Synthesis and elastic and mechanical properties of Cr 2 GeC
- PDF / 768,089 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 1 Downloads / 214 Views
Herein we report on the synthesis and characterization of Cr2GeC, a member of the so-called Mn+1AXn (MAX) phase family of layered machinable carbides and nitrides. Polycrystalline samples were synthesized by hot pressing pure Cr, Ge, and C powders at 1350 °C at ∼45 MPa for 6 h. No peaks other than those associated with Cr2GeC and Cr2O3, in the form of eskolaite, were observed in the x-ray diffraction spectra. The samples were readily machinable and fully dense. The steady-state Vickers hardness was 2.5 ± 0.1 GPa. The Young’s moduli measured in compression and by ultrasound were 200 ± 10 and 245 ± 3 GPa, respectively; the shear modulus and Poisson’s ratio deduced from the ultrasound results were 80 GPa and 0.29, respectively. The ultimate compressive strength for a ∼20 m grain size sample was 770 ± 30 MPa. Samples compressively loaded from 300 to ∼570 MPa exhibited nonlinear, fully reversible, reproducible, closed hysteretic loops that dissipated ∼20% of the mechanical energy, a characteristic of the MAX phases, in particular, and kinking nonlinear elastic solids, in general. The energy dissipated is presumably due to the formation and annihilation of incipient kink bands. The critical resolved shear stress of the basal plane dislocations—estimated from our microscale model—is ∼22 MPa. The incipient kink band and reversible dislocation densities, at the maximum stress of 568 MPa, are estimated to be 1.2 × 10−2 m−3 and 1.0 × 1010 cm−2, respectively.
I. INTRODUCTION
The Mn+1AXn phases (where M is an early transition metal, A and A-group element and X is C or N) are layered hexagonal solids with two formula units per unit cell, in which near close-packed layers of M are interleaved with layers of pure A-group elements, with the X atoms filling the octahedral sites between M layers. At this time, it is fairly well established that these phases have an unusual and sometimes unique combination of properties. They are excellent electrical and thermal conductors, thermal shock resistant, and damage tolerant.1,2 Despite being elastically stiff, they are all readily machinable with nothing more sophisticated than a manual hacksaw.3,4 Moreover, some of them are fatigue, creep, and oxidation resistant.5–10 The ternary carbide, Cr2GeC, is a MAX phase.10,11 Like the others, its unit cell is hexagonal (space group D46h–P63/mmc), with unit cell parameters, a ⳱ 2.954 Å and c ⳱ 12.08 Å. This phase was first discovered by
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0262 J. Mater. Res., Vol. 23, No. 8, Aug 2008
http://journals.cambridge.org
Downloaded: 18 Mar 2015
Jeitschko et al. in 1963.12 As far as we are aware, with the exception of two recent papers,13,14 there are no reports in the literature on its synthesis or characterization in bulk. In one of the papers13 we measured the functional dependences of its lattice parameters to quasihydrostatic pressures of the order of 50 GPa and found it to be high (182 ± 2 GPa); no phase transformations were observed. We also reported1
Data Loading...
