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a nominal strain rate between 1.7x 10-4 s-1 and 1.7x 10-3 s-1 during a compression test to examine the strain rate sensitivity of flow stress. The operative slip plane was determined from slip markings which were observed by an optical microscope using Nomarski interference contrast. Deformation substructure was observed by a Hitachi H-800 transmission electron microscope (TEM) operated at 200kV. Table 1. Schmid factors for possible deformation mode in Ni3Nb single crystals. Oblique lines indicate that any twins can not be available in compression. Loading axis

Deformation

[110]*

[3311*

[2701*

(016)[1001

0.491

0.465

0.225

(010)(0011 (o0I)[100]

0 0

0.174 0.145 0.047

0 0 0.133

0

0.10 0.312

mode

1211}

(001)[10010

(010)[1001

0

RESULTS DOa-type crystal structure

Figure 1 shows a unit cell of the DOa structure and the atomic arrangement on the closely packed (010) plane. Nb atoms on (010) are arranged in rectangular positions and the DOn structure is classified in an orthorhombic lattice with lattice parameters of a=0.51 nm, b=0.424nm and c=0.454nm. However, since the atomic sites on (010) are similar to those on the basal plane in an hcp lattice with two fold periodicity of the stacking sequence, the DOa structure is also regarded as an ordered structure based on hcp lattice like the DO19 structure. Moreover, the DOa structure shows similar atomic arrangement on (010) to that on f 111 in the L1 2 structure, although the stacking sequence is different; two fold periodicity in the DOa structure and three fold periodicity in the LI 2 structure. Thus, (010)[100] slip in the DOa structure may be compared with {111 1 slip in the L1 2 structure and the plastic deformation behavior will be examined focusing on the analogous slip system.

0: Ni

[01O1

o~

*: Nb 0

(3> too11

[e[100

~-9-•-O~/

11001 -------

2[CO0

0-b=1121100] [0011 t (b)

(a)

Figure 1. (a) Unit cell of the DOa structure, (b) atomic arrangement on the (010) plane. Circles with different diameter show different layers of stacking atoms. Temperature dependence of yield stress Significant plastic flow was obtained before crack initiation for three orientations even at room

temperature. Specimens with [110]* and [331* showed adequate ductility with at least 20% plastic strain at all tested temperatures. Figure 2 shows temperature dependence of yield stress (0.2% offset KK10.7.2

stress) in Ni 3Nb single crystals with three different compressive axes. Remarkably positive temperature dependence of yield stress appears for all specimens but the peak temperature for anomalous strengthening depends strongly on crystal orientation: 400 'C for [2701*, 700 'C for [331]* and 800 °C for [110]*. Yield stress at the peak for [110]* and [331]* specimens is about five times higher than that achieved at room temperature. Above the peak temperature, yield stress rapidly decreases but the slope change of the stress-temperature curve also depends strongly on orientation. A decrease in yield stress for [270]* and [331]* specimens occurs more rapidly than for