Mechanical properties and cyto-toxicity of new beta type titanium alloy with low melting points for dental applications

Beta stabilized new alloys such as Ti–29Nb–13Zr–2Cr, Ti–29Nb–15Zr–1.5Fe, Ti–29Nb–10Zr–0.5Si, Ti–29Nb–10Zr–0.5Cr–0.5Fe and Ti–29Nb–18Zr–2Cr–0.5Si have been developed for dental applications. These alloys were designed based on master alloy Ti–29Nb–13Ta–4.6Zr (TNTZ) for biomedical applications. In this research, high melting temperature element Ta was replaced with beta stabilizing elements such as Cr, Fe and Si to lower the melting temperature of the alloy.Their melting points, mechanical properties, surface reaction layers and cyto-toxicity were investigated in this study.Melting points of designed alloys fall by about 50 K to 370 K as compared with that of TNTZ, and Ti–29Nb–13Zr–2Cr has the lowest melting point of around 2050 K. Vickers hardness of the surface of each designed alloy cast into modified magnesia based investment material is in the range of 400 Hv to 500 Hv, which is lower than that of TNTZ (around 560 Hv).Balances of strength and ductility of cast Ti–29Nb–13Zr–2Cr, Ti–29Nb–15Zr–1.5Fe and Ti–29Nb–10Zr–0.5Cr–0.5Fe are nearly equal to that of cast TNTZ.Cell viability of each cast designed alloy is excellent.     

Factors affecting eutectic solidification of Cr–Ni (–Si–Mn) white cast irons

Abstract

The crystal structure and morphology of eutectic carbides are known to strongly determine the mechanical and tribological properties of Cr–Ni white cast irons. In an effort to improve these properties, investigators at the US Bureau of Mines have studied the effects of alloying additions of 0·0–1·8%Si, 0·0–6·7%Ni, and 0·0–3·2%Mn (all wt-%) and solidification rates of from 1·0 to ～500 K min^?1 on hypoeutectic irons containing ～3%C and ～8%Cr. The structure and morphology of the eutectic carbides formed were identified using electron microprobe analysis, X-ray diffraction, and scanning electron and optical microscopy. Differential thermal analysis was used to study the effects of alloying additions on the solidification reactions. The results show that these irons can have carbide structures consisting of (Fe, Cr)₃C or (Fe, Cr)₇C₃ or both. These observations are explained in terms of the effects of Si, Ni, and Mn on the liquidus surface of the metastable Fe–Cr–C phase diagram.

MST/1288     

Morphology and crystallographic phase of V–C particles formed in Fe–Cr–Ni–V–C alloys

Abstract

The morphology and crystallographic phase of V–C carbide particles formed in cast Fe–Cr–Ni–V–C alloys were investigated by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy (TEM). The combination of results obtained with these techniques revealed that cuboidal, cruciform and spherical carbide particles were formed, depending on the alloy composition, all having the cubic-VC_1?x structure (Fm-3m). Detailed TEM observations suggested that small carbide particles were initially cubic in shape and became spherical with increasing particle size. All cuboidal and spherical carbides were single crystallites with no grain boundary at any particle sizes, even after growing to 6 μm in diameter.     

Effect of Nb on long-term creep life of JIS SUS304HTB and JIS SUS347HTB steels – heat-to-heat variation and life assessment of stainless steels

Heat-to-heat variation in creep life has been investigated for the 9 heats of JIS SUS 304HTB (18Cr–8Ni steel) and also for the 9 heats of JIS SUS 347HTB (18Cr–12Ni–Nb steel) in the NIMS Creep Data Sheets, mainly taking the effect of Nb into account. The heat-to-heat variation in creep life of 304HTB is mainly caused by the variation in precipitation hardening due to fine NbC carbides at short times, while it is mainly caused by the variation in available nitrogen concentration, defined as the concentration of nitrogen free from AlN and TiN, at long times. The heat-to-heat variation in creep life of 347HTB is mainly explained by the variation of boron concentration, 3–27 ppm, but not by the variation of solution temperature, Nb/C atomic ratio and phosphorus concentration. Boron reduces the coarsening rate of fine M₂₃C₆ carbides along grain boundaries, which enhances the grain boundary precipitation hardening.     

