The MN+1AXN phases: A new class of solids

Perspectives on Titanium Science and Technology

The more than 60 ternary carbides and nitrides, with the general formula Mn+1AXn—where n = 1, 2, or 3; M is an early transition metal; A is an A-group element (a subset of groups 13–16); and X is C and/or N—represent a new class of layered solids, where Mn+1Xn layers are interleaved with pure A-group element layers. The growing interest in the Mn+1AXn phases lies in their unusual, and sometimes unique, set of properties that can be traced back to their layered nature and the fact that basal dislocations multiply and are mobile at room temperature. Because of their chemical and structural similarities, the MAX phases and their corresponding MX phases share many physical and chemical properties. In this paper we review our current understanding of the elastic and mechanical properties of bulk MAX phases where they differ significantly from their MX counterparts. Elastically the MAX phases are in general quite stiff and elastically isotropic. The MAX phases are relatively soft (2–8 GPa), are most readily machinable, and are damage tolerant. Some of them are also lightweight and resistant to thermal shock, oxidation, fatigue, and creep. In addition, they behave as nonlinear elastic solids, dissipating 25% of the mechanical energy during compressive cycling loading of up to 1 GPa at room temperature. At higher temperatures, they undergo a brittle-to-plastic transition, and their mechanical behavior is a strong function of deformation rate.

Elastic and Mechanical Properties of the MAX Phases

After almost three decades of intensive fundamental research and development activities, intermetallic titanium aluminides based on the ordered γ-TiAl phase have found applications in automotive and aircraft engine industry. The advantages of this class of innovative high-temperature materials are their low density and their good strength and creep properties up to 750 °C as well as their good oxidation and burn resistance. Advanced TiAl alloys are complex multi-phase alloys which can be processed by ingot or powder metallurgy as well as precision casting methods. Each process leads to specific microstructures which can be altered and optimized by thermo-mechanical processing and/or subsequent heat treatments. The background of these heat treatments is at least twofold, i.e., concurrent increase of ductility at room temperature and creep strength at elevated temperature. This review gives a general survey of engineering γ-TiAl based alloys, but concentrates on β-solidifying γ-TiAl based alloys which show excellent hot-workability and balanced mechanical properties when subjected to adapted heat treatments. The content of this paper comprises alloy design strategies, progress in processing, evolution of microstructure, mechanical properties as well as application-oriented aspects, but also shows how sophisticated ex situ and in situ methods can be employed to establish phase diagrams and to investigate the evolution of the micro- and nanostructure during hot-working and subsequent heat treatments.

Design, Processing, Microstructure, Properties, and Applications of Advanced Intermetallic TiAl Alloys†

Thermomechanical processing (TMP) is associated with two major requirements: (i) to produce usable shapes through primary working (ingot breakdown) and secondary mill operations (hot rolling or forging) and (ii) to optimize mechanical properties through microstructure control during the different stages of the thermomechanical process. This paper reviews the thermomechanical processing of beta titanium alloys in general and high temperature deformation mechanisms, microstructure control during TMP, and final mechanical properties in particular.

Thermomechanical processing of beta titanium alloys—an overview

The effect of process variables on flow response and microstructure evolution during hot working of Ti–6Al–4V with a colony alpha preform microstructure was established using isothermal hot compression tests. Testing was conducted on material with prior-beta grain sizes of 100 μm or 400 μm at strain rates of 0.001–10 s−1, test temperatures between 815 and 955°C, and height reductions of 40–80%. All of the flow curves exhibited a peak stress followed by moderate flow softening. The absence of a grain/colony size dependence of flow behavior, coupled with relatively low values of the strain rate sensitivity of the flow stress (∼0.05–0.30), led to the conclusion that deformation was controlled by dislocation glide/climb processes. Flow softening was interpreted in terms of deformation heating and substructure/texture evolution. The dependence on strain rate and temperature of the kinetics of dynamic globularization of the colony microstructure was complex and appeared to be of second-order importance compared to the effects of strain per se, thus suggesting the dominance of dislocation-type processes for the control of globularization as well.

Flow behavior and globularization kinetics during hot working of Ti–6Al–4V with a colony alpha microstructure

The modification of lamellar alpha phase in Ti-6A1-4V by hot working was investigated with the aim of controlling morphology (aspect ratio) and final grain size. The effect of strain was studied using forging at 955 °C (1750 °F), followed by annealing at 925 °C (1700 °F) to allow the alpha morphology to adjust. Increasing the deformation from 6.5 pct to 80 pct reduction caused the lamellar alpha morphology to become progressively more equiaxed upon annealing. TEM observations showed that annealing of material deformed to 6.5 pct resulted in recovery of the alpha, without a noticeable change in the morphology, while higher deformation resulted in plate shearing and beta cusp formation. It was found that material with an initial thin alpha plate structure (thickness — 3.4 ώm) breaks up at a lower critical strain than a material with a thicker plate morphology (thickness ≃ 6 μm). The material with thin alpha plates more rapidly forms equiaxed alpha grains separated by beta phase, while the material with a thicker plate structure exhibits more alpha/alpha boundaries after deformation and annealing. The morphology change from alpha lamellae into lower aspect ratio grains was identified to be by a break-up of the alpha lamellae, essentially by a two-step process: a formation of low and high angle alpha/alpha boundaries or shear bands across the alpha plates followed by penetration of beta phase to complete the separation. This break-up takes place during hot deformation and subsequent annealing.

Modification of alpha morphology in Ti-6Al-4V by thermomechanical processing

The hot workability of conventional titanium alloys and titanium aluminides is reviewed. For both alloy classes, the influence of hot working variables and microstructure on failure via fracture or flow-localization controlled processes is summarized. The occurrence of wedge cracking and cavitation during bulk forming of α / β alloys with Widmanstatten microstructures or γ titanium aluminides with lamellar or equiaxed structures, is examined. In particular, the effects of grain size, grain boundary second phases and process variables on failure are presented. Observations and models of flow localization and cavitation processes which lead to failure during low strain rate, superplastic, tensile-type deformation of titanium and titanium aluminide alloys with fine equiaxed structures, are also described. In the area of flow-localization-controlled failure during bulk forming, the occurrence of shear bands and other flow nonuniformities during both conventional and isothermal hot working processes is reviewed. The influence of material properties, such as flow softening rate and strain rate sensitivity and process variables, which lead to temperature and hence flow nonuniformities, is examined. The flow localization concepts are illustrated for several hot working processes.

Hot workability of titanium and titanium aluminide alloys—an overview

In this article, the thermomechanical processing of ingot-metallurgy alpha/beta titanium alloys is summarized, with special emphasis on microstructure evolution and workability considerations. Primary hot working dealing with the conversion of ingot structures to fine-equiaxed wrought structures is addressed. In this regard, the breakdown of lamellar microstructures, the occurrence of cavitation/wedge cracking, and the development of crystallographic texture are described. Secondary processes such as sheet rolling, superplastic forming, and closed-die forging are also reviewed.

Isaac Weiss

Papers

Thermomechanical processing of beta titanium alloys—an overview

Flow behavior and globularization kinetics during hot working of Ti–6Al–4V with a colony alpha microstructure

Modification of alpha morphology in Ti-6Al-4V by thermomechanical processing

Hot workability of titanium and titanium aluminide alloys—an overview

The thermomechanical processing of alpha/beta titanium alloys