations. The crystallographic MBs were exclusively aligned with {111} slip planes containing slip systems whose sum of Schmid factors was the largest among the four available slip planes. The corresponding boundaries appeared to mainly display either a large twist or a large tilt component.

Introduction

It is well known that the phase transformation taking place in industrial steels on cooling from hot working temperatures precludes a direct observation of their deformation microstructure characteristics in the austenite state. In order to obtain such characteristics as a function of processing conditions, which is important for advancing the physically-based modelling of hot working of steels, Ni-30Fe and Fe-30Ni-based model alloys retaining the austenite hot deformation microstructure at ambient temperature have been employed [1], [2], [3], [4], [5], [6], [7], [8], [9]. The use of a Fe-30Ni-Nb model steel led to the direct confirmation of previous suggestions [10], [11], [12] that the dislocation structures introduced during hot deformation provide the preferential nucleation sites for precipitation during the multi-pass hot working of microalloyed austenite [6], [13], [14]. It should be noted that the above model steel not only retains the austenite hot deformation microstructure at ambient temperature but also displays similar flow curve characteristics as industrial microalloyed steels while maintaining acceptable solubility of NbC [6], [11], [12], [13]. Some novel findings have recently been made by the present authors using this model steel that extend the current knowledge of the interaction between austenite hot deformation microstructure and precipitate particles during multi-pass hot deformation [8]. Thus, the detailed understanding of the microstructure evolution during hot deformation of austenite is crucially important not only with respect to the softening processes and flow behavior, but also in relation to the strain-induced precipitation.

It has been surmised from the microstructural studies performed using Ni-30Fe and Fe-30Ni-based model alloys [1], [2], [3], [4], [5], [6], [7], [8], [9] that, at deformation temperatures relevant to the industrial steel finish rolling range [15], [16], the austenite grains become subdivided on a coarse scale into deformation bands [17], [18], [19], [20] and on a fine scale into “microbands” (MBs) for a majority of grain orientations. MBs are elongated microstructure features bounded by pairs of parallel low-angle planar dislocation walls, often termed “geometrically necessary boundaries” (GNBs), typically characterized by approximately opposite misorientation vectors [1], [2], [3], [4], [5], [6], [7], [8], [9], [21], [22], [23], [24], [25], [26]. The GNB/MB evolution and orientation dependence have been extensively investigated for straining at ambient temperature, especially for aluminium alloys e.g. [21], [22], [23], [24], [25], [26], [27], and the MB formation mechanisms involving dislocation multiple cross-slip [28], [29] or dislocation wall splitting [30], [31], [32] have been suggested. The Risø research group have proposed that the alignment of GNB boundaries strongly depends on the crystallographic orientation of a grain for deformation in tension, compression and rolling [21], [26], [33], [34], [35], [36], [37]. Three major types of dislocation boundary structures have generally been observed [23], [26], [33]: Type 1, with extended planar boundaries aligned with the highly-stressed slip planes; Type 2, with an equiaxed dislocation cell structure without extended planar boundaries; and Type 3, with extended planar boundaries aligned with other crystallographic planes than slip planes, though still with a fixed relationship to the most stressed slip systems. Similar grain orientation dependence of the deformation substructure has recently been confirmed for Ni-30Fe and Fe-30Ni-Nb austenite subjected to hot plane strain compression [7] and uniaxial compression [8], [9], respectively. The latter studies were largely focused on the grain orientations concentrated around the stable 〈110〉 fibre, with only a small number of orientations scattered around the standard stereographic triangle being studied. The findings reported in Ref. [9] have surprisingly indicated that, in contrast to the aluminium alloys [26], 〈111〉 fibre grains in austenite hot deformed in uniaxial compression might contain both crystallographic and non-crystallographic MBs. Nevertheless, in the above work, only three grains having their tensile directions aligned close to the 〈111〉 direction were investigated. A more statistically significant, detailed study is thus required to further elucidate the frequency of the formation of MBs of each type within the above grains and to compare the characteristics of the both types.

The aim of the present work was to carry out a detailed investigation of the microstructure characteristics of 〈111〉 oriented grains in a Fe-30Ni-Nb model steel subjected to hot uniaxial compression using a strain of 0.2. The above strain level was selected as it provided a well-developed substructure, a wide range of available grain orientations and a potentially relatively short rotation path from the original grain orientation. The main emphasis of the investigation was placed on the MB characteristics.