O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 1 Tungsten Armored Ferritic Steel Glenn Romanoski & Lance Snead June 2004

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 2 Phase I : Fabrication Process and Repair Tungsten Armored Low Activation Ferritic Steel Objective: select and optimize methods for bonding tungsten to a Low Activation Ferritic Steel and assess the integrity of these coatings under IFE relevant thermal fatigue conditions. Approach: -Evaluate methods for applying tungsten coatings to F82H steel substrates. Fabricate and study adherence and thermal stability. Is this material combination viable? FY-04 Milestone. -Given W thickness (100μm to 250µ nominal) and thermal boundary conditions, assess the stability and fatigue performance of the underlying LAF. FY-04 Milestone. -Screen coupon coatings using thermal fatigue facility. Select candidate monolithic armor system or move to “engineered structure.” FY-05 Milestone.

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 3 IFE Relevant Performance Assessment is Critical to Armor Design and Material Selection  The goal of materials performance assessment is to be relevant if not equivalent.  The goal of materials selection and design is to meet or exceed the operational and durability requirements of the IFE first wall.  How close is our most relevant thermal fatigue test conditions to IFE equivalent conditions?

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 4 IR Thermal Fatigue Facility Facility has been used for interfacial fatigue of W/LAF Previously 20 MW/m2 (time average), 20 msec pulse, 10 Hz, 10 cm 2

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 5 Facility Improvements : IR Thermal Fatigue Now capable of 100 MW/m2 (time average), 2 msec pulse, 10 Hz, 5 cm 2 Phase 1 goal 1000 MW/m2 (time average), 0.1 msec pulse, 10 cm 2

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 6 Simulating HAPL Interface Stresses  The IR heat load cannot duplicate tungsten surface stresses, but it can duplicate the interface stresses  If the tungsten layer is sufficiently thick, preserving the time-averaged heating is sufficient  The key metric is the stress distribution

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 7 Comparison of Stress Distribution  7.2 MW/m2  20 ms pulse

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 8 F82H Ferritic Steel with 100µm of Tungsten Armor is the Reference Material/Design Solution Infrared fusion of tungsten powder Diffusion bonding of tungsten foil Vacuum plasma spraying powder Alternative approaches, e.g., CVD Processing Method Method of Screening Thermal stability of the interface will be assessed under cyclic and isothermal conditions. Thermal fatigue performance of the tungsten armor and substrate will be assessed with the IR plasma arc lamp. Interfacial Strength will be measured using flexural tests.

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 9 Preliminary Thermal Cycling Test Results Illustrate the Issues Pertinent to Material Selection, Processing and Testing. Processing Method Initial tests showed promise -----Coatings adhered after 1000 cycles. Tensile cracks developed in the substrate due to CTE mismatch and phase changes. Diffusion bonding below the phase transformation temperature will be tried. Dissolution of carbides in the steel at the interface indicated that the temperature probably exceeded 900ºC. Diffusion of tungsten into the steel could generate brittle phases such as FeW and Fe 2 W. Thermal management of the substrate and incident heat flux is critical to a meaningful thermal fatigue test. Diffusion bonded tungsten foil

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 10 Thermal conductivity of W and F82H defines the interface temperature for fixed geometry and thermal boundary conditions.

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 11 Microstructural stability of F82H will limit the interface temperature to about 800C  Coarsening of carbides in the F82H steel above 800C and dissolution around 900C will degrade mechanical properties.  The alpha – gamma - alpha phase transformation and CTE mismatch will impart strains at the interface.  A critical thickness of tungsten may be required to dissipate the heat pulses to maintain the interface in an acceptable temperature regime. Will that be adequate?  A thermal model of our experiment and appropriate instrumentation of specimens is the key to running meaningful experiments.

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 12 Alternative Design/Material Approaches  Given the selection of a ferritic steel as the substrate material, the interface temperature is limited to 800ºC. What thickness of W coating and back face heat flux is required to maintain a stable interface? Is 600ºC an appropriate far field temperature?  Given the selection of tungsten as the armor material, are there other substrate materials with higher temperature capability?  Group VA alloys (V, Nb or Va) could serve as an intermediate layer having higher temperature capability. Sensitivity to oxygen would be a consideration.

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY Milestone : Go/No Go on tungsten armor. Is tungsten clad F82H steel a viable material option? ! Where are we ??? Vacuum plasma sprayed W on F82H is the principal material candidate. A number of material conditions are ready for testing. Additional material conditions exploring the extremes of the W/F82H material option will be produced, e.g., thick tungsten coatings. Achieving thermal similitude between our thermal cycling test and the IFE condition is critical. Well modeled and instrumented tests are our goal. Long-term stability of interface is required. Is the temperature limits imposed by the current choice of materials and/or design too limiting for the thermal boundary conditions of the IFE first wall? Do these temperature limits result in desired system efficiency?

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 14 Development of Armor Fabrication process and repair He management Mech. & thermal fatigue testing “ Engineered Structures ” Ablation Underlying Structure bonding (especially ODS) high cycle fatigue creep rupture Armor/Structure Thermomechanics design and armor thickness detailed structural analysiis thermal fatigue and FCG Structure/Coolant Interface corrosion/mass transfer/coating Development of W/LAF : Phase 1 Effort and Milestones ! ! ! ! } ! scopingoptimizationscaling ! ! scoping & modelingoptimization !!

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 15 Comparing Data from Different Exposure Experiments  It is customary to correlate surface effects with fluence, but exposure times and deposition depths vary across the different experiments  This implies different stresses for the same fluence  If roughening is a thermomechanical phenomenon, then we must compare thermomechanical results across the experiments (peak surface temperature, peak surface temperature gradient, peak stress, plastic strain range, stress intensity factor, etc.)  We intend to use peak surface temperature, stress intensity factor, and plastic strain range for our comparisons  If the correlation across the experiments is consistent, then we will use this as the primary design criterion  It is important that we are able to deduce the damage mechanism as a result of this process Fixed fluence; surface heat

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 16 RHEPP Simulations (p, N+, N++)

O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 17 Next Step: Fracture Comparisons