1. A Study of Welding Behavior for Molybdenum With and Without

Lanthanum Oxide Additions

Todd Leonhardt* and J.E.Gould** / *Rhenium Alloys, Inc., North Ridgeville, Ohio, USA / **EWI, Columbus, Ohio, USA

 

Molybdenum seamless tubing has been used in thermocouple sheathing for decades. The sheath protects the thermocouple components from exposure in the furnace environments of up to 2200°C. Tungsten inert gas welding has been used as the welding process to close the end of the thermocouple sheath. This process increases the grain size, both due to solidification and subsequent exposure of the molybdenum sheath during operational temperature. The molybdenum-welded closure (PM-Mo) has shown to have a significant quantity of porosity, which further promoted brittle behavior. A research program was undertaken to increase the durability of the thermocouple sheath by increasing the recrystallization temperature and in turn, decreasing the ductile-brittle transition temperature (DBTT). Molybdenum lanthanum oxide (M-L) was selected and prepared as seamless tubing for use as a thermocouple sheath. During the characterization phase of the study, the M-L closure demonstrated smaller heat affected zones, as well as a refined fusion zone with little or no porosity. The results of the comparison between unalloyed molybdenum (PM-Mo) and M-L will be shown along with a discussion of the mechanisms involved in welding molybdenum alloys.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

2. Comparison of Extruded Powder Metallurgy Rhenium and Hot Isostatic

Pressed Rhenium

Todd Leonhardt* and James Ciulik** / *Rhenium Alloys, Inc., North Ridgeville, Ohio, USA / **The University of Texas at Austin,
Department of Mechanical Engineering, Austin, Texas, USA

 

This study explored the different densification/compaction processes and their effect on the microstructures and mechanical properties of hot isostatic pressed rhenium and hot extruded rhenium in order, to determine if extrusion is a viable method for manufacturing fully dense rhenium rods. Rhenium is a heavy transition metal with a melting point of 3180°C and has the highest modulus of elasticity of all the refractory metals (420 GPa). It is extremely sensitive to processing conditions because of its high work-hardening coefficient. In this study, two 45mm diameter rhenium rods were processed through sintering; one rod was subsequently hot isostatic pressed and the other rod was hot extruded to 23mm diameter. The two processing methods are compared and contrasted through the differences in the resulting microstructures, morphologies of the fracture surfaces, and mechanical properties obtained from tensile testing at room temperature and at 1927°C.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

3. Comparison of Mechanical Properties of Powder Metallurgy Molybdenum

41% and 47.5% Rhenium as Hot Isostatic Pressed and Rotary Swaged

Todd Leonhardt* and James Ciulik** / *Rhenium Alloys, Inc., North Ridgeville, Ohio, USA / **The University of Texas at Austin, Department of Mechanical Engineering, Austin, Texas, USA

 

Rhenium and rhenium containing alloys are unique with high melting points, high moduli of elasticity and excellent high temperature mechanical properties. The most common rhenium containing alloys are molybdenum 41% rhenium (Mo41%Re) and molybdenum 47.5% rhenium (Mo-47.5%Re). The focus of the study was the mechanical properties and microstructures of hot isostatic pressed versus hot rotary swaged Mo41%Re and Mo47.5%Re alloy rods (32mm diameter). Both alloys are BCC solid solution alloys with work hardening rates half that of commercial-purity rhenium. Unlike pure molybdenum, Mo41% and Mo47.5% and rhenium have excellent cold and warm plasticity. Adding rhenium to molybdenum increases the ductility as the rhenium concentration increases. The primary deformation mechanism of both alloys is twinning. Twinning occurs during cold and warm working, which enhances the mechanical properties without sacrificing ductility while maintaining good strength at elevated temperatures. The results of the differences in rhenium concentration and processing methods on the mechanical properties at room temperature and at elevated temperatures are discussed.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

4. Comparison of Processing on the Mechanical and Microstructure of Powder

Metallurgy Molybdenum 41% and 47.5% Rhenium

Todd Leonhardt, James Downs, and Joseph Johnson / Rhenium Alloys, Inc., North Ridgeville, Ohio, USA

 

