Change of Mechanical Property and Fracture Mode of Molybdenum by Carbon Addition

Abstract

Commercially available specimens of pure molybdenum were immersed in a graphite-filled Ta container and heated at a relatively low temperature of 1373 and 1473 K for 1.2 to 18.0 ks. Changes in the yield and maximum strength were evaluated by a three-point bending test at a temperature between room temperature and liquid-nitrogen temperature. Then two parameters, critical stress (apparent intergranular fracture strength) and critical temperature (DBTT), were determined. Finally, the fracture mode was examined using SEM and the apparent transgranular fracture strength was estimated from the relationship between the critical stress and the fracture mode.Results are summarized as follows. (1) Both the critical stress and the critical temperature changed drastically with carbon addition and exhibited a peak value under optimum conditions: 1373 K-3.6 ks and 1473 K-1.2 ks. (2) The fracture mode changed from intergranular to transgranular after carbon addition. (3) The change in the critical stress as a function of the increase in carbon content in this work almost agrees with that in the previous work of Hiraoka et al. (4) The apparent transgranular fracture strength was unchanged (about 990 MPa) after carbon addition of 10 mass ppm or less. This value is almost equal with that before carbon addition. The apparent transgranular fracture strength was lowered after excess carbon addition.

Fracture Behavior and Delamination Toughening of Molybdenum in Charpy Impact Tests

“…6,11,12 By adding small amounts of carbon and boron, the ductility of molybdenum can be improved. 4,11,16 However, molybdenum suffers from severe delamination, a typical splitting of grains boundaries, in the transition and upper shelf regions due to the anisotropic grain geometry which makes its processing challenging. In the past, different investigation methods have been applied to solve the problem of delamination.…”

Low-Temperature Strengths and Ductility of Various Tungsten Sheets

“…Average values are summarized also in Table 1. PIFvalues of W(r1) and KDW(r2) are as high as about 90% similar to pure molybdenum after recrystallization [5]. On the other hand, PIF-values of W(r2) and LDW(r2) are medium (about 60%).…”

“…In Figure 8, PIF-value is plotted against critical stress. Data obtained for molybdenum with grain size of 20-25 μm [5] is also plotted in the figure for reference. In addition, the green line having a slope of 0.2 represents linear relationship between the critical stress and the fracture mode for molybdenum [5,8].…”

“…In Figure 9, reciprocal of critical temperature (ductility) is plotted against critical stress. Data obtained for molybdenum [5] is also plotted in the figure for comparison. At first, it is obvious that ductility of tungsten is generally much lower than that of molybdenum.…”

Influence of Carbon Diffusion and the Presence of Oxygen on the Microstructure of Molybdenum Powders Densified by SPS

“…Many studies have been performed to increase the density of the end-product by using: (i) different sizes of molybdenum powder [7], (ii) heat treatment of the initial molybdenum powders before sintering [6], (iii) specific additives as densification aids [8], or (iv) alternative methods It is well known that the presence of some impurities in molybdenum powder can modify the mechanical behavior of the sintered product. The presence of oxygen influences the mechanical properties by segregating at the grain boundaries [14][15][16], while other impurities, such as carbon in a reasonable proportion, increase the ductility [17,18], but have a negative effect in excess. Some studies have also investigated the relationship between impurities and the final density.…”