Ultrahigh dense and gradient nano-precipitates generated by warm laser shock peening for combination of high strength and ductility Chang Ye a , Yiliang Liao a , Sergey Suslov b , Dong Lin a , Gary J. Cheng a,c,n a School of Industrial Engineering, Purdue University, West Lafayette, IN 47906, USA b School of Materials Engineering, Purdue University, West Lafayette, IN 47906, USA c School of Mechanical Engineering, Purdue University, West Lafayette, IN 47906, USA article info Article history: Received 23 January 2014 Received in revised form 30 April 2014 Accepted 3 May 2014 Available online 14 May 2014 Keywords: Warm laser shock peening Dynamic strain aging Nanostructure Precipitate Strength Ductility abstract Nanocrystalline materials generated by severe plastic deformation often come with high strength but low ductility due to the inability to accumulate dislocations and thus the low work hardening rate. In this study, a unique high strain rate deformation process – warm laser shock peening (WLSP) – is studied to generate extremely high-density nano-precipitates in precipitation hardenable alloy. Aluminum alloy (AA) 7075 was selected to evaluate the generation of ultra-high-density precipitates by WLSP and the effects on the strength and ductility. WLSP integrates the advantages of laser shock peening (LSP), dynamic strain aging (DSA) and dynamic precipitation (DP). The nanoscale precipitate particles generated by WLSP effectively block dislocations and thus increase the material strength. The precipitate–dislocation interaction has been observed by high resolution TEM (HR-TEM) and modeled by the multiscale discrete dislocation dynamic (MDDD) model. It has been demonstrated that compared with room temperature LSP, WLSP can improve material strength by 32.3% without compromising the ductility, in that elongation remains 20%. These ultra-high-density nano-precipitates greatly improve dislocation accumulation capacity and thus effectively increase ductility. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Severe plastic deformation (SPD) has been widely used to improve strength through nanocrystalline formation [1–4]. How- ever, the ductility is often compromised due to the inability for dislocations to accumulate and thus low work hardening rate in nanocrystalline structure. It has been a long time question to find out a microstructure that has high strength without sacrificing the ductility. There have been many attempts to produce materials with both high strength and high ductility [5,6], for example, by designing bi-modal microstructures [7], gradient nanostructures [8,9], by introducing coherent nanoscale twin boundaries [10,11], or by precipitate particles [12–16]. Among these methods, pre- cipitation hardening has proved to be an important method to enhance mechanical properties [16]. The effects of precipitates on property enhancement greatly depend on precipitate size and number density. There have been limited ways to generate ultra- fine and high-density precipitates. Recently, generation of nano- precipitate in aluminum alloy [17,18] and carbon steel [19,20] has been reported to have promising applications on mechanical properties enhancement, such as fatigue resistance. However, the mechanism of how these nano-precipitates could be used to improve strength and ductility remains unclear. This limits the selection of processing techniques for ideal mechanical properties. In general, precipitates can effectively strengthen material by imposing resistance to dislocation slip. On the other hand, ductility could also be favorably affected by the precipitates due to the improved dislocation accumulation capacity acquired from disloca- tion–precipitate interaction. However, in order to take full advan- tage of precipitates, their size, density and distribution need to be optimized. This study will develop a novel process, warm laser shock peening (WLSP), in order to generate ultrahigh-density nano- precipitates and significantly increase strength without loss of ductility in precipitation hardenable materials. WLSP integrates the advantages of laser shock peening (LSP), dynamic strain aging (DSA) and dynamic precipitation (DP) [17,19,20]. In order to make use of WLSP for precipitation hardenable materials with high strength and ductility, there are several critical problems to address. First, what is the technical route for extremely high-density nano- precipitates and dislocations in metals? Second, how these nano- precipitates and dislocations interact and thus affecting the strength and ductility of materials? Third, how does the distribution of the nano-precipitates affect the mechanical properties? In order to address these problems, WLSP of aluminum alloy (AA) 7075 will be carried out in the following aspects. First, the effects of processing conditions of WLSP on dislocation density, precipitate Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A http://dx.doi.org/10.1016/j.msea.2014.05.003 0921-5093/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ765 4945436. E-mail address: gjcheng@purdue.edu (G.J. Cheng). Materials Science & Engineering A 609 (2014) 195–203