Effects on microstructure and tensile properties of a zirconium addition to a Cu-Al-Ni shape memory alloy
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X-ray analyses showed that both alloys 1 and 2 were fully martensitic, with the martensite being a mixture of M18R and N2H types. [9,1~ Figure 3 shows a transmission electron micrograph and electron diffraction pattern with their analyses for the M18R-type martensite, in which the M18R martensite is composed of internal stacking faults. [9] Figure 4 shows a transmission electron micrograph and electron diffraction pattern with their analyses for the N2H-type martensite, which is composed of internal twins. [~~ Microstructural examination showed that the M18R-type martensite was the major phase, while the N2H-type martensite was the minor phase, in both alloys. Figure 5 shows typical stress-strain curves of both alloys tested at room temperature. The test specimens were recrystallized for 2 minutes at 850 ~ The results show that the Zr-added fine-grained alloy 2 exhibits a significantly higher fracture stress and strain compared to the coarse-grained Zr-free alloy 1. Fracture strength increases from 380 to 830 MPa, and fracture strain increases from 4 to 7.5 pct by adding 0.54 wt pct Zr to the Cu-13.4A1-3.8Ni base. It was generally observed that the fracture stress and strain increased with decreasing grain size, according to a Hall-Petch type relationship. [5,6] The variations of fracture stress and strain at
Table I. Chemical Compositions (Weight Percent) and Ms Temperatures of the Investigated Alloys
Alloy Alloy 1 Alloy 2
Cu bal. bal.
A1 13.4 13.6
Ni 3.8 3.0
Zr -0.54
M, (~ 116 67
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I. Recrystallization J.W. KIM, D.W. ROH, E.S. LEE, Graduate Students, and Y.G. KIM, Professor, are with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, Korea. Manuscript submitted July 5, 1989. METALLURGICAL TRANSACTIONS A
1[. Grain Growth Fig. 1--Variation of grain size with increasing solution treatment time at 850 ~ for alloys 1 and 2 (log-log plot). VOLUME 21A, MARCH 1990--741
Fig. 2 - - O p t i c a l micrographs of (a) the Zr-free alloy 1 and (b) the Zr-added alloy 2.
Fig. 3--Transmission electron micrographs showing Ml8R-type structure after solution treatment (10 min, 850 ~ and followed by water quenching (25 ~ in alloy 2: (a) bright-field image and (b) electron diffraction pattern and its indexing of variant A, Z =[10 3 2]; lattice constants: a = 4.422 ,~, b = 5.330/~, and c = 38.019 ]k; monoclinic angle = 88.60 deg.
Fig. 4 - - T r a n s m i s s i o n electron micrographs showingN2H-type structure after solution treatment (10 min, 850 ~ and followed by wat_er quenching (25 ~ in alloy 2: (a) bright-field image of (1 2 1) twin and (b) electron diffraction pattern and its indexing of variant A, Z = [2 1 4]; lattice constants: a = 4.394/~, b = 5.324 ./k, and c = 4.223 A. 742--VOLUME 21A, MARCH 1990
METALLURGICAL TRANSACTIONS A
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