Environmentally Assisted Cracking: Theory and Practice: Environmentally Assisted Embrittlement and Cracking II
Sponsored by: TMS Structural Materials Division, TMS: Corrosion and Environmental Effects Committee, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Bai Cui, University of Nebraska–Lincoln; Raul Rebak, GE Global Research; Sebastien Dryepondt, Oak Ridge National Laboratory; Srujan Rokkam, Advanced Cooling Technologies

Wednesday 2:00 PM
March 1, 2017
Room: 31C
Location: San Diego Convention Ctr

Session Chair: James Burns, University of Virginia; Ilaksh Adlakha, Arizona State University

2:00 PM  Invited
The Effect of Composition, Temper, and Crack Orientation on the Stress Corrosion Cracking Behavior of Al-Mg Alloys: James Burns1; Amber Lass1; Michael McMurtrey1; Matthew McMahon1; Patrick Steiner1; Sarah Fakler1; 1University of Virginia
    Al-Mg alloys are susceptible to intergranular stress corrosion cracking (IGSCC) following precipitation of an active β phase on grain boundaries during exposures at temperatures >50°C. Fracture mechanics-based testing evaluates the influence of material composition, temper, and crack orientation on KISCC and da/dtII. 5083 is only slightly more susceptible than 5456 at a constant temper. For a constant composition the temper demonstrates a more pronounced influence with H131 being the most susceptible followed by H116 then a solution heat treated and quenched condition. Trends are analyzed based on strength differences, Mg segregation, and inaccuracies of proxy used to describe the level of β precipitation. SL cracking is substantially more susceptible than TL. Fracture surface fissures are observed parallel to the loading axis in TL samples, the beneficial effect of this delamination toughening on the H-embrittlement mechanism is rationalized through decohesion-based micro-mechanical models.

2:40 PM  
Effect of Mechanical Stresses on the Pitting Corrosion Behavior of an Al7075 Alloy: Scott Turnage1; Ilaksh Adlakha1; Amm Hasib1; Sridhar Niverty1; Nikhilesh Chawla1; Kiran Solanki1; 1Arizona State University
    Lightweight alloys such as Al7075 are regularly used in aviation due to their excellent mechanical performance with minimal weight penalty. However, in the saline environments experienced by naval aircraft, microstructural heterogeneities in the Al7075 alloy make it susceptible to corrosion damage thereby compromising the structural integrity. Therefore, in this work, the effect of mechanical loading on the pitting corrosion behavior of Al7075 alloy at various pre-strained levels and different chloride concentrations will be discussed with the help of cyclic polarization curves along with ex situ X-ray tomography. Further, the susceptibility of pitting corrosion and re-passivation under increasing chloride concentrations and mechanical load will be discussed. The study will be complemented with corrosion morphology analysis using ex situ X-ray tomography to provide insights into the 3-D microstructural influence on the growth of pits. Overall, the electrochemical behavior under conditions promoting stress corrosion cracking will be determined.

3:00 PM  
Relationships between the Galvanic Driving Force and Strain Energy Density Accumulation: Andrea Nicolas1; Alberto Da Silva Mello Junior1; Michael Sangid1; 1Purdue University
    Galvanic corrosion is the primary failure mechanism for sea-based structures, as it plays an important role in material degradation and structural integrity. We are currently in need of life prediction tools that unify chemistry with mechanics. To begin developing these tools, we need simulations capable of representing the actual heterogeneity inside the material, i.e. the microstructure, and thus can establish the critical points inside the material during galvanic corrosion. Therefore we are using an elasto-viscoplastic fast fourier transform (EVP-FFT) formulation to model the microstructure of the material, digital image correlation (DIC) to measure the surface strains, and optical profilometry to measure the surface degradation in columnar grain materials. The combined experiments and modeling is used to identify relationships between the microstructure and local micromechanical fields with the surface damage from galvanic corrosion given a variety of loading and environmental conditions.