Rhenium and rhenium containing alloys are unique metals with high melting points, high moduli of elasticity and high temperature mechanical properties. The most common rhenium containing alloys are molybdenum-41wt% rhenium (Mo41%Re) and molybdenum-47.5wt% rhenium (Mo47.5%Re). The focus of this study is to examine the mechanical properties and microstructures of hot isostatic pressed, hot rotary swaged, hot rolled and hot rolled and annealed Mo41%Re and Mo47.5%Re alloys. Both alloys are BCC solid solution alloys with work hardening rates half of commercial-purity rhenium. Unlike pure molybdenum, Mo41% and Mo47.5% and rhenium have excellent cold and warm plasticity. Adding rhenium to molybdenum increases the ductility as the rhenium concentration increases. The primary deformation mechanism of Mo47.5 Re alloys is twinning. Twinning occurs during cold and warm working, enhancing the mechanical properties without sacrificing ductility yet maintaining good strength at elevated temperatures. This study will discuss the results of the differences in rhenium concentration and processing methods on the texture, microstructure, and the mechanical properties at room temperature and at elevated temperature.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

5. Consolidation Methods for Spherical Rhenium and Rhenium Alloys

Todd Leonhardt*, Carole Trybus** and Robert Hickman*** / *Rhenium Alloys, Inc., North Ridgeville, Ohio, USA / **Concurrent Technologies Corp., Johnstown, PA, USA / ***Plasma Process, Inc., Huntsville, AL, USA

 

The development of a high-density spherical rhenium and spherical tungsten-rhenium powders has enabled the use of advanced consolidation techniques for the manufacture of refractory metal components. The consolidation techniques that were investigated are vacuum plasma spraying (VPS) and powder metal injection molding (PIM) to produce net shape components. The required particle size distributions for these applications vary. VPS utilizes a large powder particle size (< 75) while PIM requires a fine particle size (<30 m). The major advantages of spherical powders over traditional powders in plasma spraying are the high density of the powder particles and the good flow characteristics. These two factors combine to produce high density sprayed formed parts. PIM requires that the powder particles be dense as well as fine. Powder particles must be small enough to be entrapped in the binder but without being so small (< 0.5 m) that they become difficult to utilize easily. Binders for PIM are tailored around the powder’s characteristics, including size, distribution, shape and reactivity. Binder systems are developed to provide maximum packing while maintaining their ability to be molded, and enable molding de-binding without chemically reacting with the powder. A discussion of the powder production process and the general characteristics of spherical refractory powders as well as the alloys produced to date will be discussed. Further, both consolidation techniques will be discussed in depth, focusing on the role powder’s attributes play in each technique Spherical rhenium and spherical tungsten-rhenium powders are presently being used in non-erosion throats and other propulsion system applications.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

6. Deformation Hardening and Process Annealing as Fundamental Elements

in Rhenium Forming Technology

Todd Leonhardt and Jan-C. Carlén / Rhenium Alloys, Inc., North Ridgeville, Ohio, USA

 

A rhenium structure with iridium coating for oxidation protection is an ideal material combination for efficient performance in radiation-cooled liquid rocket engine thrust chambers with a maximum operating temperature of 2500K. Because of the high degree of work-hardening caused also by limited cold working, the properties of rhenium are sensitive to processing conditions, which may induce nucleation of micro cracks and make surface cracks propagate resulting in material failure. Annealing during processing constitutes a major and important part of the fabrication of rhenium in order to ensure complete retention of the highly desirable properties which enhance integrity and promote consistent quality of rhenium parts incorporated in flight engines.

The initial approach of this study was to analyze and summarize data on the strain-hardening behavior of rhenium, which are sparse and scattered throughout the literature. This would demonstrate the change of strength and ductility of the material during cold working with all important fabrication variables taken into consideration. Because of the scarcity and considerable spread of available post-anneal information, it was deemed necessary to study the effect of process heat treating on the structure and properties of various semi-finished products of rhenium.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

7. Past, Present, and Future Developments of Tungsten 25% Rhenium Tool Material

for Friction Stir Welding of Steel and Hard Metals

Todd Leonhardt* and Brian Thompson** / *Rhenium Alloys, Inc., North Ridgeville, Ohio, USA / ** EWI, Columbus, Ohio, USA

 

Rhenium Alloys Inc. (RAI) was introduced to the friction stir welding (FSW) process by The Welding Institute (TWI) in early 1999 with the application of FSW of commercially pure titanium. At that time, RAI only produced tungsten 25% Rhenium (W25%Re) wire at (0.25 -0.5 mm) diameter for type C and D thermocouple wire.