Effect of heat treatment on microstructure and mechanical properties of Cr–Ni–Mo–Nb steel

Abstract

The microstructure and mechanical properties of a medium carbon Cr–Ni–Mo–Nb steel in quenched and tempered conditions were investigated using transmission electron microscopy (TEM), X-ray analysis, and tensile and impact tests. Results showed that increasing austenitisation temperature gave rise to an increase in the tensile strength due to more complete dissolution of primary carbides during austenitisation at high temperatures. The austenite grains were fine when the austenitisation temperature was <1373 K owing to the pinning effect of undissolved Nb(C,N) particles. A tensile strength of 1600 MPa was kept at tempering temperatures up to 848 K, while the peak impact toughness was attained at 913 K tempering, as a result of the replacement of coarse Fe rich M₃C carbides by fine Mo rich M₂C carbides. Austenitisation at 1323 K followed by 913 K tempering could result in a combination of high strength and good toughness for the Cr–Ni–Mo–Nb steel.     

Effect of niobium on creep and creep crack growth of cast Ni–Cr austenitic steel

Abstract

An investigation of the effect of Nb on creep properties and creep crack growth rate in a 25Cr–35Ni–0·4C (wt-%) cast steel at 871 and 950°C was carried out. Tensile tests were also carried out at room temperature, 871, and 950°C. The tensile strength and elongation increased with an increase in Nb content at high temperatures. There existed an optimum Nb content for the creep properties and creep crack growth rate. Creep crack growth is controlled by creep deformation.

MST/1222     

Precipitation in V bearing microalloyed steel containing low concentrations of Ti and Nb

Abstract

The paper describes the precipitation behaviour in a thermomechanically processed V bearing microalloyed steel containing small additions of Ti and Nb (0·007–0·008 wt-%) using analytical transmission electron microscopy. An intriguing aspect is the significant precipitation of titanium and niobium at these low concentrations, contributing to strength. A high density of multimicroalloyed precipitates of (V, Nb, Ti)(C, N) are observed instead of simple TiN, TiC, and NbC precipitates. They are characterised as cuboidal (45–70 nm), spherical (20–45 nm), irregular (20–45 nm), and fine (10–20 nm). Estimation of solubility products of carbides and nitrides of V, Nb, and Ti implies that the precipitation of titanium occurs primarily in austenite. Interphase precipitation of niobium occurs during austenite to ferrite transformation, while complete precipitation of vanadium takes place in the austenite–ferrite region close to completion of transformation. Substoichiometric concentrations of Ti and Nb, the presence of nitrogen, and the mutual extensive solubility of microalloying carbonitrides explains the formation of core shell (triplex/duplex) precipitates with highly stable nitrides ((Ti, Nb, V)N) in the core and carbides ((Ti, Nb, V)C) in the shell. The qualitative stochiometric ratios of triplex and duplex carbonitrides were Ti_0·53Nb_0·35V_0·12 and Ti_0·6V_0·4, Nb_0·51V_0·49 and Ti_0·64Nb_0·36. Extensive precipitation of fine carbides on dislocation substructures, and sub-boundaries occurred. They were generally characterised as vanadium carbide precipitates with ordered cubic L1₂ structure and exhibited a Baker–Nutting orientation relationship with the ferrite matrix. M₄C₃ types of carbides were also observed similar to the steel, having high concentrations of Ti and Nb.     

Refining effect of nitrogen on M7C3 carbides in hypereutectic Fe–25Cr–4C–0.5Ti–0.5Nb hardface coatings

Hypereutectic Fe–Cr–C–Ti–Nb coatings with N additives were developed by surface-hardening welding (hardfacing). The experimental results showed that the primary M₇C₃ carbides were refined by the N additives in the coatings. Based on the micro-morphologies of M₇C₃ and (Ti,Nb)(C,N), the (Ti,Nb)(C,N) was present inside the primary M₇C₃ carbides, and they were tightly combined. The mismatch between the (010) crystal plane of M₇C₃ and the (110) crystal plane of (Ti,Nb)(C,N) was 6.15%, which indicated that (Ti,Nb)(C,N) was moderately effective as a heterogeneous nucleus of M₇C₃ carbides. Therefore, the preferentially precipitated (Ti,Nb)(C,N) in the Fe–Cr–C–Ti–Nb coating was the heterogeneous nucleus of the primary M₇C₃ carbides and thereby refined the primary M₇C₃ carbides .     

