Effect of initial microstructures on grain refinement in a stainless steel by large strain deformation
Introduction
Submicrocrystalline metallic materials with grain sizes of tens to hundreds of nanometers offer improved mechanical properties continuously, which attract the attention of materials scientists [1], [2]. Recently, a large strain plastic working at relatively low temperatures was utilised as a novel processing method to develop ultra-fine-grained structural metals and alloys with a grain size on a submicron scale [3], [4], [5], [6], [7], [8], [9], [10]. The evolution of new grain structures occurring under plastic working is frequently discussed as a dynamic recrystallisation (DRX). The main characteristics of DRX during warm to hot deformation have been clarified [11], [12], [13], [14], [15], [16], [17]. Two types of DRX based on the operating structural mechanisms that result in new grain development are commonly discussed in literature. The new grain evolution that takes place in materials with low to medium stacking fault energy (SFE) is associated with the discontinuous DRX. The formation of a new grain structure results from the operation of a grain boundary bulging, namely grain boundary serration and migration consuming the strain hardened substructures [11], [12], [13], [14]. The other type is the continuous DRX, which is discussed as the mechanism that is responsible for the formation of new grains in materials with high SFE. The new grains develop as a result of the gradual increase in the misorientations between the subgrains that are caused by the plastic deformation [11], [12], [15], [16], [17].
A considerable refinement of the microstructure due to DRX can be obtained by decreasing the deformation temperature. On the other hand, the critical strain required for the initiation of DRX and especially for its full completion should be incredibly large at relatively low temperature and makes the study of ultra-fine-grain evolution during cold to warm deformation difficult. Although there is some debate, the structural mechanism responsible for the evolution of new fine grains under cold-to-warm deformation is thought to be roughly similar in most metals and alloys and can be considered as a kind of continuous DRX, i.e. the evolution of geometrically necessary dislocation subboundaries [21], [22] and a gradual rise of misorientations among them resulting in the development of ultra-fine-grains with medium- to high-angle boundaries [8], [9], [18], [19], [20].
Subboundaries that evolved near the original grain boundaries have been reported to have higher misorientations compared to those in the grain interiors and can rapidly transform to high-angle boundaries [18], [23]. This suggests that the grain boundaries have a priority as nucleation sites for the new grains. Thus, alloys with smaller initial grains should exhibit higher kinetics for grain refinement under severe deformation. Therefore, the aim of the present work, is to study the kinetics of grain refinement during large strain deformation, especially the effect of the initial microstructure on the evolution of strain-induced (sub)boundaries with high-angle misorientations.
Section snippets
Experimental procedure
A 304 type austenitic stainless steel (0.058% C, 0.7% Si, 0.95% Mn, 0.029% P, 0.008% S, 8.35% Ni, 18.09% Cr, 0.15% Cu, 0.13% Mo (all in wt%) and the balance Fe) was used as the starting material. Four sets of specimens with different initial microstructures were used (Fig. 1), namely annealed samples with grain sizes of D0=15 and 2.2 μm, and dynamically recrystallised ones with D0=3.5 and 1.5 μm. The DRX behaviour of the same steel has been described elsewhere [24], [25]. In the present study,
Stress–strain behaviour
Fig. 2 shows the four sets of stress–cumulative strain curves that are plotted for several compression passes of the samples with different initial microstructures. The flow curves obtained for the samples with a relatively coarse-grained annealed microstructure of are represented by the solid lines and those for the relatively fine-grained specimens, i.e. are shown by the dashed lines. The common flow behaviour is similar to that controlled by dynamic recovery. The
Discussion
The present results clearly show that the initial grain size has a great effect on the ultra-fine-grain development in the studied material during multiple deformation. In other words, the kinetics of grain refinement by large strain deformation depends significantly on the starting structural conditions. The initial grain boundaries should be taken into account when conducting a detailed analysis on the misorientation evolution of strain-induced (sub)grain boundaries. The former ones could be
Conclusions
Submicrocrystalline grain structure evolution taking place in a 304 stainless steel with various initial microstructures was studied in multiple multiaxial compressions at 873 K (0.5Tm). The main results can be summarised as follows:
- 1.
The deformation behaviour is similar to that controlled by a recovery. Flow stresses rapidly increase at early deformation, approaching a saturation level followed by a steady-state-like flow at cumulative strains of above 1.0. The samples with smaller initial
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- 1
On leave from the Institute for Metals Superplasticity Problems, Ufa, Russia.