Dominant factors for fracture at the micro-scale in electrodeposited nickel alloys

Abstract

Two different LIGA electrodeposited nickel alloys displayed distinct fracture modes after meso-scale tensile testing. The Ni-Co alloy failed in a ductile manner, while the Ni-Fe alloy failed in a more brittle-appearing manner. Various factors affecting the fracture are discussed; it was determined that the fracture mode did not depend upon the strain rate but did depend upon the sample geometry. The difference in the microstructure is likely the cause of the difference in fracture mode, as the Ni-Co alloy is fine-grained, while the Ni-Fe alloy is nano-grained and likely failed by a creep-like mechanism.

Keywords: Ductile fracture; Electrodeposited nickel; Fracture mechanisms; Microstructure; Nickel alloys.

Fig. 1.

Photograph of representative samples showing the four specimen geometries [22]. “x” is the tensile direction, and “z” is the growth (through-thickness) direction.

Fig. 2.

TEM micrographs showing microstructure of two alloys, (a) Alpha and (b) C, in the x-z plane. Inserts show selected area electron diffraction patterns. In Alpha, the FCC ring pattern includes bright spots along the rings suggesting that certain orientations were preferred. In C, the grains are large enough to produce single-crystal diffraction patterns, with the bright center grain producing the [011] zone-axis pattern shown in the inset.

Fig. 3.

Representative tensile curves for the S1 geometry samples of both materials at 0.001/s strain rate, known as the quasi-static tests (QS), and at 1/s strain rate, known as the MTS tests.

Fig. 4.

Representative Alpha alloy SEM fractographs from the four different sample geometries: (a) S1 – square cross-section, (b) S2 – medium rectangular cross-section, (c) S3 largest rectangular cross-section, and (d) S4 – smallest rectangular cross-section. Note that (a)-(c) show centered “brittle” fracture surrounded by cup-cone failure, while (d) shows 45° shear failure with a small amount of centered “brittle” fracture.

Fig. 5.

Side-view SEM micrographs of a) Alpha S1 geometry with asymmetric shear lips and b) Alpha S4 geometry with 45° shear failure.

Fig. 6.

High magnification SEM fractograph of center portion of Alpha failure. Sample tilted at 45°.

Fig. 7.

C alloy SEM fractographs showing the variety of fracture features observed. Left column shows microvoid failures under the different geometries. Right column shows knife-edge failures. (a) and (b) show microvoid and knife-edge failures in S1 geometry samples. (c) and (d) show microvoid and knife-edge failures in S3 geometry samples. (e) shows an S2 sample which failed by microvoid coalescence, as all S2 geometry C samples did. (f) shows an S4 geometry sample knife-edge failure, as all S4 geometry C samples failed by knife-edge.