RAI began its next phase of development in the early 2000s with an introduction into the Dual Use Science and Technology (DUST) program, which applied FSW process to HSLA-65 steel. Rhenium Alloys was brought into the program by the MTS Corporation to use W25%Re to demonstrate the use of a tungsten-based material for FSW of ship components. On the DUST Program, W25%Re material was used to join a test article to demonstrate the use of FSW for a marine application. Extensive characterization was performed on the W25%Re material and the FSW welds. The W25%Re wire bar process did not produce a dense rod for FSW process, so RAI scaled up the production to produce a dense large diameter W25%Re rods. Increasing the diameter from a wire bar to large diameter W25%Re rods was a difficult process, due to limitation of equipment to process large diameter W25%Re rods.

In 2005, RAI obtained ultra high temperature sintering facilities, which significantly increased the density and properties of W25%Re rods. Subsequently, RAI acquired both large-scale powder compaction capabilities and rotary swaging facilities along with sintering capabilities. These facilities’ significantly enhanced the capabilities to produce 100% dense rods with worked structures. RAI then undertook an extensive research program in late 2007 to investigate the processes and properties of W25%Re and W25%Re with the addition of hafnium carbide (HfC). Microstructures and mechanical properties were examined for the two alloy composition at three differing testing temperatures (21°C, 1371°C, and 1926°C). The mechanical test results were analyzed and compared between the different process versus results of microstructures and mechanical properties.

In early 2001, EWI began more vigorously working in the area of welding hard metals with FSW. At the time commercially pure tungsten alloys, and the like, were used as FSW tool materials. Working as a participant in a Metals Affordability Initiative (MAI) early in the 2000’s, EWI began working with various tungsten alloys for welding titanium, followed by other hard metals. In 2005, the EWI started modeling the W25%Re for joining steel using the FSW process based on the property data produced by RAI. That model was used to development the advanced tool designs using tungsten-based alloys for steels and hard metals.

Today, RAI has acquired both the extrusion and forging capabilities to produce large diameter FSW refractory metal rods and FSW tools that have customized mechanical properties to meet the demand application. RAI is moving from a manufacturing of raw material to a manufacturer of refractory metal FSW tools, with EWI as the technology partner. The demand for joining thicker and thicker sections continues to grow; RAI must be able to produce tools and tool material in increasingly larger diameters. The refractory metal FSW tool material must withstand softening at elevated temperatures without significant wearing issues or catastrophically failing during FSW.

In the future, the EWI-RAI partnership will be improving the FSW tool materials and designs for welding Fe-, Ti-, and Ni-based, with a low cost, long lasting tool tailored to meet the requirements of future demands.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

8. Investigation of Mechanical Properties and Microstructure of Various

Molybdenum-Rhenium Alloys

Todd Leonhardt, Jan-C. Carlén, and Martin Buck*; Charles R. Brinkman, Weiju Ren, and C.O. Stevens** / *Rhenium Alloys, Inc., North Ridgeville, Ohio, USA / **Lockheed Martin Energy Research Corporation, Oak Ridge National Laboratory, Oak Ridge, TN, USA

 

Powder metallurgy products made from molybdenum alloys with a rhenium content above 40 weight percent are traditionally used for many high temperature applications. The resistance to thermal shock and high temperature strength of the molybdenum-rhenium alloys make them often the best choice for applications such as: heat sinks, heating elements, thermocouple sheathings, reflectors, vacuum furnace components, electron tube components, hydrazine thrusters, and other important industrial and aerospace applications. The physical characteristics of the molybdenum-rhenium alloys produce a material with excellent life in extremely harsh service environments. The effect of the rhenium addition to molybdenum in powder metallurgy sheet product near and exceeding the saturation point of rhenium in molybdenum was investigated. Alloys of 41, 44.5, 47.5 and 51% rhenium were produced to examine the mechanical properties at room temperature, 1073K, and 1473K. Microstructural analysis was performed to correlate mechanical properties with the microstructures present in each alloy. This study shows that an optimum rhenium content can be achieved to maximize room temperature properties and elevated temperature performance.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

9. Low Temperature Mechanical Behavior of a Molybdenum-Rhenium Alloy

S. R. Agnew* and Todd Leonhardt** / *University of Virginia, Charlottesville, Virginia, USA / **Rhenium Alloys, Inc., North Ridgeville, Ohio, USA

 

The positive effects of rhenium additions, up to the solubility limit, on the mechanical behavior of group VI-A refractory metals have been known for decades. In particular, the high temperature strength and creep resistance as well as the low temperature ductility are enhanced. This article outlines the low temperature mechanical behavior and discusses the possible role of mechanical twinning.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

