Deformation of an alloy with a lamellar microstructure: experimental behavior of individual widmanstatten colonies of an

  • PDF / 1,510,216 Bytes
  • 9 Pages / 594 x 774 pts Page_size
  • 70 Downloads / 206 Views

DOWNLOAD

REPORT


M A N Y alloys possess a microstructure which contains colonies comprised of a mixture of phases with a lamellar morphology. Some examples are the a-fi alloys in the B-annealed condition, steels containing pearlite, and some directionally solidified eutectics. While certain feawres of the deformation of such alloys are well established, relatively little is known about the deformation behavior of individual colonies. It is well known that some, but not all, colonies with a lamellar microstructure are susceptible to very planar, nonuniform slip which can lead to easy crack initiation and propagation along such localized bands. This is particularly so in a-fl Ti alloys ~ and pearlitic steels. 7-n Even though it may be confined to a few isolated colonies, such behavior can limit the overall ductility and fatigue resistance of a component. There has been no experimental study which identifies the relationships between a microstructure consisting of aligned lamellae of two deformable phases and the resulting deformation behavior for individual colonies. Utilizing the a-fl Ti alloy, Ti8A1-1 Mo-1 V, this investigation examines the deformation behavior of thirty-seven individual colony test samples. The yield strength, work hardening behavior, and the slip chracteristics are related to several microstructural parameters, such as the orientation of the K. S. CHAN and C. C. WOJCIK, formerly Graduate Students, Department of Metallurgical Engineering,MichiganTechnological University, are currently with the Department of Materials Science and Engineering,Stanford University,Palo Alto, CA 94305 and TeledyneWall Chang, P.O. Box 460, Albany, OR 97321, respectively. D. A. KOSS is Professor,Department of Metallurgical Engineering, Michigan TechnologicalUniversity,Houghton, MI 49931. Manuscript submitted December29, 1980.

colony to the stress axis and the crystallography of slip. Since the fl phase can be transformed to a martensite if a colony is quenched from 1200 K or retained as bcc fl if quenched from 1033 K, ~3-~6the influence of heat treatment is also explored. Particular attention is given to relating these results to the calculations of Ankem and Margolin on the role of elastic interaction stresses on the onset of plastic flow for oriented two ductile phase structures) 6 The Ti-8AI-IMo-IV alloy ("Ti-811") is used as a model system because it is relatively well characterized; the nature of the individual phases can be manipulated, and it is much easier to grow rather large a-fi Widmanstatten colonies in this alloy than in other a-fl alloys, such as Ti-6AI-4V. EXPERIMENTAL PROCEDURE The preparation of large colonies consisting of relatively well aligned a and fi lamellae is difficult in Ti alloys because the microstructure is a result of a solid state reaction on cooling from the fl transus at ~ 1320 K into the a + fl two phase field. Strips of Ti-811 approximately 76 m m long by 25 m m wide were cut from 1.27 m m thick sheet. The strip length was parallel to the rolling direction. After cleaning, large a-fl colonies were gro