Spherical Nanoindentations and Kink Bands in Ti 3 SiC 2
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We report for the first time on load versus depth-of-indentation response of Ti3SiC2 surfaces loaded with a 13.5 m spherical tipped diamond indenter up to loads of 500 mN. Using orientation imaging microscopy, two groups of crystals were identified; one in which the basal planes were parallel to, and the other normal to, the surface. When the load-penetration depth curves were converted to stress-strain curves the following was apparent: when the surfaces were loaded normal to the c axis, the response at the lowest loads was linear elastic—well described by a modulus of 320 GPa—followed by a clear yield point at approximately 4.5 GPa. And while the first cycle was slightly open, the next 4 on the same location were significantly harder, almost indistinguishable, and fully reversible. At the highest loads (500 mN) pop-ins due to delaminations between basal planes were observed. When pop-ins were not observed the indentations, for the most part, left no trace. When the load was applied parallel to the c axis, the initial response was again linear elastic (modulus of 320 GPa) followed by a yield point of approximately 4 GPa. Here again significant hardening was observed between the first and subsequent cycles. Each cycle resulted in some strain, but no concomitant increase in yield points. This orientation was even more damage tolerant than the orthogonal direction. This response was attributed to the formation of incipient kink bands that lead to the formation of regular kink bands. Remarkably, these dislocation-based mechanisms allow repeated loading of Ti3SiC2 without damage, while dissipating significant amounts of energy per unit volume, Wd, during each cycle. The values of Wd measured herein were in excellent agreement with corresponding measurements in simple compression tests reported earlier, confirming that the same mechanisms continue to operate even at the high (≈9 GPa) stress levels typical of the indentation experiments. I. INTRODUCTION
The carbide Ti3SiC2 is a hexagonal layered compound belonging to a family of over 50 ternary carbides and nitrides with the general formula MN+1AXN, where n ⳱ 1 to 3, M is an early transition metal, A is an A-group (mostly IIIA and IVA) element, and X is C and/or N1. These compounds—best described as thermodynamically stable nanolaminates—possess an unusual combination of properties.1 The most studied, and best understood, to date is Ti3SiC2. It has the same density as Ti, but is almost three times as stiff and yet is readily machinable.1–3 It is a better electrical and thermal conductor than Ti metal3; highly resistant to damage,4,5 thermal shock,3,4 fatigue,6 and creep.7 It exhibits an increasing R-curve behavior,6 with toughness values as high as 16 MPa⭈√m. It is now fairly well-established that the Vickers microhardness values in Ti3SiC2 decrease with increasing loads down to a plateau value.2,3,5,8,9 Furthermore, it is not possible to induce cracking from the corners of VickDOI: 10.1557/JMR.2004.0148 J. Mater. Res., Vol. 19, No. 4, Apr 2004
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