The weakest size of precipitated alloys in the micro-regime: The case of duralumin
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Rui Gu Materials Characterization and Preparation Center, Southern University of Science and Technology, Shenzhen, People’s Republic of China
Alfonso H.W. Ngana) Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, People’s Republic of China (Received 4 January 2017; accepted 14 April 2017)
In the microsize regime, all crystalline metals studied to-date exhibit a “smaller-is-stronger” size effect. Here, we report an unusual weakest-size phenomenon in the precipitated alloy duralumin 2025, i.e., below a critical size of ;7 lm, the strength increases as the size decreases, while above this size, the strength increases toward the bulk value with increasing size. At the critical size, strain-hardening is also slowest and the room-temperature creep is fastest. Interestingly, the reduction of strength at the weakest size is more significant for the peak-aged state of duralumin 2025 than its naturally aged state. Theoretical modeling shows that at the weakest size, both strengthening mechanisms of precipitation hardening and dislocation starvation are ineffective. The present results indicate that the conventional wisdom of precipitation hardening is not applicable in the micro-regime, and the common “smalleris-stronger” understanding is incorrect when material microstructures impose internal length scales that can affect strength.
I. INTRODUCTION
The phenomenon of the “smaller-being-stronger” size effect of materials has attracted a lot of attention since first reported in the 1950s via tensile tests on metallic whiskers.1,2 The past decade has seen the unveiling of the defining characteristics of such a phenomenon through compression tests on micropillars of metals fabricated by means including focused ion-beam (FIB) milling.3–5 For metals with a monolithic, single crystalline microstructure in the submicron size regime, the condition of “dislocation starvation” generally prevails, in which the deforming specimen is continuously depleted of dislocations due to their ease of zipping out of the specimen volume.5–8 Generation of new dislocations is continuously needed to sustain the deformation, leading to successive emission of strain bursts.9–11 In this regime, the strength data scatter significantly, precluding clear-cut conclusion on whether strength depends on size to be drawn, although theoretically it has been suggested that a weak stronger-being-smaller size effect should exist, as
Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2017.167
a smaller specimen should contain fewer nucleation sites for dislocations.12,13 In the size regime of over a micron, the strength is found to vary with the specimen size according to a power law,4,5,12,14,15 and plastic flow still occurs in a jerky manner through the continuous emission of strain bursts.12,13,16 In the “source truncation” mechanism,17–19 single-arm dislocation sources truncated by free surfaces determine the stren
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