THE AMPLITUDE DEPENDENT DISLOCATION DAMPING IN HIGH PURITY MOLYBDENUM CRYSTALS

The amplitude dependence of the y peak shows anomalous behaviour f o r low ampl i tudes, and part ly i r revers ib le ef fec ts a t larger amplitudes. This hinders a d i rec t determination of the activation volume. However, an estimate of the actual activation volume can be obtained from the observed peak broadening. The resul t i s compatible with a kink-pair formation process. Dislocation damping peaks in ul tra-high purity molybdenum crystals (residual res is t ance r a t io 230 000) were investigated by torsion-pendulum technique. The y peak (400-490K, 1Hz) shows recovery a f t e r heating u p t o 600 K , compare curve 1 and 2 in f ig.1, ( 1 ) ( 2 ) . The amplitude dependent damping was obtained during one heating up by online data processing of the osc i l la t ion amplitudes (2) . Fip. 1: Damping (om') vs. T (temperature), ul tra-hi gh purity Mo-single crystal (RRR=32.000, spec. axis < I l l > ) . (1) following 1% tension (200K)+0.1% torsion (293K); (2) 2nd run following (1) a f t e r heating t o 600K (f=0.5 Hz, Amp1 i tude 1,.5x10-5) Fig.2 shows the amplitude dependence of the y peak fo r two frequencies in a range of low amplitudes (2xl0-6-1,2xl0-5 re la t ive surface s t r a i n ) . The behaviour observed in th i s amplitude range i s anomalous: the damping peak becomes s m l l e r , and s h i f t s t o higher temperaturzs with increasing amplitudes (curves 1...6, in f ig .2) . For larger ampl i tudes (1x101 . 2 x 1 0 ~ p l i tude dependence becomes normal ( f i g.3). The dependence Q ~ vs. amplitude ( A ) for the y peak i s shown in f ig.4 f o r two amplitude ranges: ~<1.5x10-4 and ~ ~ 1 . 5 ~ 1 0 5 . The curve obtained from the large s t a r t ing ampl i tude ( ~ ~ " 1 . 5 ~ 1 0 ~ ) mets exactly toggther w i t h the damping c u r v a b t a i n e d from a lower s t a r t ing amplitude (Ao=1.5x10) as long as the temperatures are below 460 K. For higher temperatures t h i s t e s t shows i r revers ib le changes in the specimen s t a t e fo r the larger amplitudes in a time scale of one decay. In the following we r e s t r i c t ourselves to the amplitude range below 1 . 5 ~ 1 0 ~ and temperatures, where irreversible ef fec ts are small in the time scale of one decay, and consider the broadening of the y peak due to internal stresses. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19839106 C9-706 JOURNAL DE PHYSIQUE F ig .2 :Theymax imum:Ampl i tudedepenF i g . 3 : A s f i g . 2 , b u t f o r l a r g e r a m p l ~ t u d e s -for small amplitudes (A) a t 2 fre0 . 1 ) ~ = 1 x 1 0 ~ , (2)(6) AA=2xlO6 quenci es . (6) ~=1.2x10-4. "Normal " amp1 i tude depen(1) ~ = 2 x 1 0 ~ , ( 2 ) (6) ~ ~ = 2 x 1 0 ~ dence. (6) A=I. 2x10-5. "Anomalous" ampl i tude dependence.