Room Temperature Tensile Ductility in Polycrystalline B2 NiAl

Abstract

Binary polycrystalline alloys of B2 NiAl, ranging from 47 to 51.5 at% Al, were prepared by casting and extrusion. These were tested in tension at temperatures ranging from 300 K to 873 K. Significant tensile ductility is observed in the near-stoichiometric alloy at room temperature as well as at 473 K in spite of the presence of only <100> dislocations. This anomaly appears to be related to the recrystallized texture, to the lower yield strength and to the presence of residual dislocations at grain boundaries in this alloy.

Ductility in Intermetallic Compounds

“…NiAl actually becomes stronger as the stoichiometry deviates from 50 at.-% Al. The RT yield strength of stoichiometric NiAl is only about 230 MPa, [20] but strength rises to over 1200 MPa as Al content increases to 53 at.-%. [21] This hardening effect in Al-rich material is thought to result from the large numbers of vacancies present in the lattice.…”

“…Ni-rich NiAl is also stronger than stoichiometric NiAl; yield strength increases to over 800 MPa as Al content drops to 42 at.-%. Unfortunately, deviations from stoichiometry in NiAl actually reduce ductility; what little ductility polycrystalline NiAl possesses at RT (2 % elongation or less [20] ) drops to zero ductility for both Al-rich and Ni-rich compositions.…”

Physical and mechanical properties of the B2 compound NiAl

“…o the extrusion axis exhibited the highest flow strengths while specimens oriented longitudinally or transverse to the extrusion direction have lower flow stresses, however, the actual differences in strength were relatively small. 169 The reason that these orientation effects were relatively minor compared with what is observed in single crystals is that the textures observed in extruded NiAI are fairly weak, being a factor of only 3-5 times random. 23o ,231…”

“…164 For orientations other than [100J and [110], only single slip of the type (OOl){lIO} was observed in these studies.156.164However, (001) slip on cube planes also was observed by Loretto and Wasilewskp30.163in [112] NiAI single crystals is unique compared with soft orientations and therefore, a significant amount of work has been spent on analysing and understanding the operative slip systems in hard crystals through experimental means. 168 Vedula et ai., 169 and Cotton. 172 Isolated dislocation segments with Burgers vectors other than <001) have been observed in as extruded NiAI alloys.129,172,180,181 However, the presence of non-(OO1) dislocations in these studies .does not constitute an alternative deformation mechanism; they are probably formed as a result of dislocation interactions between gliding ao (001) dislocations 134 ,180 and are generally observed only after extrusion.…”

Physical and mechanical properties of the B2 compound NiAl

“…NiAI should have a (100) slip vector and {OIl} slip plane with the possibility for slip also occurring on {001}type planes. 164 For orientations other than [100J and [110], only single slip of the type (OOl){lIO} was observed in these studies.156.164However, (001) slip on cube planes also was observed by Loretto and Wasilewskp30.163in [112] 168 Vedula et ai., 169 and Cotton. 172 Isolated dislocation segments with Burgers vectors other than <001) have been observed in as extruded NiAI alloys.129,172,180,181 However, the presence of non-(OO1) dislocations in these studies .does not constitute an alternative deformation mechanism; they are probably formed as a result of dislocation interactions between gliding ao (001) dislocations 134 ,180 and are generally observed only after extrusion.…”