High temperature embrittlement of NI and Ni-Cr alloys by trace elements
- PDF / 6,187,889 Bytes
- 16 Pages / 603.28 x 788 pts Page_size
- 61 Downloads / 171 Views
I.
Ni3S2-Ni interface, allowing the sulfide to "wet" the primary nickel grain boundaries. The beneficial effects of certain malleabilizing elements, such as magnesium, manganese, and zirconium, were attributed to their ability to precipitate stable high melting point sulfides, thereby lowering the amount of sulfur available to form low melting Ni3S2.1-4,9 More recent studies suggest that a low melting grain boundary phase is not necessary for impurity-induced HTIGC. Lozinskiy et a l 3 reported significant HT-IGC upon increasing the sulfur content in nickel from 10 to 20 ppm (see Figure 1). This ductility loss occurs in a temperature range where the data of Brigham et a110 indicate that Ni3S2 should not form in Ni + 20 ppm S. Other results on single phase nickel alloys containing antimony, tin, and arsenic also indicate that impurity-induced HT-IGC can occur without the presence of low melting point phases at the grain boundaries. 5'6 An alternate explanation of the potent embrittling effects of these metalloid impurities is that they segregate to nickel grain boundaries without precipitation of a second phase. Several of the impurities known to induce HT-IGC in nickel
INTRODUCTION
HIGH purity nickel, or nickel containing certain "ductilizing" elements, normally fails in a ductile transgranular manner in tensile tests at 0.4 Tm tO 0.7 Tm (Tin = absolute melting temperature). 1-4 Details of the ductile failure often indicate extensive deformation of the grains, recrystallization, and sometimes grain growth in the necked portion of tensile specimens. In contrast to high purity nickel, nickel containing small concentrations of metalloid* impurities (e.g., S, Se, As, *The various definitions of "metalloid" usually include elements having some properties of both metals and nonmetals.8 Of the elements mentioned here, only sulfur is questionable as a metalloid. Because of its chemical similarity to Se and Te, as well as for convenience, we will use the generic term "metalloid" to include sulfur.
Sb, Bi, and Sn) often exhibits poor ductility in the 0.4 Tmto 0.7 Tmtemperature range.~-7 These metalloid impurities induce extensive cracking and cavitation on grain boundaries normal to the local tensile stress. Low-ductility failure generally occurs when these grain boundary cracks and cavities link up. This "high-temperature intergranular cavitation and cracking" (henceforth denoted as HT-1GC) is distinct from impurity induced low temperature grain boundary fracture (LT-GBF) phenomena such as temper embrittlemerit, although sometimes the same impurity may induce both phenomena. Sulfur is the most common embrittling impurity in nickel, and early work by Merica and Waltenberg I attributed its effects to the formation of a Ni-Ni3S2 eutectic film along the primary nickel grain boundaries. Such a grain boundary film would be near its melting point at 0.5 Tm for pure nickel, making the grain boundary regions very weak. The potent embrittling effect of sulfur could then be explained by its low solubility in nickel, and low interfa
Data Loading...
