Question

Problem A lungsten d 10um, ρ 4x1010/cm2 Figure I: Deformation-mechanism map for tungsten showing constant strain-rate contours from M. F. Ashby, Acta Metallurgica 20 (1972) 887. The shear modulus is G-160 GPa Theoretical strength Dislocation glide co10-2 Dislocation creep 10 O 10-4 10-4 Coble creep 10-6 10 10 Nabarro 10 /s cre 10-8 0.6 1.0 0.2 0.4 0.8 Homologous temperature, T/TM Tungsten is being used at half its melting point (TMa 3,400°C) and a stress level of 160 MPa. An engineer suggests increasing the grain size by a factor of 4 as an effective means of reducing the creep rate at this temperature. Using the deformation-mechanism map for tungsten shown in Figure 1, (1) (2) (3) Do you agree with the engineer? Why? What if the stress level were equal to 1.6 MPa? What is the predicted increase in length of the specimen after 10,000 hours at 160 MPa if the initial length is 10 cm?

Answer 1

DISLDCATION -1 GLİDE Deformatfon-Mechanism emap for Tungsten DISLOCATION 108 CREEP Cable CREEP lo Nabar Nabartu ecp 0.5 Given: stress level 160MPa G= 160GPa i) From the Deformation-mechanism, kde come to knoro that the governing Crèep mechanism is Dislocation- driven The Suggestion of engineer will not be acceptable as increasing the grain-Size leads to reauction in grairn-boundary count there by fevouring the mot on of cslocation. As the dielocatior thorg clearly explains that presence of srain boundarfes are hinderonce to motfon of dislocatfon. In order to arest the notifon e need matertal woith diner grains vohich leads to more no of grain boundaries or we can have a rmaterial with higher strength. Engineer's Suggestfon oill be help Ful in Nabarro creep vohich ?s diffusion-driven (2) IF stress level reduced to 6MPa Noro the governing creep m echonism le Coble creep ohich ia in volves stress driven diffusion of atoms along grain boundarfes-to elongate graio along stress axis which is called grain-boundary diffusion ine grained materials are more vulnerable to coble creep: Hence engineet's su ggestion roill work here Increasing grain size reduces aFfinity for coble creep for 3 and (3) From map of creep mechanism we read strain rare (é lo/sec Ex Li-36 x 10cm 360cm -t> Tncrease in length-360cm. ,'。 AL