Dislocation structure, deformation structure, strengthening mechanisms and embrittlement causes of ordered Ni₄Mo alloy

Author

Hwa-Perng Henry Kao

Date of Award

3-1986

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

C. R. Brooks

Abstract

The primary purpose of the present research was to investigate the strengthening mechanisms, deformation mechanisms and embrittlement causes of ordered Ni₄Mo from the microstructural and especially the dislocation structure viewpoint.

The following experiments were performed for samples aged at 725°C and 775°C for up to 100,000 minutes. (1) The long-range order parameter and the domain size as a function of aging time were measured. (2) The microstructure for various aging treatments was characterized using optical microscopy, SEM and TEM. (3) The tensile yield strength, elongation and microhardness were measured at 25°C as a function of aging time. (4) The fracture surface of the tensile samples were characterized by SEM. (5) The fracture surfaces for one aging treatment were analyzed by Auger spectroscopy.

Yield strength and elongation were measured at temperature from 300 to 1000°C. The fracture surfaces were characterized by SEM. TEM was used to characterize the dislocation structure of undeformed aged samples, of aged samples after deformation, of aged samples deformed in situ in the TEM and of samples tensile tested at high temperatures.

Upon aging, the strength approximately doubled, but the alloy became very brittle (e.g., less than 4% elongation), fracturing intergranularly along the high angle, prior a boundaries. It was found that the degree of long-range order is much more important in strengthening than is the domain size.

The dislocation structure of deformed Ni₄Mo consisted mainly of overlapping {111} intrinsic stacking faults, which were bounded by widely separated 1/6<112>, single Burgers vector, type dislocations. Upon sufficient deformation, these bands thickened into twins. The dislocations easily passed through the ordered domains, but piled up at the high angle grain boundaries, with which they were incompatable. Continued deformation caused these boundaries to separate, causing brittle intergranular fracture.

The main strengthening factors are the development of long-range order, which limits the slip systems, and the development of a dislocation structure consisting of widely separated partial dislocations, which cannot cross slip. This is probably due to a great reduction in the stacking fault energy upon ordering. The Auger analysis did not indicate any impurity segregation which might have caused grain boundary weakness.

Recommended Citation

Kao, Hwa-Perng Henry, "Dislocation structure, deformation structure, strengthening mechanisms and embrittlement causes of ordered Ni₄Mo alloy. " PhD diss., University of Tennessee, 1986.
https://trace.tennessee.edu/utk_graddiss/12274