Crystal Growth and Plastic Deformation of Solid Helium.
The first measurements of acoustic reflection, together with acoustic transmission at the liquid-solid interface of ('4)He have been made at 10 MHz and over the temperature range, .8 K to 1.8 K. For the range .8 K to 1.46 K the total, relative acoustic energy loss, (epsilon), due to interface mechanisms only, has been determined. Comparison in the temperature range .9 K < T < 1.45 K with the only available theory was at best ambiguous since the relation, R(,Ls) = T(,Ls) - 1, which explic- itly holds in this theory, was not confirmed for the measured R(,Ls) and T(,Ls) over this temperature range. (Here R(,Ls) and T(,Ls) are the liquid to solid reflection and transmission coefficients, respectively). An unexpected relaxational behavior was observed in (epsilon) over the above temperature range. Based on the parameters of this relaxational behavior in (epsilon) and the assumption of a temperature dependent relax- ational time, a new mechanism for limiting the ('4)He solid-liquid inter- face mobility based on the accomodation of mobile vacancies in the vicinity of the interface is proposed. Measurements of the plastic deformation of bcc ('3)He near the melting curve (T (GREATERTHEQ) 0.8 T(,m)) over the temperature range, 0.65 K (LESSTHEQ) T (LESSTHEQ) 1.17 K, and for a range of strain rates, (epsilon), 2 x 10('-6)/sec < (epsilon) < 2 x 10('-4)/sec, have been made. The stress and strain relations, strain rate dependence of the flow stress at constant temperature and the temperature dependence of the strain rate at constant flow stress have been determined. The change in ultrasonic attenuation has been measured during plastic deformation. Comparison of these results with those of similar experiments is made. The present results are interpreted in terms of dislocation climb and vacancy diffusion in solid He. Values of the vacancy diffusion rate and of the bandwidth of delocalized vacancies are deduced from the observed temperature dependence of the strain rate.
- Pub Date:
- Physics: Condensed Matter