Fundamental aspects of superplastic deformation☆
Introduction
Initially superplastic (SP) deformation was considered as unique phenomenon inherent only to several alloys. However, systematic studies displayed its more general character as compared to conventional deformation. In reality, this effect can be observed not only in metals but in intermetallides and ceramics as well [1], [2] which, as known, are characterized by brittle failure under common conditions and display no features of plastic flow.
The physical nature of this phenomenon is very complex. Just recently [3], [4], [5] it seemed that SP deformation could be explained by the operation of usual mechanisms of deformation, namely, grain boundary sliding (GBS), intragranular dislocation slip and diffusion creep. However, the latest data have shown that this phenomenon is conditioned by the operation of a specific mechanism of deformation—cooperative grain boundary sliding (CGBS) [6]. The operation of this deformation mechanism does not depend on the crystal lattice type and dislocations present. It depends on the long-range area and structure of grain boundaries in a polycrystal.
The aim of the present work is to analyse experimentally the mechanism of SP deformation on microscopic, mesoscopic and macroscopic scales and to present a physical model of this phenomenon.
Section snippets
Materials and experimental procedure
The task set was solved using various model alloys and special objects of investigation to reveal features of various deformation mechanisms and their operation. The results of the investigations of the classic Zn-22%Al alloy, Zn bicrystals and Al tricrystals as well as commercial alloys are given in the present work.
The main methods of investigations were: optical and electron microscopy, observation of deformation relief, local analysis — photographs of one and the same area taken before and
Micromechanisms of SP deformation
GBS brings the main contribution to SP deformation. Therefore, it is very important to determine a micromechanism of this type of deformation. The investigation was performed on special model objects — Zn bicrystals. The bicrystal boundary was at an angle of α=45° to the loading axis. In this case there is no intragranular dislocation slip: the basal planes are situated normally and parallelly to the operating load (‘pure’ GBS). The development of GBS on a bicrystal with the tilt boundary at an
Conclusions
Interaction of lattice dislocations with grain boundaries leads to an acceleration of GBS and development of ‘stimulated’ GBS.
CGBS bands are formed during SP deformation and their interaction with mechanical properties is established.
In Zn-22%Al alloy and Al tricrystals it is shown that local migration of grain boundaries near a triple junction is necessary for the formation of CGBS.
A physical model describing SP deformation as a phenomenon conditioned by the operation of the specific mechanism
References (10)
- et al.
Scripta Metall. Mater.
(1991) - et al.
Scripta Metall. Mater.
(1997) - A.K. Mukherjee, in: H. Mughrabi (Ed.), Plastic Deformation and Fracture of Materials, vol. 6, Materials Science and...
- et al.
Cited by (36)
Effect of texture and mechanical anisotropy on flow behaviour in Ti–6Al–4V alloy under superplastic forming conditions
2021, Materials Science and Engineering: ACitation Excerpt :The deformation mechanism and hence the mechanical properties of Ti-64 are often seen to be influenced by the materials initial crystallographic texture. Previous studies have related the anisotropy in plastic flow at elevated temperature to different factors including (i) texture and preferred orientation of grains [21,22], (ii) active slip systems [23,24], (iii) banded microstructures [25–28] and (iv) grain size/shape/fraction [29]. Other reports suggest that all these factors co-exist such that their contributions cannot be individually distinguished [30,31].
Effect of recrystallization on hot deformation mechanism of TA15 titanium alloy under uniaxial tension and biaxial gas bulging conditions
2017, Materials Science and Engineering: ACitation Excerpt :For both the initial rolled sheet and as-welded tube, the materials had high initial dislocation density, therefore substantial recrystallization happened at the early stage of deformation, which led to a higher fraction of HAGBs and smaller average grain size. It is widely accepted that grain boundary sliding (GBS) with associated accommodation mechanisms is the main deformation mechanism for titanium alloys during superplastic forming [35,36]. Zelin et al. [29] found that HAGBs and fine grain size were beneficial to GBS.
Tensile flow and work hardening behaviors of ultrafine-grained Mg-3Al-Zn alloy at elevated temperatures
2016, Materials Science and Engineering: ASuperplasticity from viscous flow in high Pb ternary alloy
2016, Materials Science and Engineering: ACitation Excerpt :Several shear pathways oriented 30° to 45° from the loading axis are present. Shear interfaces can result from grain boundary migration (GBM), grain rotation, and intra-granular dislocation motion (IDM) [31–34]. An example of accommodation of an unfavorably oriented grain through IDM, which creates shear interfaces, is shown in Fig. 7b–d. Fig. 7c clearly shows the formation of a shear interface in the grain interior.
Effect of the grain/subgrain size on the strain-rate sensitivity and deformability of Ti-50at%Ni alloy
2015, Materials Science and Engineering: ACitation Excerpt :Thus, it can be concluded that the smaller the grain size of Ti–Ni alloy, the lower the temperature and the higher the strain rate leading to superplasticity. Generally, grain boundary sliding contributes to the superplastic behavior of metallic materials [30,44,45]. From our experiments, the activation volume ΔV/b3 is calculated as [46] ΔV/b3=√3kT/mσb3 (where m is the strain-rate sensitivity exponent, k is the Boltzmann constant, T is the testing temperature, σ is the steady-state flow stress and b is the Burgers vector) (in our case b≈2.61 Ǻ of B2-austenite), equals to 3–4 for the creep test at 400 °C after ECAP-400, which corresponds to the diffusion and grain boundary sliding deformation mechanisms.
- ☆
This paper is dedicated to Professor Pavel Lukáč on the occasion of his 65th birthday.