10. Molybdenum-Rhenium Implantable Medical Devices

Todd Leonhardt* and Udayan Patel** / *Rhenium Alloys, Inc., North Ridgeville, Ohio, USA / **ICON Interventional Systems, Inc., Atlanta, GA, USA

 

Molybdenum 47.5% rhenium (Mo47.5%Re) has gained interest as material for medical devices. Mo47.5%Re has high ductility with high strength that is ideal for implants that require a low profile. Compatibility with surrounding tissue is of prime importance in selection of implant materials. Mo47.5%Re being inert and nickel free, has excellent biocompatibility with MRI inertness and, does not cause artifacts in a CT image. One of major advantages of Mo47.5%Re is its durability which, allows it to be use in orthopedic rods, plates and screws for spinal repair, maxillary and facial reconstruction, Volar fixation for extremities, cranial covers that allows reconstruction of bone without interfering with bony cell growth. The ability to design implants with low profile has opened up the possibilities of performing reconstructive surgery by percutaneous minimally invasive techniques. This study will discuss the medical applications and mechanical properties required to produce the desired effect for the medical implant.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

11. Near Net Shape of Powder Metallurgy Rhenium Parts

Todd Leonhardt and James Downs / Rhenium Alloys, Inc., North Ridgeville, Ohio, USA

 

In this paper, a description of the stages of processing necessary to produce a near-net shape (NNS) powder metallurgy (PM) rhenium component through the use of cold isostatic pressing (CIP) to form a complex shape will be explained. This method was primarily developed for the production of the 440 N and 490 N liquid apogee engine combustion chambers used in satellite positioning systems. The CIP to NNS process has been used in the manufacture and production of other rhenium aerospace components as well. Cold isostatic pressing (CIP) to a near net shape utilizing a one or two–part mandrel greatly reduces the quantity of rhenium required to produce the component, and also significantly reduces the number of secondary machining operations necessary to complete the manufacturing process. Further, the developments in near-net shape powder metallurgy rhenium manufacturing techniques have generated significant savings in the area of both time and budget. Overall, cost declined by as much as 35 % for the rhenium chambers, and manufacturing time was decreased by 30-40 %. The quantity of rhenium metal powder used to produce a rhenium chamber was reduced by approximately 70 %, with a subsequent reduction of nearly 50 % in secondary machining operation schedules. Thus, it is apparent that the overall savings provided by the production of near-net shape powder metallurgy rhenium components will be more than merely another aspect of any project involving high temperature applications, it will constitute significant benefit.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

12. Near-Net Shape Powder Metallurgy Rhenium Thruster

Todd Leonhardt, Mark Hamister, and Jan C. Carlén*; James Biaglow and Brian Reed** / *Rhenium Alloys, Inc., North Ridgeville, Ohio, USA / **NASA Glenn Research Center, Cleveland, Ohio

 

This paper describes the development of a method to produce a near-net shape (NNS) powder metallurgy (PM) rhenium combustion chamber of the size 445 N (100 lbf) used in a high performance liquid apogee engine. These engines are used in low earth orbit and geo-stationary orbit for satellite positioning systems. A research program was established which was funded under a NASA Phase I and II SBIR. The goal was to establish an alternative manufacturing method for the production of powder metallurgy rhenium combustion chambers. The task to reduce the quantity of rhenium required to produce rhenium chambers was undertaken to lower the cost of the chambers. This paper will focus on the evolution of manufacturing techniques for rhenium thruster chambers. Several manufacturing methods used to produce a near-net shape rhenium chamber are discussed, including the research and development work needed to proceed from cylindrical, sintered, hot isostatic pressed (HIP) rhenium ingots through the manufacturing of rhenium tubes and eventually to make a complex near-net shape rhenium chamber. Several breakthroughs in processing were achieved during this research study. There were improvements in the design of the mandrel, fixture of the mandrel in cold isostatic pressing (CIP) latex molds, altering wet-bag CIP parameters to achieve the complex shape, thermal processing for controlling shape and properties, container-less HIP, and the reduction of machining steps. One of the major breakthroughs was the two-part mandrel with a complex shape for CIP processing which enabled the compaction of rhenium metal powder. Another important innovation was hot isostatic pressing (HIP) without a canning material around the chamber. This innovation had a major impact on the surface finish and cost of the chambers. A study of spin forming was undertaken to try to obtain rolled rhenium mechanical properties on a NNS rhenium thruster. The spin forming method alters the HIP rhenium microstructure to a rolled rhenium structure while decreasing wall thickness of the NNS thruster to close to final dimensions. The developments in near-net shape powder metallurgy rhenium combustion chambers will reduce manufacturing cost of the rhenium chambers by 25%, and reduce the manufacturing time by 30-40%. The quantity of rhenium metal powder used to produce a rhenium chamber is reduced by approximately 70% and the subsequent reduction in machining schedule and costs is nearly 50%. The overall savings provided by the production of near-net shape powder metallurgy rhenium chambers will become an important benefit when increase of performance, reliability, and cost reduction will be required to compete in the international market for satellite propulsion components.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

