Development of Microstructures with Improved Cryogenic Toughness through Local Variations in Stress State: Aluminum-Lith
- PDF / 2,532,039 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 42 Downloads / 186 Views
DEVELOPMENT OF MICROSTRUCTURES WITH IMPROVED CRYOGENIC TOUGHNESS THROUGH LOCAL VARIATIONS IN STRESS STATE: ALUMINUM-LITHIUM ALLOYS K. T. VENKATESWARA RAO AND R. 0. RITCHIE
Center for Advanced Materials, Lawrence Berkeley Laboratory and Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. ABSTRACT
Microstructurally-induced changes in the local stress state (triaxial constraint) and their effect on fracture-toughness behavior are examined at ambient and cryogenic temperatures in an Al-Li-Cu-Zr alloy, processed in the form of 12.7 mm-thick "naturally laminated" plate containing aligned-weak interfaces and 1.6 mm-thin unlaminated sheet. It is shown that marked improvements in long-transverse (L-T) toughness can be achieved in the plate material at cryogenic temperatures by promoting through-thickness delamination along these interfaces, which relaxes local constraint and promotes a fracture-mode transition from global plane strain to local plane stress. Conversely, in thin sheet material, the absence of such interface delamination leads to a reduction in toughness with decrease in temperature, consistent with the greater degree of crack-tip constraint. INTRODUCTION
In addition to intrinsic properties such as crystal structure, grain size, slip character and second-phase particle distribution, the fracture behavior of a material is greatly influenced by the local stress-state under which failure occurs [1-4]. For example, the load necessary to generate a given plastic strain in a structure can be higher in the presence of a notch compared to an unnotched sample, which leads to strain-controlled failures in ductile materials being associated with notch strengthening. Conversely, tensile stress-controlled cracking processes occur more readily at notches, because crack-opening stresses, a are elevated by triaxial constraint [1]. Consequently, transgranular cleavage cracking in mil' steels is greatly enhanced by triaxial stresses; similarly, boron-doped Ni 3 AI intermetallic alloys exhibit a predominantly intergranular fracture in the presence of a notch, compared to transgranular tearing-type failures under uniaxial tensile loading conditions [5). The state of stress within the deformation or fracture process zone in metallic materials can be strongly influenced by thickness of the specimen [1-4]. In thin samples where the extent of plasticity is comparable to specimen thickness, the spread of local yielding restricts constraint at the crack tip (ayy max9aay, the yield strength, and through-thickness stresses, arzzs0), thereby promoting primarily plane-stress conditions. Such effects are largely confined to the surface in very thick samples; maximum ayy crack-tip stresses in the interior may approach over 5ay in the presence of strain hardening [6,7]. Such plane-strain conditions represent a more severe of state of stress in triaxial tension. Measured (crack-initiation) fracture-toughness values in plane strain (maximum constraint) are therefore generally significantly l
Data Loading...
