An examination of class A to class M transition in Pb-9Sn and other alloys
- PDF / 649,003 Bytes
- 7 Pages / 594 x 774 pts Page_size
- 80 Downloads / 169 Views
INTRODUCTION
Table I.
THE high
temperature creep behavior of metallic solid solution alloys was considered to be different from that of pure metals. Sherby and Burke I have pointed out, however, that a great number of alloys behave similarly to pure metals at all concentrations across the phase diagram and proposed that solid solution alloys be divided into two categories. The characteristics of these two classes (here designated M and A) have been documented in the literature, ~'2'3and may be briefly summarized in Table I. Cannon and Sherby 3 were the first to predict the classes of creep behavior of solid solution alloys. The factors chosen to classify the types of creep behavior were Young's modulus of the solvent and the size difference between the solute and solvent atoms. It was concluded that climb (class M) was favored in alloys having a large elastic modulus whereas viscous glide (class A) was favored in alloys having a large atom misfit ratio. However, although this approach had a degree of success in predicting the class of creep behavior of solid solution alloys, there were several additional significant parameters which needed to be considered. Mohamed and Langdon4 have refined their analysis and developed a semi-empirical criterion for class A solid solution alloys as
\b--7:
kec 1/2Gb 3j
'
Class A Micro creep a ~3 Little primary creep p a o-z No subgrains No F effect
Class M pure metal behavior k a ~r45 large primary creep p a o-z distinct subgrains F effect
e.g., AuNi,/3-brass
NiCu, a-brass Fe-Si, austenitic SS
A1Mg, PbSn, NaCI-KC1
In this criterion, B was employed to determine the boundary of climb-glide creep behavior. However, there are controversies about constant B to be cleared up. Firstly, Pahutov~i et a l ) have pointed out that the quantity B is dependent on temperature. Secondly, because the value of B is strongly dependent on the relative diffusivities governing the climb (class M) and glide (class A) creep mechanisms and there are limited amounts of suitable diffusion data, different values of B have been proposed. 4'5'6 As far as diffusivities is concerned, Fuentes-Samaniego et al. 7"s have proposed modified diffusivities for climb and glide in lieu of Herring 9 and Darken m diffusivities (see Table II), and Table II.
Ill
where B is a dimensionless constant, k the Boltzmann constant, F the stacking fault energy, G the shear modulus, b the Burgers vector, T the absolute temperature, e the atomic misfit factor, c the concentration of the solute, and (De, D 8) the diffusion coefficients for climb and glide, respectively.
Solid-Solution Alloys
Various Proposed Diffusion Coefficients
DADs Dc = (XaD* + XBD*)f
Herring diffusion coefficient
D e = (XAD* + X B D * ) ( 1 + aln Fa) O lnXA
Darken diffusivity
1
= 7(XAD* + TSANG-TSE FANG, formerly a Graduate Student in the Department of Materials Engineering, North Carolina State University, Raleigh, NC, is Associate Professor, Department of Metallurgy and Materials Engineering, National Cheng Kung University, Tainan, Taiwan, Republ
Data Loading...