13. Overview of the Recent Developments in Rhenium and Rhenium-Containing Alloys

Todd Leonhardt, Don Mitchell, James Downs, and Nancy Moore / Rhenium Alloys, Inc., North Ridgeville, Ohio, USA

 

Several advanced consolidation techniques were investigated with regard to the processing of rhenium and rhenium-containing powders. These processes include: powder metal injection molding (PMIM), direct hot isostatic pressing (D-HIP), direct vacuum hot pressing (D-VHP), laser additive manufacturing (LAM), vacuum arc melting (VAM), and hot extrusion. A focus of this paper will be the relationship between microstructures and mechanical properties, when compared to all the various processing methods investigated. Other notable advances involve the powder production of spherical molybdenum-rhenium and tungsten-rhenium alloys for the advanced consolidation techniques and other powder metallurgy processing, as well as the production of seamless molybdenum-rhenium seamless tubing. This paper will be an encapsulation of research and developments during the past 5 years that has focused on rhenium and rhenium containing alloys.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

14. Processing and Properties of Tungsten 25% Rhenium With and Without

Hafnium Carbide

Todd Leonhardt / Rhenium Alloys, Inc., North Ridgeville, Ohio, USA

 

Historically tungsten 25% rhenium alloy has been produced into wire for the thermocouple market, but recent demands for high temperature structural components have forced the development of novel processing techniques for tungsten-rhenium and tungsten-rhenium with hafnium carbide. Tungsten - rhenium alloys with a melting point of 3050°C and a recrytallization temperature near 1900°C are being used in aerospace, temperature measuring and friction stir welding applications. The mechanical properties of the tungsten 25% rhenium and tungsten 24.5% rhenium with 2% hafnium carbide at room temperature, 1371°C and 1926°C in different processing conditions with varying microstructures will be discussed in detail.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

15. Properties of Tungsten 25% Rhenium and Tungsten-Rhenium With Hafnium Carbide

Todd Leonhardt / Rhenium Alloys, Inc., North Ridgeville, Ohio, USA

 

Historically tungsten 25% rhenium alloy has been manufactured into wire for the thermocouple market, but recent demands for high temperature structural components has forced the development of novel processing techniques for tungsten-rhenium and tungsten-rhenium with hafnium carbide. With a melting point of 3050°C, and a recrytallization temperature near 1900°C, tungsten - rhenium alloys are being used in aerospace, temperature measuring and friction stir welding applications. The mechanical properties of the tungsten 25% rhenium and tungsten 25% rhenium with hafnium carbide at ambient, 1371°C and, 1926°C in different processing conditions with microstructures will be discussed in detail.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

16. Prototype Rhenium Component for Stirling Engine Power Conversion

Todd Leonhardt* and Frank Ritzert** / *Rhenium Alloys, Inc., North Ridgeville, Ohio, USA / **NASA Glenn Research Center, Cleveland, Ohio 44135

 

The Stirling engine power conversion concept is a candidate to provide electrical power for deep space missions. A key element for qualifying potential flight hardware is the long-term durability assessment for critical hot section components of the power converter. One such critical component is the power converter heater head, which is a high-temperature pressure vessel that transfers heat to the working gas medium of the converter. Rhenium is a candidate material for the heater head application because of its high melting point (3453 K), high elastic modulus (420 GPa), high yield and ultimate tensile strengths at both ambient and elevated temperatures, excellent ductility, and exceptional creep properties. Rhenium is also attractive due to the potential of near-net-shape (NNS) manufacturing techniques that allow components to be produced using less material, which lowers the overall cost of the component. The objective of this research was to demonstrate the manufacturing method using rhenium for this high-temperature power conversion application to provide space power system designers with generally applicable technology for future applications.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