Effect of selective oxidation of chromium on creep strength of Alloy 617

Abstract

In high temperature alloys, the selective oxidation of chromium to form a chromia scale leads to subsurface depletion of chromium in the alloy which in turn leads to the dissolution of chromium carbide in the depleted zone. The effect of this carbide depleted subsurface zone on the creep properties of Inconel Alloy 617 (Ni–22Cr–12Co–9Mo–1Al–0·08C; wt-%) has been determined. The specimens were subjected to heat treatments before creep testing to simulate long term service exposure of a thin walled heat exchanger tube operating at high temperatures. It was found, surprisingly, that in creep tests carried out at 900°C, specimens having extensive chromium depleted and carbide free subsurface zones exhibit higher creep strength than specimens thermally aged for the same durations, but having no chromium depleted zone. As chromium was removed from the matrix owing to selective oxidation, the carbon, released as the carbides in the chromium depleted zone dissolved, migrated to the centre of the specimen, producing enhanced carbide precipitation. This led to an increase in the creep strength of the specimen core, which offset the loss in creep strength of the subsurface zone. The expected detrimental effect of chromium depletion was therefore not observed.

MST/1487     

The development of aligned precipitates during internal carbonitridation of Fe–Ni–Cr alloys

Abstract

An austenitic Fe–25Cr–19Ni stainless steel alloy was carbonitrided at 1,000°C in an atmosphere with a_c=1 and P(N₂)=0.9atm to form two discrete precipitation zones. Local equilibrium between the precipitates and the austenite matrix carbon activity was achieved throughout the reaction zone. Small, globular Cr₇C₃ particles were formed immediately beneath the surface. High aspect ratio Cr₂₃C₆ lamellar plates were formed deeper in the precipitation zone and were found to have a cube–cube orientation relationship with the austenite matrix. The inward growth of these carbides was facilitated by the formation of an austenite/depleted austenite grain boundary at the precipitation front, which transformed the austenite to a more appropriate orientation and accelerated the segregation of chromium to the carbide tips.     

Microstructural evolution in bainite,martensite, and δ ferrite of low activation Cr–2W ferritic steels

Abstract

The microstructural evolution in (2–15)Cr–2W–0·1C (wt-%) firritic steels after quenching, tempering, and subsequent prolonged aging was investigated, using mainly transmission electron microscopy. The steels examined were low induced radioactivation ferritic steels for fusion reactor structures. With increasing Cr concentration, the matrix phase changed from bainite to martensite and a dual phase of martensite and δ ferrite. During tempering, homogeneous precipitation of fine W₂C rich carbides occurred in bainite and martensite, causing secondary hardening between 673 and 823 K. With increasing tempering temperature, dislocation density decreased and carbides had a tendency to precipitate preferentially along interfaces such as bainite or martensite subgrain boundaries. During aging at high temperature, carbides increased in size and carbide reaction from W₂C and M₆C to stable M₂₃C₆ occurred. No carbide formed in δ ferrite. The precipitation sequence of carbides was analogous to that in conventional Cr–Mo steels.

MST/1049     

Use of titanium and zirconium in centrifugally cast heat resistant steel

Abstract

Low carbon 25Cr–35Ni steel (HP type steel) modified with titanium and zirconium has been produced by centrifugal casting. The different phases present in the as cast and aged conditions were described by light optical and scanning electron microscopy with secondary electron imaging and energy dispersive spectroscopy. Results suggest that the use of titanium as a microalloying element reduces secondary precipitation during aging. Moreover, secondary precipitates in the microalloyed steel are much finer and more evenly distributed. On the other hand, zirconium oxides was found to be potential nucleation sites for primary titanium rich carbides contributing to an optimum distribution of these carbides in the tubes. These differences together with the higher stability of the titanium containing primary carbides are responsible for the improvement on ductility and creep resistance found in the present work.     

