Deformation mechanisms of low-temperature superplasticity in 8090 Al-Li alloys

Conference · Sun Dec 31 00:00:00 EST 1995

OSTI ID:194183

Pu, H P; Huang, J C ^[1]

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1. National Sun Yat-Sen Univ., Kaohsiung (Taiwan, Province of China). Inst. of Materials Science and Engineering

The 8090 Al-Li alloys, after special thermomechanical processes, exhibited low-temperature superplasticity (LTSP) from 350 to 450 C, behaving differently from the conventional high-temperature superplasticity (HTSP). The LTSP sheets after {approximately} 700% elongation at 350 C and 8 {times} 10{sup {minus}4} s{sup {minus}1} still possessed fine (sub)grains 3.7 {micro}m in size and narrow surface Li-depletion zones 11 {micro}m in width. It results in a post-SP T6 strength of {approximately} 500 MPa, which is higher than that of the conventional superplastic 8090 alloys tested at 525 C or above. The tensile behavior and deformation mechanisms of the LTSP and HTSP sheets were investigated over the strain-rate range 10{sup {minus}5}--10{sup {minus}2} s{sup {minus}1} and strain range 0.5--1.0. At {var_epsilon} 0.5, the strain rate sensitivity (m-value) for the LTSP and HTSP materials was found to be {approximately} 0.33 and 0.50, respectively. The activation energy was extracted to be 92 kJ/mole for the LTSP sheets and to be 141 kJ/mole for the HTSP sheets. As straining to {var_epsilon} = 1.0 the m-value of the LTSP materials increased to 0.37 and the activation energy decreased slightly to 82 kJ/mole. By SEM examinations, the movement of surface grains in LTSP samples confirmed the role of grain boundary sliding (GBS). TEM observations revealed that the deformation mechanism consists of a large amount of dislocation motion resulting in the subgrain formation and rotation; and TEM observations from the transverse section might explain the anisotropic deformation behavior during the initial superplastic strain. The primary and rate-controlling deformation mechanisms for the HTSP and LTSP sheets are considered to be GBS and dislocation creep, respectively.

• OSTI ID: 194183
• Report Number(s): CONF-950201-; ISBN 0-87339-300-7; TRN: IM9611%%6
• Resource Relation: Conference: Annual meeting and exhibition of the Minerals, Metals and Materials Society (TMS), Las Vegas, NV (United States), 12-16 Feb 1995; Other Information: PBD: 1995; Related Information: Is Part Of Superplasticity and superplastic forming, 1995; Ghosh, A.K. [ed.] [Univ. of Michigan, Ann Arbor, MI (United States)]; Bieler, T.R. [ed.] [Michigan State Univ., East Lansing, MI (United States)]; PB: 290 p.
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ALUMINIUM BASE ALLOYS
THERMOMECHANICAL TREATMENTS
MICROSTRUCTURE
TENSILE PROPERTIES
LITHIUM
COPPER
MAGNESIUM ADDITIONS
ZIRCONIUM ADDITIONS
RECRYSTALLIZATION
PLASTICITY
ELONGATION
GRAIN SIZE
STRAINS
STRESSES
FLOW STRESS
STRAIN RATE
ACTIVATION ENERGY
SCANNING ELECTRON MICROSCOPY
TRANSMISSION ELECTRON MICROSCOPY
GRAIN BOUNDARIES
DISLOCATIONS