H. M. Flower

Martensitic transformations in Ti-Mo alloys

A detailed investigation has been made of the structure of alloys of the Ti-Mo system containing up to 10wt% Mo, water-quenched from theβ-phase region. With increase in molybdenum content, the martensite structure changes from hexagonal (α′) to orthorhombic (α″) at ∼4 wt% Mo, and at 10 wt% Mo, the structure is completely retained β. For alloy compositions <4 wt% Mo, there is a diffusional component in the transformation of the β-phase at the quench rates employed. There is a transition, with increase in molybdenum content, in morphology (from massive to acicular) and in substructure (from dislocations to twins). However, the transitions in crystallography, morphology and sub-structure are not directly related to one another except for an abrupt loss of dislocation substructure at theα′/α″ transition. The α toα″ crystallographic transition has the characteristics of a second order transformation, and evidence has been obtained of the existence of a spinodal within the metastable orthorhombic system. The orthorhombic martensites of Ti-6 and 8 wt% Mo decompose during quenching producing a fine modulated structure within the martensite plates, consistent with a proposed spinodal mode of decomposition.

Silicide precipitation in the Ti−Zr−Al−Si system

Silicide precipitation has been studied in a number of martensitic Ti−Si alloys and ternary and more complex alloys containing zirconium and aluminum. The precipitation has been shown to be strongly influenced by composition. Heterogeneous nucleation of Ti5Si3 occurs at, and above, 500°C in binary Ti−Si alloys containing up to 2.4 pct Si and little conventional precipitation hardening can be achieved. Zirconium additions are incorporated into the silicide and produces finer, more stable particle dispersions accompanied by an improved aging response. If critical concentrations of zirconium (approximately 5 pct) and silicon (approximately 1 pct) are present, matrix nucleation of G.P. zones occurs at, and below 550°C. This is accompanied by a significant aging response. Additions of aluminum reduce the rate of precipitate growth while having little effect on the nature of the precipitates produced. It has been shown that typical commercial heat treatments of “near α” silicon-bearing alloys (0.5 pct Si max) do not produce silicide dispersions.

The laser surface-alloying of iron with carbon

Observations are reported on the structure of iron, laser surface-alloyed with carbon. Repeated laser surface-melting of iron pre-coated with DAG graphite has produced layers containing up to ∼6 wt % C, showing fine-scale white iron structures. Eutectic regions (interlamellar spacing ∼0.5μm) have been shown by transmission electron microscopy to consist of Fe3C + ferrite, the latter having formed by decomposition of austenite during solid state cooling. Regions of fine pearlite (spacing ∼55 nm) have also been observed. Carbon diffusion into the substrate during alloying produces a zone containing austenite and martensite.

The β ⇄ α transformation in dilute Ti-Mo alloys

An investigation has been made of the various phase transformations which occur when dilute Ti-Mo (0.5 and 1.0 wt% Mo) alloys are cooled from theβ phase field and are subsequently tempered in theα +β phase field. The morphology of the decomposition products varied with cooling rate and can be correlated with the competition between the formation ofα′ by martensitic and by diffusional processes. On isothermal transformation below theMs, or upon tempering,β phase precipitates grow heterogeneously from the supersaturatedα′ plates formed by shear. The precipitate habit plane is irrational and corresponds to the {334}β plane.

The effect of Si,Zr, Al and Mo on the structure and strength of Ti martensite

The structure and strength of martensite in “near α” titanium alloys have been studied in the composition range (wt %) up to 10% Zr, 6%, Al, 1/2% Mo, 2.4% Si. [0001], 1/3 〈11¯20〉 dislocations, 1/3 〈10¯10〉 stacking faults and approximately {10¯11} twin related martensite plates are found to be common features of the martensite. Martensite “midribs” consist either of finely transformed material between martensite plates, or regions of low dislocation density within martensite plates.The martensite morphology is related to the alloy composition, changing from “massive” to “plate-like” with increasing solute content. The strength of the martensite is controlled largely by solid solution strengthening.

The deformation characteristics of metastable ? -phase in a Ti-15 wt % Mo alloy

An investigation has been made of the deformation characteristics of metastableβ-phase in a commercial Ti-15 wt% Mo alloy within the temperature range −196 to + 146° C. Deformation occurred by slip, and by mechanical twinning on the {332} 〈113〉 system. Twinning occurred preferentially with decrease in deformation temperature. Enhanced ductility in tension, observed in the range − 94 to + 20° C, was attributed to the inhibition of “necking” resulting from strengthening by twin formation. A thinninginduced transformation occurred during foil preparation for electron microscopy.

The effect of silicon on the structure and mechanical properties of an α+Β titanium alloy

Titanium 4 wt% Al-4 wt% Mo-2 wt% Sn containing 0, 0.25 and 0.5 wt% Si has been solution-treated in the α+Β phase field at 900‡ C. The microstructures obtained at room temperature after cooling from 900‡ C at various rates have been determined using transmission electron microscopy and the partitioning of the elements between the phases has been established using X-ray energy dispersive analysis on the thin foils. The degree of partitioning increases with decreasing cooling rate: aluminium partitions to the α-phase, molybdenum and silicon to the Β-phase and tin remains uniformly distributed. Silicon is found to inhibit the partitioning of molybdenum: this has a profound effect on the stability of the Β-phase and the resultant microstructure. In quenched material containing transformed Β, substantial age hardening can be obtained in the range 350 to 600‡ C and is associated with precipitation within the orthorhombic martensite phase, possibly occurring via a spinodal mechanism. Silicon has little effect on the microstructure of air-cooled samples but contributes to high-temperature strength via dynamic strain ageing.

Martensite Formation and Decomposition in Alloys of Titanium Containing β-Stabilizing Elements

An investigation is reported of martensite formation in certain Ti-Mo, Ti-V and Ti-Nb alloys and of the decomposition of martensite on ageing. A transition from hexagonal to orthorhombic martensite occurred with increase in solute content. On ageing the orthorhombic martensite structures, spinodal decomposition took place, accompanied by substantial strengthening. Reversion of α” to β can also occur in some alloys.

The structure of Ti-6Al-4V laser surface alloyed with carbon

Laser processing has been used by Ayers et al to surface harden and to improve the wear resistance of Ti-6A1-4V alloy by introducing carbide particles eg TiC, into the laser melt pool. When sufficient solution of TiC particles occurs to produce melt zones high in carbon content, fine dendrites of TiC form during subsequent solidification. Refined dendrites of TiC have also been produced by the laser melting of Fe-TiC alloy. Research on ferrous substrates has used another approach to surface hardening by carbide formation, namely, to precoat the substrate with DAG graphite prior to laser melting; repeated cycles of coating and laser melting have been found to introduce high carbon levels (up to about6 wt%C). This same procedure has been applied in the presently reported work to obtain a high degree of carbon solution into titanium substrates with accompanying surface hardening; some observations on carbide structure and morphology are briefly reported here. Laser surface alloying of a Ti-6A1-4V substrate with carbon can produce structures containing substantial proportions of TiC dendrites in a predominantly martensitic matrix. The TiC contains only small amounts of Al and V. Crack-free alloyed zones can be produced for improving wear resistance.

The effect of an addition of molybdenum on the quenched and aged structure of a Ti-1 wt % Si alloy

Titanium-1% silicon alloys with the addition of 0.5 and 1 wt % Mo, when water quenched from the β field, contain small regions of the β-phase lying between the martensitic crystals indicating that some diffusional changes occur during quenching. On ageing the 1% Mo alloy at 650° C titanium suicide precipitation occurs together with spheroidization of the β-phase.