Environmentally Assisted Cracking: Theory and Practice: Environmentally Assisted Embrittlement and Cracking I
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 8:30 AM
March 1, 2017
Room: 31A
Location: San Diego Convention Ctr

Session Chair: Reiner Kirchheim, University of Göttingen; Bai Cui, University of Nebraska-Lincoln

8:30 AM  Invited
Hydrogen Embrittlement and Stress Corrosion Cracking as Examples of the Chemomechanics of Solids: Reiner Kirchheim1; 1University of Goettingen
     Fracture and deformation of crystals occurs by formation, annihilation and motion of lattice discontinuities. Thus vacancies and dislocations cause plastic flow or new surfaces form during fracture. The energy of generating these discontinuities, γ, is affected by a chemical species A as described by the extended Gibbs Adsorption Equation(1) dγ=-Γ*dμ. Γ is the excess of A at the discontinuity and μ is the chemical potential of species A. Thus in Eq. 1 mechanics of plasticity as determined by γ is related to chemistry via Γ and μ. Eq. 1 is derived in its general way and statistical thermodynamics allows calculating the excess Γ. Emphasis is placed on new modeling and experiments on the ideal work of fracture and its changes with increasing hydrogen pressure or chemical potential, respectively. [1] R. Kirchheim, Acta Materialia 55 (2007) 5139-5148; [2] Kirchheim, R., Somerday, P.B., Sofronis, P., Acta Materialia 99 (2015) 87–98

9:10 AM  
The Role of Hydrogen-enhanced Strain-induced Lattice Defects on Hydrogen Embrittlement Susceptibility of X80 Pipeline Steel: Moeko Hattori1; Hiroshi Suzuki1; Kenichi Takai1; Yusuke Seko2; 1Sophia University; 2Tokyo Gas
    The associated factors which lead to hydrogen embrittlement have not been fully clarified yet for ferrite/bainite X80 pipeline steel. Hydrogen embrittlement susceptibility was evaluated based on fracture strain in tensile testing. Tracer hydrogen content corresponding to the amount of lattice defects was measured by a thermal desorption analysis. Hydrogen embrittlement susceptibility and the amount of tracer hydrogen significantly increased with decreasing crosshead speed. In addition, the rate of formation of hydrogen-enhanced strain-induced lattice defects became maximum immediately before its final fracture. Fracture surface of the hydrogen-charged specimen exhibited shallower dimples without nucleuses such as second phase particles compared with hydrogen-free specimen. These findings indicate that hydrogen enhances formation of the lattice defects especially just before final fracture occurred, which enhance to form shallower dimples, and probably cause the premature fracture of X80 pipeline steel at lower crosshead speeds.

9:30 AM  
Consequence of Hydrogen Desorption on Local Mechanical Properties and the Fracture Mechanisms of a Martensitic Steel: Abdelali Oudriss1; Hélène Morillot2; Rémy Milet1; Cyril Berziou1; Stephane Cohendoz1; Jean-Michel Sobrino3; Juan Creus1; Xavier Feaugas1; 1University of La Rochelle; 2CETIM-Matériaux Métalliques et Surfaces; 3CETIM-Matériaux Métalliques et Ingénierie de Surface
    Increasing the mechanical properties and corrosion protection systems of the fastening elements is a major challenge for a better strength of components. However, this approach leads to a greater sensitivity of these elements to the hydrogen embrittlement. Indeed, the hydrogen can be introduced mainly through the surface treatment process and may lead to a significant decrease of mechanical strength. To remedy this, a baking step is applied to remove the hydrogen. However, because of a metallurgy increasingly complex, and high mechanical properties, the effectiveness of this step is questionable. So, in this study we are interested in the influence of time and temperature during the hydrogen baking process on the mechanical resistance of a martensitic steel. Thus, we have established a relationship between the microstructure, the hydrogen desorption kinetics and delayed fracture mechanisms. Furthermore, by adopting a local approach of fracture, we identified local mechanical characteristics sensitive to the hydrogen.

9:50 AM  
Design of Nickel Alloys and Superalloys with a High Resistance to Hydrogen Embrittlement: Franck Tancret1; Miles Stopher2; Edern Menou1; Gérard Ramstein1; Pedro Rivera-Díaz-del-Castillo2; 1Université de Nantes; 2University of Cambridge
    In nuclear and in oil & gas applications, nickel alloys are used in conditions where hydrogen embrittlement (HE) can occur. Literature suggests, with limited evidence, that MC carbides and γ’ with a high γ/γ’ lattice misfit may act as hydrogen traps and increase the resistance to HE (RHE). Exploiting both computational thermodynamics (Thermo-Calc CALPHAD software) and a Gaussian process model for γ/γ’ lattice misfit, genetic algorithm multi-objective optimisation (GAMOO) was used to design model alloys to verify experimentally such features, by targeting alloys containing either only finely dispersed MC vanadium carbides, or only γ’ with a controlled γ/γ’ lattice mismatch. GAMOO is finally used to optimise simultaneously all the relevant structural, chemical and microstructural characteristics; new superalloys are designed, having combinations of good strength, corrosion resistance and resistance to hydrogen embrittlement, for nuclear and oil & gas applications.

