1997 TMS Annual Meeting: Monday Abstracts

LIGHT WEIGHT ALLOYS FOR AEROSPACE APPLICATION IV: Session I: Al Alloys

Session Chairperson: Eui W. Lee, Code 4342, Naval Air Warfare Center, Patuxent River, MD 20670

8:30 am

ELECTRODE POTENTIAL DEPENDENCE OF ENVIRONMENTAL CRACKING IN HIGH STRENGTH 7XXX ALUMINUM ALLOYS: L.M. Young, R.P. Gangloff, Department of Materials Science and Engineering University of Virginia, Charlottesville, VA 22903

This research aims to determine the effect of electrode potential on the kinetics of environmental crack growth in precipitation hardened 7XXX aluminum alloys. High resolution compliance and electric potential measurements of da/dt provide part of the foundation to understand the mechanisms for the agingmicrostructure dependence of shorttransverse cracking. Measured Stage II growth rates in AA7075T651, immersed in aqueous chloride, are slow and depend on electrode potential and inhibitor addition. For cracking in a molybdatechloride solution, da/dt rises from 107 mm/s at 900 mVSCE (nearopen circuit) to 105 mm/s at 750 mVSCE, but is constant or declines with more noble applied polarization. The crack path is along or near highangle boundaries, but the crystallographic and microstructure details are undefined. These results are consistent with literature findings for AA7075T6 in KI solution. The potential for the maximum in da/dt is more noble for the iodide compared to the molybdate, and is most active for cracking in bare chloride solution. This transition potential is governed by the onset of substantial localized crack tip dissolution which causes an IR difference that shields the crack from changes in anodic polarization. The importance of this potential dependence is illustrated by results for AA 7050T6 in molybdate. Crack growth is faster compared to the AA7075 case if both alloys are at their open circuit potential. Crack growth in AA7050 is slower than that in AA7075 if the comparison is made at a fixed electrode potential.

8:55 am

GRAIN SIZE AND TEMPERATURE DEPENDENCE OF INELASTIC DEFORMATION IN 5083 Al ALLOY: Yong Nam Kwon, Young Won Chang, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790784, Korea

The inelastic deformation behavior of 5083 A1 alloy has been studied to clarify the effects of grain size and temperature by conducting a series of load relaxation tests. The experimental results were then analyzed based on the recently proposed internal variable theory of inelastic deformation. The inelastic deformation of fine grained 5083 Al alloy at high temperatures is confirmed to consist of grain boundary sliding and accommodating grain matrix deformation caused by dislocation glide processes. A HallPetch type relation is found to exist between the grain size and an internal strength variable, instead of the generally used flow stress.

9:20 am

ON THE WARPAGE OF 7050 ALUMINUM PLATES: M.L. Smith, J. Foyos, E.W. Lee, C. Ho, and O.S. Es-Said, Mechanical Engineering Department, Loyola Marymount University, Los Angeles, CA 90045; Naval Air Warfare Center, Code 434200A, NAWCAD PAX, Patauxent River, MD 20670; Mechanical Engineering Department, Howard University, Washington, DC

Investigators need a consistent and accurate method to measure and report sample warpage to assess material distortion during heat treatment. Consistent and accurate sample surface profile measurement can commence only after the sample is precisely positioned. This paper proposes using a positioning fixture which exactly constrains a sample's six degrees of freedom. Mounted in this fixture, standard dimensional inspection tools such as level gauges and dial indicator are then used to measure profiles along predetermined sample surface zones. Normalizing those measurements with respect to fixture constraint points and then plotting them on a graph allow precise warpage measurement and easy distortion visualization. In this study samples of 7050 plates were machined. The samples were of 1', 1.5", and 4" thickness and the crosssectional shapes were Uchannels and I beams. Samples were solution treated and one group was quenched in water and the second group in a polyalkylene-glycol solution. The effect of the sample's thickness and shape as well as the quenching medium on the warpage behavior will be discussed.

9:45 am

DYNAMIC DEFORMATION CHARACTERISTICS OF AlLi ALLOYS: Chang Gil Lee, Ki Jong Kim, and Sunghak Lee, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790784, Korea

This paper presents a correlation study between microstructure and dynamic deformation behavior of AlLi alloys which have great potential for structural armor materials. The selected materials were a 2090 AlLi alloy, a Weldalite 049 alloy, and a 7039 Al alloy, to allow a comparative study of different strengths and microstructures. These alloys were deformed at a very high strain rate by ballistic impact. The amount and the distribution of adiabatic shear bands were examined after the ballistic impact testing, and the dynamic shear stressstrain curves were obtained from the dynamic torsional Kolsky bar (strain rate=i-103/sec) test in order to evaluate the possibility of forming adiabatic shear bands. In the vicinity of the perforated region, adiabatic shear bands were hardly observed in the 2090 and the Weldalite alloys, whereas they easily formed in the 7039 alloy. The overall observed phenomenology was interpreted using the dynamic shear stress strain curves, together with the strength level and microstructure. In the curves, the maximum shear stress and the shear strain at the maximum stress point were of primary importance in analyzing the adiabatic shear banding behavior, suggesting that the torsional Kolsky bar test is a good tool for evaluating the ballistic performance AlLi alloys.

