The mechanical properties of W-based heavy alloys are determined mainly by the composition and amount of binder phase, the content of nonmetallic impurities and the condition of post-sintering treatment. The aim of the present investigation is to study critically the effect of the above major factors in terms of microstructure and mechanical properties. Since the effect of non-metallic impurities, such as S and P, was recently studied in much detail, the other two factors-binder composition and heat-treatment-have been included in the major scope of the study.
In chapter II, the effect of Ni/Fe ratio on the microstructure and mechanical properties has been studied for 93W-7(Ni,Fe) alloys (in wt %). The alloys with the Ni/Fe ratio of 7/3, 8/2 and 9/1 have been sintered between 1460 ℃ and 1510℃ for 1 hr. With the increase in Ni/Fe ratio, the matrix volume and hardness increase due to the increase in w solubility in the matrix. The heavy alloy with Ni/Fe ratio of 8/2 shows the best mechanical properties in the studied range. Compared to the other alloys, elongation is increased by 14 %, and impact energy 25 % without reduction of ultimate tensile strength. The improvement in mechanical properties, in particular impact energy, is discussed in terms of the increase in matrix volume and matrix hardness.
In chapter III, the effect of the W-W contiguity on mechanical properties of 93W-5.6Ni-1.4Fe (wt %) has been evaluated. The W-W contiguity of the alloy has been controlled between 0.34 and 0.05 by a newly developed heat-treatment which consists of repeated isothermal holdings at 1150 ℃ and water-quenching between them. With decreasing the contiguity from 0.34 to 0.05, the ultimate tensile strength and elongation remain unchanged, contrary to previous expectation, whereas impact energy increases from 57 J to 170 J. The tensile fracture surface of specimens appears similar regardless of contiguity: predominant cleavage of W grains and dimple fracture of matrix. The fracture surface by impact, on the other hand, reveals a change of W-W interface failure towards ductile failure of W-matrix with many dimples by decreasing contiguity. The increase in the fraction of ductile failure and observed crack blunting are attributed to the increase in impact energy with decreasing contiguity.
In chapter IV, the mechanism of matrix penetration into W grainboundaries has been studied for specimens subjected to various post-sintering heat-treatments. SEM micrographs and AES analysis show that the penetration has been taken place by cyclic post-sintering heat-treatment at 1150 ℃. The volume of the matrix penetrated between W grains increases with increasing the number of heat-treatment cycle. TEM micrographs show that high density of dislocations most probably due to thermal stresses during cyclic heat-treatment is concentrated at W-matrix interface. The penetration of matrix phase by cyclic heat-treatment is therefore thought to be due to the thermal stresses arising from the difference in thermal expansion coefficient between the matrix and W. The stresses may be much relieved at the heat-treatment temperature, however, the cyclic heat-treatment may provide it in repetition, resulting in successive penetration of matrix with the cycle.