17. Spherical Rhenium Metal Powder

Todd Leonhardt, Nancy Moore, and Mark Hamister / Rhenium Alloys, Inc., North Ridgeville, Ohio, USA

 

The development of a high-density, spherical rhenium powder (SReP) possessing excellent flow characteristics has enabled the use of advanced processing techniques for the manufacture of rhenium components. The techniques that were investigated were vacuum plasma spraying (VPS), direct-hot isostatic pressing (D-HIP), and various other traditional powder metallurgy processing methods of forming rhenium powder into near-net shaped components. The principal disadvantages of standard rhenium metal powder (RMP) for advanced consolidation applications include: poor flow characteristics; high oxygen content; and low and varying packing densities. SReP will lower costs, reduce processing times, and improve yields when manufacturing powder metallurgy rhenium components. The results of the powder characterization of spherical rhenium powder and the consolidation of the SReP are further discussed.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

18. Stress-Rupture Strength and Creep Behaviour On Molybdenum-Rhenium Alloys

Dietmar Freund and Bernd Fischer*; Jan-C. Carlén and Todd Leonhardt** / *Fachhochschule Jena - University of Applied Science, Department of Materials Technology, D-07745 Jena, Germany / **Rhenium Alloys, Inc., North Ridgeville, Ohio, USA

 

It is necessary to measure strength and creep behavior to guarantee a safe and reliable usage of refractory alloys at extremely high temperatures. In the literature there is very little information available about the properties of Mo-Re alloys at temperatures higher than 1000°C. A special test facility has been designed and built for stress-rupture testing at very high temperatures (up to 3000°C) of high temperature melting metals and alloys in inert atmospheres. The stress-rupture strength as well as the creep behavior for molybdenum-rhenium alloys with rhenium contents between 41 and 51 wt.% have been determined at temperatures ranging from 1200 to 2000°C and rupture times of up to 10 hours using this facility. Previous measurements of stress-rupture strength and creep behavior of pure rhenium have been compared with the measurement results of Mo-Re alloys. The discussion of the values measured is based on metallographic test results and scanning electron microscopy (SEM) images of the Mo-Re alloy samples after stress-rupture testing.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

19. Texture, Anisotropy, and the Role of Twinning in Determining the Mechanical

Behavior of a Molybdenum-Rhenium Alloy

S. R. Agnew* and Todd Leonhardt** / *University of Virginia, Charlottesville, Virginia, USA / **Rhenium Alloys, Inc., North Ridgeville, Ohio, USA

 

The “rhenium effect” was first reported by Geach and Hughes in 1955, with seminal articles investigating the effect by Jaffee
et al. – and Lawley and Maddin published soon afterwards. The strength, creep resistance, and low temperature ductility of group VI-A refractory metals are all improved with increasing rhenium content up to the solubility limit. Early on, a connection was observed between the improved low temperature ductility and the prevalence of mechanical twinning in Mo-Re alloys in comparison to pure molybdenum. However, additional possibilities were also raised:

(i) lowering the tendency to form a weak oxide film along grain boundaries,

(ii) increased interstitial (e.g. O) solubility and/or short-range ordering of the same,

(iii) lowered resistance to dislocation slip (solid solution softening),

(iv) electronic structure arguments related to stacking faults or otherwise, and

(v) decreased surface (interfacial) energies.

The first argument is attractive as it parallels the accepted role of interstitial carbon in these refractory metals – and there has been some reported evidence in favor it. The second argument is linked with the first, since an increased solubility for oxygen or a strong bond between oxygen and ordered complexes within the grains would lower the tendency to form deleterious oxides. Although solid solution softening has been observed in some of the dilute Mo-Re alloys – this has been ruled out as an important contributor for high Re alloys, such as is investigated in the present study. The latter three arguments appear not to have much support in the literature.

This study was initiated to re-investigate the role of mechanical twinning in producing the superior low temperature ductility of a high rhenium content alloy, Mo-47.5 wt% Re, in comparison with recrystallized pure molybdenum, which undergoes a ductile to brittle transition near room temperature. Even in reports focusing on other effects that rhenium may have, the enhancement of twinning is invariably mentioned. This study speaks to a longstanding question regarding the connection (positive and negative) between twinning and fracture. Interestingly, a seminal study of Mo-51wt%Re has played an important role in this debate.

