ISSN:
1573-4803
Source:
Springer Online Journal Archives 1860-2000
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes:
Abstract Overall strength (σ)–grain size (G), i.e. σ–G-1/2, relations retain the same basic two-branched character to at least 1200–1300°C. However, some polycrystalline as well as single crystal strength shifts or deviations are seen relative to each other, and especially relative to Young's moduli versus temperature for poly- and single crystals. The variety and complexity of these deviations are illustrated mainly by Al2O3, BeO, MgO and ZrO2 for which there is considerable data. At ∼22°C, Al2O3 polycrystals show substantial strength decrease due to H2O while MgO, ZrO2 and BeO polycrystals have limited, variable decreases. Al2O3 single crystals (sapphire) also show substantial strength decreases, but ZrO2 and MgO single crystals show little or none. Sapphire's strength markedly decreases from at least −196°C to a minimum in the 400–600°C range, then rises to a maximum at≥1000°C, followed by an accelerating decrease with further temperature increase. Polycrystalline Al2O3 shows similar (but less pronounced) strength minima and maxima, or alternatively an approximate strength plateau from ∼22 to ∼1000°C interrupting the normally expected strength decreases with increasing temperature at suitably large grain size and absence of defects (e.g. pores) dominating failure. BeO crystals show a linear strength decrease with increasing temperature (T) similar to that of Young's modulus. BeO polycrystals often show a significant strength (apparently grain size and impurity dependent) maximum (at ∼500–800°C) or plateau (from ∼22 to ∼1000°C) interrupting an otherwise continuous decrease. MgO shows similar temperature behaviour to BeO, but more pronounced crystal strength decrease and less pronounced polycrystalline strength maxima. Polycrystalline ZrO2 shows more rapid Young's modulus (E), and especially strength, decreases at ∼200–500°C than single crystals. More limited data for other materials also shows greater, variable σ–T versus E–T trends, e.g. MgAl2O4 has a similar, but less pronounced decrease than ZrO2. Collectively these deviations suggest variable impacts on primarily flaw controlled σ–G-1/2 behaviour due to factors such as microplasticity, machining stresses, and thermal expansion and elastic anisotropies requiring more comprehensive testing and evaluation to better sort out these effects.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1023/A:1018630113180
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