Microstructural characterization of a rapidly solidified ultrahigh strength Al 94.5 Cr 3 Co 1.5 Ce 1 alloy
- PDF / 1,880,532 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 83 Downloads / 201 Views
I. INTRODUCTION nanophase composites such as rapidly solidified Al-RE-TM alloys (RE: rare earth; TM: transition metal) receive considerable research interest because of their extremely high tensile strength exceeding 1200 MPa.[1,2,3] For example, Al-Y-Ni alloys containing 85 to 90 at. pct Al can be quenched into a fully amorphous state from the melt.[4] By annealing, the nanoscale a-Al particles are uniformly dispersed in the amorphous matrix, and remarkably high tensile strength is obtained when the volume fraction of the a-Al is optimized to around 20 pct.[4–7] The origin of the high strength was attributed to the ultrafine grain structure free of dislocations, which is presumed to be difficult to deform.[4,8] On the other hand, recent investigations by Zhong et al.[9] reported that the hardness of the a-Al/amorphous nanocomposite material is essentially the same as that of the remaining amorphous phase with the same composition; thus, they attributed the ultrahigh strength of the nanocomposite Al alloys to the solute enrichment in the remaining amorphous phase. A different type of nanocomposite microstructure composed of nanoscale amorphous particles dispersed in relatively large crystalline grains is obtained by directly quenching Al-V-TM (TM 5 Fe, Co, or Ni) or Al-TM-RE alloys with the Al concentration over 90 at. pct.[10,11,12] These include high-strength Al-based alloys consisting of a nanoscale mixture of a-Al and amorphous phases or a mixture of a-Al, amorphous, and icosahedral phases in Al-Mn-Ce, Al-Mn-Ln (Ln: Lanthanide metals), and Al-Cr-Co-Ce systems. For example, Inoue et al.[13] reported that the tensile
AL-BASED
D.H. PING, Researcher, and K. HONO, Head of 3rd Laboratory, are with Materials Physics Division, National Research Institute for Metals, Tsukuba 305-0047, Japan. A. INOUE, Professor, is with Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Manuscript submitted November 20, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
strength of an as-melt-spun Al94V4Fe2 alloy is as high as 1390 MPa. The microstructure is composed of nanoscale amorphous regions trapped within the dendritic a-Al grains.[14] In the Al-Cr-Co-Ce system, a-Al and icosahedral phases with nanoscale grains were reported to coexist in the rapidly solidified Al95.5-xCr3Co1.5Cex alloys.[15] Additions of Ce from 0.5 to 3 at. pct were reported to increase the precipitation tendency of the icosahedral phase and to enhance the good bending ductility and high tensile strength of this alloy. However, few studies have been made on the microstructural evolution in these nanocomposite high strength aluminum alloys. The aim of this study was to obtain a better understanding of the mechanism of the nanoscale microstructural evolution during the rapid solidification processes in an Al-Cr-Co-Ce alloy by atom probe field ion microscopy (APFIM) and high resolution transmission electron microscopy (HREM). II. EXPERIMENTAL A quaternary alloy ingot, with a composition of Al94.5Cr3Co1.5Ce1, was prepared by inductively m
Data Loading...