3:20 PM  
The Effects of Alloy Chemistry on Localized Corrosion of Austenitic Stainless Steels: David Sapiro1; Bryan Webler1; 1Carnegie Mellon University
    This study investigated the effect of alloying on localized corrosion behavior of austenitic stainless steels in stressed and unstressed conditions. While austenitic stainless steels are widely used in corrosive environments, they are vulnerable to pitting and stress corrosion cracking (SCC), particularly in chloride-containing environments. The corrosion resistance of austenitic stainless steels is closely tied to the alloying elements chromium, nickel, and molybdenum. Polarization curves were measured for five commercially available austenitic stainless steels of varying chromium, nickel, and molybdenum content in 3.5 wt% and 25 wt% NaCl solutions. The alloys were also tested in tension at slow strain rates in air and in a chloride environment under different polarization conditions to explore the relationship between the extent of pitting corrosion and SCC over a range of alloy content and environment.

3:40 PM Break

4:00 PM  
Intergranular Hydrogen Embrittlement: Hydrogen Diffusion in Nickel Singles Crystals and Bi-crystals: Jiaqi Li1; 1University of La Rochelle
    Hydrogen Embrittlement (HE) is one of the causes mainly evoked in premature rupture of industrial components exposed to aggressive environment. Many studies have been conducted in order to understand the mechanisms involved during this degradation. The effects of the defects in FCC materials remain a controversy, although a number of theories have been proposed to describe the role of grain boundaries (GBs) for hydrogen diffusion and segregation, none of them is able to give an exact answer. In addition, there is little suitable experimental data available. Therefore, we have studied the hydrogen diffusion in single crystals and bi-crystals with different GBs (Σ11-50°30<110>{311}, Σ11-129°30<110>{332}, Σ3-70°30<110>{111} and Σ5-37°<100>{310}). We evaluate the hydrogen diffusion and trapping mechanisms using the electrochemical permeation (EP) coupled to the thermal desorption spectroscopy (TDS). Molecular Dynamics (MD) have become also a useful method to comprehend the becoming of hydrogen in these types of GBs.

4:20 PM  
Diffusion, Trapping Mechanisms and Some Implications on Local Approach of Fracture in Martensitic Steel: Stéphane Cohendoz1; Cyril Berziou1; Christelle Rebere1; Remy Milet1; Catherine Savall1; Abdelali Oudriss1; Jamaa Bouhattate1; Juan Creus1; Xavier Feaugas1; 1Université de La Rochelle
    Hydrogen diffusion and trapping mechanisms have been revisited in martensitic steels with a large variability of microstructural parameters. Electrochemical permeation set-up associated with thermal desorption spectroscopy and elastic theoretical calculations was methodically used to find a relationship between physical parameters associated with diffusion and trapping and microstructure evolutions. Using FEM calculations and a new design of permeation testing under tensile loading until fracture we revisit the question of hydrogen embrittlement of martensitic steel. We explore the impact of mobile and trapped hydrogen on ductile and brittle fracture of martensitic steel. We highlight that trapped hydrogen promotes ductile fracture and in opposite, that the mobile hydrogen conduces to a quasi-clivage fracture mode at a scale of martensitic laths. Finally, the contribution of plasticity and hydrostatic state on damage is discussed using local approach of fracture.

4:40 PM  
Effect of Chemical Composition on Embrittlement of High Manganese TWIP Steel: Young-Ha Kim1; Tae Jin Song1; Sung Kyu Kim1; Il Jeong Park1; Yon-Kyun Song1; 1POSCO
    TWIP (Twinning Induced Plasticity) steels with high strength and excellent ductility show good formability and crash worthiness of car bodies in automotive industry. On the other hand, TWIP steels with high amount of alloying elements can hardly be free from Environmentally Assisted Cracking such as Hydrogen Embrittlement (HE) and Liquid Metal Embrittlement (LME) when Zn is coated on the steel surface to endure in corrosive environment. In this study, we investigate the effect of Al addition on embrittlement of the high manganese TWIP steels, which POSCO has developed, with tensile strength of 1180MPa and elongation of more than 35%. By varying Al content in the TWIP steels, change in HE and LME of the Zn coated TWIP steels will be examined.

5:00 PM Concluding Comments Speaker: Prof. Ian Robertson / Bai Cui