Martensitic transformations alloys

Precipitation Hardening. With the exception of ferritic steels, which can be hardened either by the martensitic transformation or by eutectoid decomposition, most heat-treatable alloys are of the precipitation-hardening type. During heat treatment of these alloys, a controlled dispersion of submicroscopic particles is formed in the microstmeture. The final properties depend on the manner in which particles are dispersed, and on particle size and stabiUty. Because precipitation-hardening alloys can retain strength at temperatures above those at which martensitic steels become unstable, these alloys become an important, in fact pre-eminent, class of high temperature materials. 

Another property pecuHar to SMAs is the abiUty under certain conditions to exhibit superelastic behavior, also given the name linear superelasticity. This is distinguished from the pseudoelastic behavior, SIM. Many of the martensitic alloys, when deformed well beyond the point where the initial single coalesced martensite has formed, exhibit a stress-induced martensite-to-martensite transformation. In this mode of deformation, strain recovery occurs through the release of stress, not by a temperature-induced phase change, and recoverable strains in excess of 15% have been observed. This behavior has been exploited for medical devices. 

THE MARTENSITIC TRANSFORMATION IN IRON-NICKEL ALLOYS A MOLECULAR DYNAMICS STUDY... 

Many metals and metallic alloys show martensitic transformations at temperatures below the melting point. Martensitic transformations are structural phase changes of first order which belong to the broader class of diffusion js solid-state phase transformations. These are structural transformations of the crystal lattice, which do not involve long-range atomic movements. A recent review of the properties and the classification of diffusionless transformations has been given by Delayed... 

A martensitic transformation from a cubic CsCl-type structure by 110 (lT0> type shears occurs for NlxAli. alloys in the composition range 0.615 < x < 0.64. Precursive... 

THE MICROSTRUCTURE AND MARTENSITIC TRANSFORMATION IN A (POTENTIALLY) SHAPE MEMORY Ni-AI-Ti-B ALLOY... 

The characteristic temperatures of the martensitic transformation, as well as the (Af -Mf) temperature range were strongly influenced by the microstructure of the alloy. Applied predeformation lowered both and temperature range of transformation while the ageing at 510 K that followed decreased the width of the transformation. 

Detailed consideration of the structure of many of the advanced and complex alloys which are of considerable technological importance (high-strength titanium alloys, nickel-base superalloys, etc.) is beyond the scope of this section, other than to point out that no new principles are involved. Certain titanium alloys, for example, exhibit a martensitic transformation, while many nickel-base superalloys are age hardening. Similarly, cast irons, although by no means advanced materials, are relatively complex they are considered in Section 1.3 where graphitisation is discussed. 

The Ms temperature, at which the diffusion-less martensitic transformation starts, depends on the alloy considered (its composition, etc.) it can be above or below room temperature. For the so-called austenitic steels Ms < < ambient temperature, whereas Ms > ambient temperature for the martensitic steels. 

Figure 5.30. Schematic drawing showing the construction of an isothermal transformation diagram from measurements of the progress of the transformation at various constant temperatures. This may be done, for instance, by metallographic examination of several specimens, quenched from the 7-field quickly enough to miss the nose of the C-curve and then isothermally annealed for various length of time. Notice that curves for the transformation of different samples may be shown on the same diagram and that more complex trends may be observed in real diagrams of specific alloys. In the example reported, Ms is the temperature at which the alloy will begin to show the martensitic transformation, Mf indicates the temperature below which no additional martensite forms.

Examples of martensitic transformation. This kind of transformation has been observed in a number of substances elements, compounds, alloys, minerals, metallic and ceramic materials. A few examples of systems showing a... 

Another important group of alloys involved in martensitic transformation is represented by several plutonium alloys. The martensitic nature, for instance, of the 6 to a transformation has been clearly established in Pu-Ga and Pu-Al alloys and a behaviour similar to that shown in Fig. 5.30 has been observed. For a systematic description of plutonium alloys, the stability of the different phases and their transformations see Hecker (2000). 

Chandrasekaran, L. (1980) Ordering and martensitic transformations in Cu-Zn-Mn shape memory alloys , Ph.D. Thesis, University of Surrey, Guildford, UK. 

Martensitic transformations occur in a variety of nonferrous alloy systems, particularly for allotropes that transform at low temperatures. The reasons for this are... 

Such transformations have been extensively studied in quenched steels, but they can also be found in nonferrous alloys, ceramics, minerals, and polymers. They have been studied mainly for technical reasons, since the transformed material often has useful mechanical properties (hard, stiff, high damping (internal friction), shape memory). Martensitic transformations can occur at rather low temperature ( 100 K) where diffusional jumps of atoms are definitely frozen, but also at much higher temperature. Since they occur without transport of matter, they are not of central interest to solid state kinetics. However, in view of the crystallographic as well as the elastic and even plastic implications, diffusionless transformations may inform us about the principles involved in the structural part of heterogeneous solid state reactions, and for this reason we will discuss them. 

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