James C. Williams

Deformation behavior of HCP Ti-Al alloy single crystals

Single crystals of Ti-Al alloys containing 1.4, 2.9, 5, and 6.6 pct Al (by weight) were oriented for 〈a〉 slip on either basal or prism planes or loaded parallel along the c-axis to enforce a nonbasal deformation mode. Most of the tests were conducted in compression and at temperatures between 77 and 1000 K. Trace analysis of prepolished surfaces enabled identification of the twin or slip systems primarily responsible for deformation. Increasing the deformation temperature, Al content, or both, acted to inhibit secondary twin and slip systems, thereby increasing the tendency toward strain accommodation by a single slip system having the highest resolved stress. In the crystals oriented for basal slip, transitions from twinning to multiple slip and, finally, to basal slip occurred with increasing temperature in the lower-Al-content alloys, whereas for Ti-6.6 pct Al, only basal slip was observed at all temperatures tested. A comparison of the critically resolved shear stress (CRSS) values for basal and prism slip as a function of Al content shows that prism slip is favored at room temperature in pure Ti, but the stress to activate these two systems becomes essentially equal in the Ti-6.6 pct Al crystals over a wide range of temperatures.Compression tests on crystals oriented so that the load was applied parallel to the c-axis showed extensive twinning in lower Al concentrations and 〈c+a〉 slip at higher Al concentrations, with a mixture of 〈c+a〉 slip and twinning at intermediate compositions. A few tests also were conducted in tension, with the load applied parallel to the c-axis. In these cases, twinning was observed, and the resolved shear for plastic deformation by twinning was much lower that that for 〈c+a〉 slip observed in compression loading.

Corrosion Behavior of Ti-6Al-4V with Different Thermomechanical Treatments and Microstructures

Abstract The corrosion behavior of four different microstructures of Ti-6Al-4V with varying volume fractions of primary α (0, 10%∼20%, 40%∼50%, and ∼90%) was investigated in sodium chloride (NaCl) ...

Thermo-mechanical processing of high-performance Ti alloys: recent progress and future needs

Abstract Over the past 10–15 years, improved thermo-mechanical processing of high-performance Ti alloys has been responsible for improving the reliability and performance of products that use them. Improvements in melting, ingot conversion practice and forging practice all have been contributing factors to this improvement. Modeling of the forging process coupled with hot die forging has led to significant improvements in uniformity of properties. Taken together, these factors have enabled improved reliability of critical components for aerospace applications. This paper describes these improvements and summarizes their benefits. There is room for further improvement, however, the most critical need is reduction of cycle time required to qualify a new forging or a change in forging practice. It is argued that improved models that permit prediction of the microstructure and resulting properties would be of great value. An image of some of the ingredients in improved models will allow this predictive capability will be outlined.

Alpha + Beta Alloys

In this chapter on α+β alloys the basic correlations between processing, microstructure, and properties of the whole group of α+β alloys listed in Table 2.6 will be outlined including the so-called “near α” alloys designated for applications at high temperatures. Only the special features of the latter group of alloys in the area of processing, microstructure, and properties will be described in Chap. 6.

Processing, Structure, Texture and Microtexture in Titanium Alloys

We review the effect of processing on structure and texture in titanium alloys, focusing on the understanding of this relationship that has evolved over the last decade. Thermomechanical processing cycles for these alloys involve deformation and heat treatment in single phase β and two phase, α+β, phase fields, and involves a complex interplay between deformation and recrystallization textures of the individual phases, textures arising from the crystallographic relationship between the two phases, and the scale of microstructure evolution. We explore these interactions and trace the strong dependence of thermomechanical pathways on the final structure and texture.

Current Status of Ti PM: Progress, Opportunities and Challenges

Utilization of titanium components made by powder metallurgy methods has had limited acceptance largely due to the high cost of titanium (Ti) powder. There has been renewed interest in lower cost economical powders and several Ti reduction methods that produce a particulate product show promise. This talk summarizes work done at Oak Ridge National Laboratory to consolidate these economical powders into mill products. Press and sinter consolidation, hot isostatic pressing (HIP) and direct roll consolidation to make sheet have been explored. The characteristics of the consolidated products will be described as a function of the consolidation parameters.

