Influence of Initial Microstructure on Microstructural Stability and Mechanical Behavior of Cryorolled A356 Alloy Subjec
- PDF / 4,755,467 Bytes
- 17 Pages / 593.972 x 792 pts Page_size
- 47 Downloads / 233 Views
INTRODUCTION
ULTRAFINE-grained (UFG)/nanostructured (NS) materials have received more attention in recent years because of their superior mechanical properties over their coarse-grained counterparts.[1–3] Severe plastic deformation (SPD) methods are found to be successful in producing UFG/NS materials in bulk form. Equal channel angular pressing (ECAP),[4] high-pressure torsion (HPT),[5] accumulative roll bonding,[6] and multiaxial forging[7] are some of the wellknown SPD techniques that produce bulk UFG materials with no shape change involved during the SPD processes. Cryorolling is a relatively young process first investigated by Wang et al.,[8] in which the material is subjected to rolling at liquid nitrogen temperature. The low processing temperature leads to suppression of dynamic recovery; therefore, the grain refinement resulting from cryorolling is far superior to other SPD techniques.[9,10] Though SPD processes are efficient in refining the microstructure to ultrafine and nano regime, the stability of the resultant microstructure is still questionable.
R.J. IMMANUEL and S.K. PANIGRAHI are with the Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036, India. Contact e-mail: [email protected] Manuscript submitted November 13, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The intrinsic grain boundary energy stored in the material (E) is related to the grain size (D) per the following expression[2]: E¼
3c ; D
½1
where c is the specific grain boundary energy. It is obvious from Eq. [1] that the stability of the UFG grains (of about 100 nm) is about 1000 times lower than that of the conventional coarse-grained material with average grain size of 100 lm. Further, apart from the fine grain structures with high-angle grain boundaries (HAGBs), the SPD-processed materials possess fine dislocation networks with a large amount of stored dislocations[11] that contribute significantly to their extraordinary properties. These dislocation networks are even more unstable than the grain structures mentioned previously.[12] Therefore, a thorough understanding of the thermal stability of these UFG materials processed by any SPD technique is necessary before considering them for any engineering application. The thermal stability of aluminum alloys subjected to different SPD processes were studied by various researchers. Horita et al.[13] studied the thermal stability of aluminum alloys with different chemical compositions (alloy series of 1100, 2024, 3004, 5084, 6061, and 7075) subjected to ECAP, and they found that the refined microstructures are stable up to an annealing temperature of 473 K (200 C) with the presence of
submicrometer grains. Zhilyaev and Langdon[5] highlighted that most of the HPT–processed materials retain their UFG microstructure up to the temperature ranging between 423 K (150 C) and 473 K (200 C). In the case of cryorolled materials, exposure to high temperature leads to recrystallization of the deformed microstructure, which enhances the mechanical properties
Data Loading...