Distribution of strong carbide forming elements in hard facing weld metal

Abstract

Carbides and the matrix microstructure in high carbon hard facing weld metals reinforced with strong carbide forming elements were investigated by means of electron probe microanalysis (EPMA), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). The thermodynamics and the effect on the matrix of the formation of carbides were discussed. It was revealed that Nb, Ti, and V strongly influenced the distribution and existing state of carbon, induced obvious depletion of carbon in the matrix, and precipitation of carbides. However, when only V was alloyed as the carbide forming element, carbides were scarce and distributed along the grain boundary, and the hard facing metalwas easily hardened. The hard facing alloy reinforced with Nb, V, and Ti can form finely dispersed carbides and low carbon martensite matrix.     

Interface temperature measurement of M2C and M6C eutectic carbides in the Fe–Mo–C system

The High speed cast iron, which is used for hot rolling parts, needs high fracture toughness and wear resistance. To improve these properties, the control of eutectic carbides, M₃C, M₇C₃,M₆C and MC is important by adding elements such as Cr, W, V and Mo.

The aim of this study is to estimate which carbide will solidify under certain solidification conditions and compositions. This prediction criterion can be gained by measuring the interface temperature of each carbide in various samples with different solute elements, composition and growth rate.

In this report, the solidified temperature of γ + M₂C and γ + M₆C eutectic carbide in the Fe–Mo–C ternary system in the composition range near to the eutectic monovariant line, was measured during the unidirectional solidiication process. The relationship between solidified interface temperature and growth rate was obtained. In eutectic solidification along the γ + M₆C monovariant line, a coefficient of undercooling, the k value, was obtained.

The authors have already measured the k values of other eutectic carbides, such as γ + M₃C, austenite + M₇C₃, and γ + VC in Fe–Cr–C and Fe–V–C system. The paper also discusses the relationships between these properties of eutectic carbides.     

Effect of molybdenum on microstructure and wear properties of Fe–Ti–Mo–C laser clad coatings

Abstract

The AISI?1045 steel surface was alloyed with preplaced ferrotitanium (Fe–Ti), ferromolybdenum (Fe–Mo) and graphite powders using a 5 kW CO₂ laser. In situ carbide reinforced Fe based surface composite coating was fabricated. The results showed that (Ti,Mo)C particles with flower-like and cubic shapes were formed during laser cladding process. The growth morphology of the reinforcing (Ti,Mo)C carbide has typically faceted features, indicating that the lateral growth mechanism is still the predominant growth mode under rapid solidification conditions. Increasing the amount of Fe–Mo in the reactants led to a decrease in carbide size and an increase in volume fraction of carbide but increased the crack sensitivity of the coating. The multiple carbides of (Ti,Mo)C created a higher microhardness and excellent wear resistance than TiC alone under dry sliding wear test condition.     

Mechanical properties of Nb based multiphase alloy studied by nanoindentation and atomic force microscopy

Abstract

The mechanical properties and deformation behaviours of as cast and heat treated Nb–21Ti–4C–xAl (x: 0, 5, 10 and 15 at-%) alloys are comprehensively investigated using nanoindentation and atomic force microscopy. For the Nb–21Ti–4C alloy, nanoindentation tests are performed for the Nb solid solution (Nbss) matrix, carbide and the interface between them. The results show that the hard carbide, which has a strong bonding with the Nbss matrix, can enhance the alloy before and after heat treatment, and the eutectoid transformation of the large sized carbide after heat treatment leads to less possibility for the forming of cracks on the carbide surface, which in turn improves the toughness. For the Nb–21Ti–4C–(0, 5, 10, 15)Al alloys, the hardness of the Nbss matrix increases significantly with increasing Al fraction for both as cast and heat treated alloys. However, deviations of the elastic modulus are inconspicuous with the Al fraction for the as cast and heat treated alloys.     