10:10 AM Break

10:30 AM  
Corrosion of Nickel-Titanium, C110, and Al6061 in Gallium-based Liquid Metal Alloys: Jacob Mingear1; Darren Hartl1; 1Texas A&M University
    Nickel-titanium is one of the most common shape memory alloys (SMAs). SMA-based actuators are limited by their actuation response frequency, specifically slow cooldown. Polyalphaolefin and water have been looked into as fluids for active cooling mechanisms. [1] However, liquid metal can act as a more effective coolant due to its high thermal conductivity. Currently, there is no literature on liquid metal corrosion on nickel-titanium, which is crucial to understanding the viability of using liquid metal as an active coolant for SMA-based actuators. Specimens of nickel-titanium will be placed in liquid metal-filled crucibles of at 400°C for 300 hours. Liquid metals will include gallium, gallium-indium eutectic, and gallium-indium-tin eutectic. Specimens will be characterized using scanning electron microscopy and energy-dispersive spectroscopy to investigate the penetration of liquid metal, penetration rates, and reaction layer chemistry. [1] A. M. Coppola, Advanced Engineering Materials, Mar. 2016. DOI: 10.1002/adem.201600020

10:50 AM  
Sensitization Effects on Tensile Behavior in 5XXX Series Aluminum Alloys: Macro- and Mesoscale Observations: Benjamin Palmer1; John Lewandowski1; 1Case Western Reserve University
    The desirability of 5XXX series Aluminum-Magnesium alloys for Naval use is tempered by their propensity to sensitize in the application environment. A contributor to this is the β phase, Al3Mg2, and non-equilibrium variants that preferentially precipitate at grain boundaries with various thermal exposures. These phases are anodic with respect to the matrix and can corrode and/or provide a path for environmentally enhanced cracking. However, the mechanisms behind this and other phases which may contribute in industrial alloys are less well known. In this study, interrupted tensile tests and microscopy are used to observe how strain accommodation is affected by sensitization in samples from the Short-Transverse (ST) direction of 5083-H116 rolled plates. The ST direction is chosen as it suffers the most severe mechanical degradation from sensitization. Additionally, Differential Scanning Calorimetry is used to characterize materials with different thermal exposures to search for phases that contribute to sensitization in such alloys.

11:10 AM  
Strain Rate Effects on the Stress Corrosion Cracking Behavior of Ni and Co Based Superalloys for Marine Applications: Allison Popernack1; James Burns1; 1University of Virginia Center for Electrochemical Science and Engineering
    The effect of strain rate on stress corrosion cracking (SCC) susceptibility is poorly understood for fracture mechanics based characterization, thus yielding an incomplete view of possible crack growth behavior experienced in service. This study aims to quantify the effect of applied stress intensity rate on the measured crack growth rate (da/dt) in Co- (MP98t) and Ni-based (Monel K-500) superalloys, known to be susceptible to H-embrittlement. Slow-rising stress intensity testing conducted under cathodic polarization in NaCl solution using real-time crack growth measurements, via direct current potential difference (DCPD), establishes the effect of strain rate on SCC behavior. Results demonstrate a clear increase in Stage II crack growth rate (da/dtII) for both materials as the applied strain rate increased. Monel K-500 exhibits distinct upper and lower da/dtII bounds. These data provide mechanistic understanding and inform the protocol necessary for accurate, conservative fracture mechanics based characterization of SCC susceptibility.

11:30 AM  
Stress-corrosion Cracking in Ti-8Al-1Mo-1V: Sheng Cao1; Chao Voon Samuel Lim1; Su-Ming Zhu1; Xinhua Wu1; 1Monash University
    In SCC of Ti-8Al-1Mo-1V, there was a fracture mode transition from ductile dimples during pre-crack to flat facets during SCC. In order to understand the SCC mechanisms, T-EBSD and TEM analysis were applied on FIB lift-out lamellas cut from pre-cracking and SCC fractured regions to investigate the difference in the dislocation substructure underneath the fracture surface. It has been found that: • In pre-cracking region, basal α slip was the dominate slip system. While, basal α and c+a slip were observed underneath SCC facets. • An increase in dislocation density was observed underneath SCC facets compared to that below the pre-crack dimples. • A combined AIDE and HELP could be the SCC mechanism.