Internal Friction in Plastically Deformed High-Purity NiAl Single Crystals

  • PDF / 413,733 Bytes
  • 6 Pages / 416.34 x 643.86 pts Page_size
  • 73 Downloads / 218 Views

DOWNLOAD

REPORT


KK8.2.1 Mat. Res. Soc. Syrup. Proc. Vol. 552 0 1999 Materials Research Society

1d

400

Al

6

"'otl5O

f0

I,

I

I

T 20I

if,•

I

.

/

.

I

.,





2 0-

400

600

800

1000

400

T [K]

600

800

1000

T [K]

Figure 1. Internal friction, Q-, of oriented single crystal for different degrees of torsional deformation with surface strains of oriented single crystal. The specimen was first measured up to 700 K, then with decreasing temperature to about 530 K. This was followed by a measurement with rising temperature up to 900 K and downwards to 650 K, and then by an upward measurement to 1050 K and downwards in temperature again. These stepwise measurements indicate clearly that strong annealing effects occur already in the temperature ranges of the two relaxation maxima. The annealing kinetics of both maxima were investigated by isothermal internal friction measurements of the relaxation strength in the temperature range from 500 to 650 K and from 750 to 950 K, respectively. Fig. 3 shows the internal friction and modulus defect as a function of time for a oriented single crystal. The time scale begins by switching on the heater. The internal friction and the modulus defect increase until the temperature of 575 K is reached. During the isothermal measurement at this temperature the internal friction decreases due to annealing effects in the temperature region of the first maximum. Furthermore, the decrease of the modulus defect indicates a hardening of the specimen. After 60000 s the temperature is rapidly increased to 800 K and the internal friction and modulus are increasing. During the isothermal measurement both quantities decrease due to annealing effects in the temperature region of the second maximum. After 120000 s the heater is turned off and the specimen cools down to room temperature. Several of these measurements with different temperatures of the isotherms could be performed since after each annealing treatment the relaxation maxima could be reproduced by an additional torsional deformation at room temperature. ANALYSIS OF THE ISOTHERMAL MEASUREMENTS The internal friction and modulus defect of dislocations can be described by a mesoscopic string model or so called 'line tension model' [10]. The major result of this approach is that the modKK8.2.2

0

30 Figure 3. Internal friction and modulus defect for an isothermal measurement at 575 and 800 K of a oriented single crystal after torsional deformation with a surface strain of 3% (frequency 10 Hz).

-10, -- 0 b, 20

"T-575K1C2=500K

0

-20

internal friction

10

0

5x104

- 30

105

t [S] ulus defect and the internal friction depend on the dislocation density A and the average distance between two pinning centers of the dislocation 10in the following way

did ACA

6

0

ann

A/O

(1)

respectively. Microscopically the dislocation motion is due to the thermally activated formation of kink pairs [2] leading to the same dependence as given in eq. (1). If the crystal contains atomic defects, e.g. interstitial impurity atoms, self-interstitials or

Data Loading...