Molybdenum-rhenium alloys are also prized for their improved formability; therefore, the crystallographic texture and anisotropy of the sheet material were investigated in both the cold-rolled and recrystallized conditions. Previous studies have documented the effect of temperature on the deformation behavior. This study also examines the strain rate sensitivity in the quasi-static rate regime (i.e. 5x10-5 to 20 s-1).

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

20. Tungsten 25% Rhenium

Todd Leonhardt and James Downs  / Rhenium Alloys, Inc., North Ridgeville, Ohio, USA

 

Tungsten - rhenium wire has been historically used for the Type C thermocouple to measure precise temperatures up to 2300°C. The wire ranged in size from 0.127mm (0.005”) to 0.5080 (.020”) in diameters. Today tungsten rhenium alloys are used in new applications including tooling for friction stir welding and specific medical devices as well as thermocouples. Tungsten 25% rhenium was selected because of the alloy’s enhanced physical and mechanical properties. These specific properties were high modulus of elasticity, good ductility, lower ductile brittle transition temperature, specific electrical properties, and a high recrystallization temperature. An in-depth discussion of the processing, properties and microstructures will be presented here today.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

21. Understanding the Manufacturing Process of Molybdenum 47.5% Rhenium Sheet

Through Examination of Microstructures at Steps of Processing

Todd Leonhardt and Joseph L. Johnson / Rhenium Alloys, Inc., North Ridgeville, Ohio, USA

 

Rhenium and rhenium containing alloys are unique metals with high melting points, high moduli of elasticity, and excellent high temperature mechanical properties. The most common rhenium containing alloy is molybdenum-47.5wt% rhenium (Mo-47.5%Re). The purpose of this paper is to discuss manufacturing the alloy from powder production through finished material. Some of the primary uses of molybdenum-rhenium alloys include rocket motors for the aerospace and defense industries, cardiovascular stents, cathodes for the electronics field, and various other extreme temperature and medical applications. The focus of the study was to examine the physical characteristics of the pure molybdenum and rhenium powders prior to blending and the blended molybdenum rhenium powders through the use of particle size analysis and scanning electron microscopy. The powders were also analyzed as it relates to ASTM Specification B859-03 (Standard Practice for De-Agglomeration of Refractory Metal Powders and Their Compounds Prior to Particle Size Analysis). An examination of the die compacted “green” powders, microstructures of the pre-sintered and sintered compact, hot rolled sheet and the annealed microstructure of molybdenum 47.5% rhenium will also be discussed. The alloy is body center cubic (BCC) solid solution alloy with work hardening rates half that of commercial-purity rhenium. Unlike pure molybdenum, Mo47.5% and rhenium have excellent plasticity at both cold and warm temperatures. The primary deformation mechanism of the alloy is mechanical twinning. The mechanical twinning occurs in Mo/Re during both cold and warm working. This form of deformation enhances the mechanical properties without sacrificing the material’s ductility, while maintaining good strength at elevated temperatures. This will also show the results of the study of molybdenum 47.5% rhenium powder and sheet manufacturing by examining the processing methods, microstructures, and the role that sigma phase plays in processing and the material’s mechanical properties. Also being discussed are fractography of the mechanical test specimens, and the mechanical properties at both room and elevated temperatures.

Contact Todd Leonhardt for full report: 440-309-2072 | Email

 

 

22. Welding Behavior of Molybdenum Tubing With and Without Lanthanum Oxide

Todd Leonhardt / Rhenium Alloys, Inc., North Ridgeville, Ohio, USA

 

Molybdenum sheath protects the thermocouple components from exposure in furnace environments up to 2200°C. To close the end of a thermocouple sheath, tungsten inert gas welding is used. The welding process increases the grain size, both due to solidification and subsequent exposure during operational. The molybdenum welded closure had a quantity of porosity, which further promoted brittle behavior. A program was undertaken to increase the durability of the thermocouple sheath by increasing the recrystallization temperature and in turn, decreasing the DBTT. A molybdenum lanthanum oxide (M-L) was produced into a seamless tube for use as thermocouple sheath. During the characterization program, the M-L closure demonstrated a smaller heat affected zones, as well as a refined fusion zone grain structure with little or no porosity. The results of the comparison between unalloyed molybdenum and M-L will be shown along with a discussion of the mechanisms involved in welding molybdenum alloys.

Contact Todd Leonhardt for full report: 440-309-2072 | Email