Vikram Bedekar

A Preliminary Study of Chemical Solubility of Ultra-Hard Ceramic AlMgB14 in Titanium: Reconciliation of Model with Experiment

Abstract This article investigates the chemical wear behavior of the ultra-hard ceramic AlMgB14 and cemented tungsten carbide for machining aerospace alloys. The chemical interdiffusivity of AlMgB14 against pure Ti and Ti-6Al-4V, in comparison with cemented carbide (WC-6%Co) cutting tool was investigated by means of diffusion couple experiments. The chemical composition profiles of various tool-workpiece combinations were determined by electron probe microanalysis after exposing the couples to 1000°C for 120 h in vacuum. Thermodynamic calculations of the chemical solubility of AlMgB14 show that the experimental diffusion results are in reasonable agreement with the predicted behavior. It is shown that AlMgB14 is significantly less soluble in titanium under static diffusion conditions, and therefore, shows considerable promise as a potential cutting tool for machining Ti alloys.

In-situ neutron diffraction and crystal plasticity finite element modeling to study the kinematic stability of retained austenite in bearing steels

Abstract This work integrates in-situ neutron diffraction and crystal plasticity finite element modeling to study the kinematic stability of retained austenite in high carbon bearing steels. The presence of a kinematically metastable retained austenite in bearing steels can significantly affect the macro-mechanical and micro-mechanical material response. Mechanical characterization of metastable austenite is a critical component in accurately capturing the micro-mechanical behavior under typical application loads. Traditional mechanical characterization techniques are unable to discretely quantify the micro-mechanical response of the austenite, and as a result, the computational predictions rely heavily on trial and error or qualitative descriptions of the austenite phase. In order to overcome this, in the present work, we use in-situ neutron diffraction of a uniaxial tension test of an A485 Grade 1 bearing steel specimen. The mechanical response determined from the neutron diffraction analysis was incorporated into a hybrid crystal plasticity finite element model that accounts for the martensites crystal plasticity and the stress-assisted transformation from austenite to martensite in bearing steels. The modeling response was used to estimate the single crystal elastic constants of the austenite and martensite phases. The results show that using in-situ neutron diffraction, coupled with a crystal plasticity model, can successfully predict both the micro-mechanical and macro-mechanical responses of bearing steels while accounting for the martensitic transformation of the retained austenite.

Tackling Characterization Challenges in High Deformation/Stress Steel Alloys Using Transmission Kikuchi Diffraction (TKD)

Although steels have been extensively studied, the application of traditional characterization methods to investigate the microstructure still poses significant challenges. One example is White Etched Areas (WEA) that are microstructural alternations in bearings induced by dynamic loading conditions [1]. Another example is ‘white layers’ at machined steel surfaces, which are generated by hard turning processes [2]. Both involve formation of nanostructured features at the surface that may lead to significant influence on surface-initiated damage, such as corrosion, fatigue and wear surface deformation. These highly deformed regions have grains that range in size from a few nanometers to 100nm and may consist of small pockets of retained austenite. In some cases preexisting carbides are no longer present in the deformed regions. In other processes such as low temperature carburization/nitridation, the challenges are not as much the structural refinement but primarily the very high level of lattice deformation and formation of nanometer size nitrides [3]. Here as well, the large stresses and very high level of interstitial alloying can result in local phase transformation that is not easily identified by scanning electron microscopy (SEM) or conventional transmission electron microscopy (TEM) without extensive effort. In these materials, sample preparation adds to the characterization challenge. Preserving the original microstructure without introducing any mechanical damage during preparation is critical. At the same time, producing adequate samples for investigating these nanometer scale features demands sample thickness and quality similar to high resolution TEM.