Seetha R. Mannava

Surface amorphization of NiTi alloy induced by Ultrasonic Nanocrystal Surface Modification for improved mechanical properties

We report herein the effects of Ultrasonic Nano-crystal Surface Modification (UNSM), a severe surface plastic deformation process, on the microstructure, mechanical (hardness, wear), wettability and biocompatibility properties of NiTi shape memory alloy. Complete surface amorphization of NiTi was achieved by this process, which was confirmed by X-ray diffraction and high-resolution transmission electron microscopy. The wear resistance of the samples after UNSM processing was significantly improved compared with the non-processed samples due to increased surface hardness of the alloy by this process. In addition, cell culture study demonstrated that the biocompatibility of the samples after UNSM processing has not been compromised compared to the non-processed sample. The combination of high wear resistance and good biocompatibility makes UNSM an appealing process for treating alloy-based biomedical devices.

Application of laser shock peening for spinal implant rods

Purpose – The current industry standard rigid spinal implants suffer fatigue failures due to bending and torsion loads. The purpose of this program was to design novel prototype flexible titanium alloy spinal implant rod with machined features, and then apply the laser shock peening (LSP) process to restore the fatigue strength debit due to these features.Design/methodology/approach – A flexible prototype rod was designed with flat section at the center of the rod. The flat section was laser shock peened. Static compression tests were conducted as per American Society of Testing Materials standards for three‐ and four‐point bending tests and “vertebrectomy” constructs. Finite element models were developed to aid in the design of LSP and also to guide the experiments.Findings – The test results indicated a ∼3X improvement in flexibility and a reduction in fatigue load ratio, defined as applied load divided by the yield load; from 72 to 68 percent. This rod was LSPs on the flat sections, and tested again. ...

High spatial resolution, high energy synchrotron x-ray diffraction characterization of residual strains and stresses in laser shock peened Inconel 718SPF alloy

Laser shock peening (LSP) is an advanced surface enhancement technique used to enhance the fatigue strength of metal parts by imparting deep compressive residual stresses. In the present study, LSP was performed on IN718 SPF alloy, a fine grained nickel-based superalloy, with three different power densities and depth resolved residual strain and stress characterization was conducted using high energy synchrotron x-ray diffraction in beam line 1-ID-C at the Advanced Photon Source at the Argonne National laboratory. A fine probe size and conical slits were used to non-destructively obtain data from specific gauge volumes in the samples, allowing for high-resolution strain measurements. The results show that LSP introduces deep compressive residual stresses and the magnitude and depth of these stresses depend on the energy density of the laser. The LSP induced residual stresses were also simulated using three-dimensional nonlinear finite element analysis, with employment of the Johnson-Cook model for describing the nonlinear materials constitutive behavior. Good agreement between the experimental and simulated data was obtained. These various results are presented and discussed.

Continuum-based and cluster models for nanomaterials

As a result of the rapid advances in the synthesis of nanomaterials, developing robust modeling methods for evaluating the physical properties of nanomaterials is becoming increasingly important. Nanomaterials are distinguished by their near-perfect molecular structures at the nanometer scale and outstanding mechanical properties defined at the macroscopic level. Correspondingly, hierarchical modeling approach has the unique advantage in its ability to describe the structure-property relations. In this work, two different hierarchical approaches for describing the mechanics of nanomaterials are presented. The first model is a continuum-based model derived from the theory of crystal elasticity. The second is the virtual atom cluster (VAC) model recently proposed in [1, 2]. In the proposed continuum-based model, it is shown that the deformation measures introduced in the model strongly depend on the underlying atomistic model. As a result, the significances of these measures are fundamentally different from those in the classical continuum theory. The VAC model, on the other hand, is a discrete model that is extracted directly from the atomistic model. Following the presentation of the two models, a systematic comparison is presented in terms of the accuracy and range of applicability. Finally, the robustness of both models and their link to the atomistic model are discussed and shown through benchmark problems.

Effects of Ultrasonic Nanocrystal Surface Modification on the Residual Stress, Microstructure, and Corrosion Resistance of 304 Stainless Steel Welds

In this study, ultrasonic nanocrystal surface modification (UNSM) of 304 stainless steel welds was carried out. UNSM effectively eliminates the tensile stress generated during welding and imparts beneficial compressive residual stresses. In addition, UNSM can effectively refine the grains and increase hardness in the near-surface region. Corrosion tests in boiling MgCl2 solution demonstrate that UNSM can significantly improve the corrosion resistance due to the compressive residual stresses and changes in the near-surface microstructure.

Effects of Ultrasonic Nano-Crystal Surface Modification on the Microstructure and Properties of 304 Austenitic Stainless Steel

In this study, the effects of Ultrasonic Nano-crystal Surface Modification (UNSM) on the microstructure changes and the mechanical properties of austenitic stainless steel 304 were studied. Due to the dynamic impacts induced by the multiple strikes during UNSM, surface nanocrystallization and transformation to martensite has been achieved. The work-hardened surface layers (3.5 times the original hardness) lead to significant improvement in the mechanical properties as measured by nano-indentation and tensile test. The results demonstrate that UNSM is a powerful surface processing technique that can improve component mechanical properties and performance.Copyright