Commercially Pure (CP) Titanium and Alpha Alloys

This chapter describes the processing, microstructure, and properties of D titanium alloys, with emphasis on the various grades of commercially pure (CP) titanium (often referred to as CP-Ti). A definition of D titanium alloys was included in Chap. 2, but it may be useful to include a more detailed one here. All D titanium alloys are based on the low temperature, hexagonal allotropic form of titanium. These alloys can contain substitutional alloying elements (Al or Sn) or interstitial elements (oxygen, carbon, or nitrogen) that are soluble in the hexagonal D phase. These alloys also contain limited quantities of elements that have limited solubility such as Fe, V, and Mo. Table 2.6 lists a representative group of α titanium alloys and grades of CP titanium, along with representative selections of alloys belonging to the α+β and β classes. The grade designations are taken from the American Society for Testing and Materials (ASTM). As the beneficial application of this class of titanium alloys has been recognized, the use has increased. Further, specific alloys have been formulated to improve to environmental resistance of CP titanium and α titanium alloys or to provide comparable performance at reduced cost where expensive additions such as palladium are involved. Consequently, there has been a proliferation of alloy grades. There are now at least 16 identifiable alloys or grades.

Titanium Based Intermetallics

Intermetallic compounds1, especially those formed between light elements such as Ti and Al are attractive, because of their low density and good elevated temperature strength. However, the formation of intermetallic compounds also usually reduces the symmetry of the parent metal lattice. In turn, this places additional restrictions on the available deformation modes. These restrictions usually are manifested as increased strength, at least at elevated temperatures, reduced ductility and fracture toughness. Historically, the issues associated with reduced ductility and fracture toughness have been viewed as outweighing the benefits of increased strength. Consequently, the use of intermetallic compounds in structural applications has been very limited.

A realistic approach for qualification of PM applications in the aerospace industry

Abstract This chapter provides a brief discussion of the evolution of titanium powder metallurgy technology and its intended use in aerospace applications, which are among the most demanding uses of structural materials. Insights into some of the pitfalls that have been encountered along the way are also presented. Included is a brief discussion of powder production technology including prealloyed powder and unalloyed Ti particulate. Prealloyed powders are primarily made by hydride–dehydride, plasma rotating electrode process (PREP), and gas atomization methods. It also provides a brief oversight on the current status of the principal PM near-net-shape making methods, including, for the press and sinter approach, cold isostatic pressing and sintering and die pressing and sintering, with or without hot isostatic pressing (HIP). Fabrication of titanium components using prealloyed powder for demanding applications via multiple additive manufacturing (or 3-D printing) processes and HIP near-net shapes produced from molds are also discussed. An emphasis of this chapter is a discussion of some of the considerations that must be taken into account to obtain qualification to manufacture components for manned aerospace vehicles.

A Comparison of Friction Stir Processing of Mill Annealed and Investment Cast Ti-6Al-4V

Friction stir processing (FSP) of cast and mill annealed Ti-6Al-4V was studied to determine whether this thermomechanical processing technique is feasible for local microstructural modification to improve fatigue properties for aerospace engine applications. The ability to produce a friction stir processed surface layer is thought to provide a microstructure that is more resistant to fatigue crack initiation than the as-cast microstructure of the bulk. FSP resulted in a microstructure of either very fine equiaxed α or a colony microstructure with refined prior β grains when processing occurred below the β transus or above the β transus, respectively. The final microstructure observed in the stir zone was related to the processing conditions and was independent of the initial cast or wrought microstructures. Electron backscatter diffraction was used to assess the presence of microtexture in the stir zone. There was very little microtexture observed in sub-transus processed material. A moderate strength α phase texture was observed in the cast and wrought materials processed above the β transus. Fatigue tests performed on 4-point bend samples revealed a dramatic increase in fatigue life after FSP in both mill annealed (presented here) and cast + hot isostatic pressed materials (previously presented) although only a limited number of samples were available. Substantial tool wear is an issue and tungsten contamination is readily observed when FSP temperatures were below the β transus. This could be a concern in high temperature applications or for applications requiring postprocessing heat treatment.