10:10 am

COMPETITION BETWEEN METASTABLE AND STABLE PHASES DURING SOLIDIFICATION OF AlFeVSi ALLOYS: Hyang Jin Koh, Woo Jin Park*, Sangho Ahn* and Nack J. Kim, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, San 31, Hyojadong, Pohang 790784 Korea, and *Research Institute of Industrial Science and Technology, San 32, Hyojadong, Pohang 790-390, Korea

AlFeVSi alloys have been produced by strip casting and spray casting. These processes offer relatively fast solidification rates and can produce near net shape products such as thin gauge strips or billets with the fine microstructure free from segregation. It has been shown that the microstructure of the alloys consists of various phases; bcc phase, microquasicrystalline phase, icosahedral phase, and the newly found hexagonal phase. All the phases are structurally related and have close orientation relationships among them. The volume fraction and distribution of these phases depend on the solidification rate and the degree of undercooling. The degree of recalescence occurring during solidification also affects the final microstructure, in that the less thermally stable phases transform to the stable phase.

10:35 am

THE EFFECTS OF AGING ON THE DIFFUSION AND TRAPPING OF HYDROGEN IN AN AL6.2ZN2.3MG2.3CU ALLOY: George A. Young Jr., John R. Scully, Center for Electrochemical Science & Engineering, Materials Science & Engineering, The University of Virginia, Charlottesville, VA 22903

Thermal desorption spectroscopy was used to investigate the diffusion and trapping of hydrogen in a Al6.2Zn2.3Mg2.3Cu alloy and in pure aluminum. Both the peak aged (T6) and overaged (T74) tempers were investigated for the alloy. Hydrogen was uniformly charged into rodshaped samples via exposure to water vapor saturated air. Isothermal desorption scans were utilized to determine the apparent activation energy for hydrogen diffusion. Constant heating rate thermal desorption scans were performed to identify hydrogen trap sites and quantify trap binding energies. In all desorption experiments, a quadrupole mass spectrometer was used to distinguish the desorption of hydrogen from that of other atomic or molecular species. Results from isothermal experiments show that bulk diffusion of hydrogen in the overaged alloy is appreciably slower than in the peak aged temper or in pure aluminum. Constant heating rate experiments reveal multiple trapping sites in both the alloy and in pure aluminum. The effects of the number and binding energy of trap sites on the diffusivity of hydrogen is quantified and the implications of these results on the hydrogen environment assisted cracking of precipitation hardened AlZnMg alloys are discussed.

11:00 am

EVOLUTION OF MICROSTRUCTURE, FRACTURE TOUGIINESS AND FATIGUE RESISTANCE DURING PROCESSING OF 7X50 ALUMINU1\I ALLOYS: F.D.S. Marquis, Department of Metallurgical Engineering, College of Chemical, Physical, and Materials Science & Engineering, South Dakota School of Mines and Technology, Rapid City, SD 57701

The evolution of microstructures was studied in 7X50 aluminum alloys, as a function of the manufacturing variables, during laboratory and industrial processing conditions. The fracture toughness, fatigue crack initiation and fatigue crack propagation of typical microstructures was measured. Low fracture toughness values and low resistance to fatigue crack growth were observed as a result of a high degree of recrystallization and a large volume fraction of both particles and hydrogen porosity. The resistance to fatigue crack propagation was higher in microstructures consisting of deformed grains with significant substructure. The morphology of various types of particles, volume fraction of the recrystallized microstructures, the grain and subgrain morphologies were observed to influence considerably the fatigue resistance. These microstructural parameters could be controlled effectively by the amount of total strain and the strain of the final pass, and the deformation temperatures during controlled rolling and subsequent double aging during final thermomechanical processing. Relationships between microstructure/fracture toughness/fatigue resistance will be discussed.

11:25 am

EFFECT OF PRE-AGE STRETCHING ON MECHANICAL AND HIGH CYCLE FATIGUE PROPERTIES OF THE 2024T851 ALUMINUM ALLOY FOR AIRCRAFT: Han-Cheng Shih, New-Jin Ho*, Steel and Aluminum Research & Development Dept. *Institute of Materials Science and Engineering, National Sun Yat-Sen University

Cold-stretching prior to artificial aging could increase the yield strength of the 3024 aluminum alloy due to the enhanced nucleation of the S' precipitates. However, the S' phase itself became weakened in resistance to dislocating due to thinning and was thus more easily sheared off. Consequently, the Orowan's strengthening mechanism could only occur in specimens undergoing longtime overaging. The strength of this alloy were thus a compromise of dislocationinduced hardening and precipitate-induced softening. Also, its high cycle fatigue performance became more and more deteriorated with increasing pre-aging strain on the basis of the similar reason.