Effect of interfacial segregation of magnesium on high carbon (18%Cr) cast steel

Abstract

The effect of Mg on the microstructure and properties of a high carbon cast steel (nominal composition, wt-%: 18Cr–2Ni–0·75Mo–Mn–Si–Fe_bal.) is investigated. It is shown that the microaddition of Mg refines the primary carbide (Cr_0·51Fe_0·49)₇C₃ and promotes an equiaxial dendritic structure, and the resulting structure refinement improves significantly the impact toughness of the alloy. Using Auger electron spectroscopy and electron probe microanalysis, it is shown that there is an unusually high Mg segregation (~70 at.-%) at the carbide interface region. It is proposed that the Mg enrichment at the carbide interface is primarily a result of the liquid phase separation that occurs in liquid Fe–Mg alloys when the Mg level attains a minimum value (~1–2 at.-%), and that this initial enrichment is a result of a combination of Mg rejection by the carbide and uphill diffusion of Mg to the carbide interface region. A thermodynamic analysis is presented to show the degree of component (Cr, Ni) segregation required for uphill diffusion of Mg to occur, which is in qualitative agreement with that observed.

MST/1046     

Microstructure of Fe-TiC surface composite produced by cast-sintering

Fe-TiC surface composite was prepared in situ on a surface of cast steel by means of cast-sintering technique. The microstructure of this material was investigated by means of SEM, electron probe and XRD. Results show that the TiC and (Fe,Cr)₇C₃ carbides in an iron matrix were achieved on the surface of cast steel during cast-sintering. From the top surface of sample to that of the master-steel, the concentration of Ti, Cr and Ni as well as the quantity of both TiC and (Fe,Cr)₇C₃ carbides decreased gradually, and the morphology of (Fe,Cr)₇C₃ transforms from strip-chunky into make-and-break reticulation. There was an excellent metallurgy-bond between the surface composite layer and the master-steel.     

Influence of microalloying additions on thickness of grain boundary carbides in ferrite–pearlite steels

Abstract

For a series of plain C and microalloyed steels at two levels of Mn, the growth of grain boundary carbides has been monitored after heating to 920°C and cooling at 40 and 150 K min^?1 through the austenite–ferrite/pearlite transformation down to room temperature. In pearlite free steels, on cooling to room temperature, all the C in solution in the ferrite is able to precipitate as carbides at the boundaries and the grain boundary carbide thickness is dependent on the number of nucleation sites for precipitation. Increasing the cooling rate increases the number of sites and reduces the carbide thickness. In ferrite–pearlite steels, the grain boundary carbides form the ‘tails’ to the pearlite colonies. The thickness of the grain boundary carbide is related to the pearlite reaction, since the temperature at which this occurs controls both the thickness of the carbide nuclei and the amount of C available for precipitating out on these tails. Increasing the cooling rate and Mn content causes a decrease in the transformation temperature and leads to finer carbides. The pearlite nose transformation temperature must be ≦600°C to produce fine (≦0·2 μm) carbides. The austenite grain size, which controls the pearlite colony size, is also very important in determining the thickness of carbides, since the finer the grain size, the greater the carbide density and,for a given amount of C available for precipitation, the finer the resulting carbides. Faster cooling or a higher Mn content refine the pearlite colony size leading to finer carbides. Compared with C–Mn–Al steels, Nb and Ti microalloying additions result in coarser carbides and higher carbide densities. The increased carbide density is due to the finer austenite grain size and the coarser carbides are due to the finer grain size raising the transformation temperature. The implications of these observations on impact behaviour are discussed.

MST/1858     

Synthesis and characterisation of TiC reinforced CoCrFeNi composite by hot-pressing sintering

ABSTRACT

Ti, Co, Cr, Fe, Ni and graphite powders were used to fabricate TiC reinforced CoCrFeNi composite by mechanical alloying and consequently hot pressing sintering at 1200°C for 1?h. Results indicated that Co, Cr, Fe and Ni powders were deformed, cold welded and crushed repeatedly during milling and an face-centred cubic-structured solid solution was obtained after milled for more than 10?h. Nano-sized TiC and micron-sized Cr₇C₃ type carbides were formed and embedded in the CoCrFeNi matrix dispersedly after sintering. The hardness and compressive fracture strength of the sintered composite reached 501 HV and 2.55?GPa, respectively, which could be ascribed to the presence of large amount of in-situ formed TiC and Cr₇C₃ type carbides in the composite.