Creep and stress-rupture of high strength zirconium alloys

Abstract

Zirconium-based alloys with small additions of tin, molybdenum, niobium and aluminum are being developed as candidate materials for high-strength pressure tubes in advanced CANDU reactors. The out-reactor creep and stress-rupture properties of 12 alloys based on these elements have been determined and correlated with their metallographic structures and substructures and thermomechanical histories. An alloy of composition Zr-3 wt% Sn-1 wt% Mo-1 wt% Nb was the best for high creep strength, low neutron capture cross section and reasonable corrosion resistance.

Résumé

Des alliages à base de zirconium et con tenant de faibles additions d'étain, de molybdéne, de niobium et d'aluminium sont en voie de développement comme matériaux à haute résistance pour des tubes de force pour le réacteur CANDU. Les propriétés de 12 de ces alliages ont été évaluées hors réacteur quant au fluage et à la rupture; des corrélations ont été établies entre ces propriétés et la structure métallographique, et les traitements thermomécaniques subis. Le meilleur alliage pour sa résistance au fluage, sa faible section de caputre des neutrons et sa résistance à la corrosion, a une composition de: Zr-3% Sn-1% Nb (% pondéral).     

Development of high strength wrought aluminum-base alloys

Work is reported on small-scale laboratory techniques described for development of high strength wrought aluminum alloys and on structure and properties of alloys produced. Alloy screening tests were on splat cooled samples and mechanical property measurements were from cast and rolled thin section cast plates. The highest tensile strengths obtained were in excess of 115,000 psi; highest yield strenghts were in excess of 110,000 psi. Four different alloys tested showed yield strengths in excess of 90,000 psi with elongations in excess of 5 pct.     

Creep of zirconium and zirconium alloys

Cumulative zirconium and zirconium alloy creep data over a broad range of stresses (0.1 to 115 MPa) and temperatures (300 °C to 850 °C) were analyzed based on an extensive literature review and experiments. Zirconium obeys traditional power-law creep with a stress exponent of approximately 6.4 over stain rates and temperatures usually associated with the conventional “five-power-law” regime. The measured activation energies for creep correlated with the activation energies for zirconium self-diffusion. Thus, dislocation climb, rather than the often assumed glide mechanism, appears to be rate controlling. The common zirconium alloys (i. e., Zircaloys) have higher creep strength than zirconium. The stress exponents of the creep data in the five-power-law regime were determined to be 4.8 and 5.0 for Zircaloy-2 and Zircaloy-4, respectively. The creep strength of irradiated Zircaloy appears to increase relative to unirradiated material. It was found that the creep behavior of zirconium was not sensitive to oxygen in the environment over the temperature range examined.     

Development of the manufacturing process of aluminum alloys with high strength and conductance

A new manufacturing process of high-strength electroconductive aluminum alloys and plane samples which is based on the immersion casting method with the action of weak current pulses (WCPs) and pulsed magnetic fields (PMFs) with the subsequent multicycle rolling is proposed. The objects of the investigation were alloys containing 97.7–98.3% Al and alloying additives (Cu, Mg, Mn, Si, Fe, Zn, Sc, and Zr). The considered technology enabled us to fabricate the samples 0.20–0.25 mm thick with a nanodimensional structure (d < 100 nm). It is established that the PMF and WCP effects differently affect the physical characteristics of the alloy. The WCP treatment enables one to attain higher strength (up to 430 MPa) and conductance (up to 55% IACS) than the PMF effect, but is the alloy plasticity is lower. As the aluminum content in the alloy decreases by 0.5%, its conductance lowers by 6–14% and its largest drop (up to 14%) is observed for the WCP-treated samples, while the smallest one (up to 6%) is observed for the PMF-treated samples.     

The influence of hydride size and matrix strength on fracture initiation at hydrides in zirconium alloys

The effects of matrix strength (yield stress) on hydride fracture and alloy ductility have been studied as a function of stress state, hydride content, hydride size, and precipitation stress. Uniaxial and triaxial states of stress were investigated by using smooth and notched tensile specimens, respectively, containing 0.18 or 0.90 at. pct H, with the longest hydride platelet dimension varying from 5 to 400 μm. The majority of the hydrides in the specimens had their plate normals oriented parallel to the tensile axis direction. Crack initiation at hydrides was monitored using acoustic emission, finiteelement calculations were employed to determine the stresses and strains in the notched specimens, and metallographic and fractographic analyses were carried out to determine the state of fractured hydrides/voids near and on the fracture surface. These techniques showed that, up to a hydride platelet length of ∼50 to 100 μm and regardless of the stress state, a critical plastic strain, independent of matrix strength, controls the initiation of fracture in hydrides. The amount of plastic strain needed to fracture hydrides decreases as (a) the average hydride length increases and (b) the axiality of stress increases. The equivalent plastic strain to fracture small hydrides is ∼ 1 pct under a triaxial as opposed to ∼5 pct under a uniaxial state of stress. When the average hydride platelet lengths are longer than ∼50 to 100 μm, negligible plastic deformation is required to fracture hydrides. A critical applied stress then is the governing factor in all three materials, ranging from 750 to 850 MPa, depending on the stress state.     

Thermomechanical treatments on high strength Al-Zn-Mg(-Cu) alloys

An investigation was carried out to determine the metallurgical properties of Al-Zn-Mg and Al-Zn-Mg-Cu alloy products processed according to newly developed Final Thermomechanical Treatments (FTMT) of T-AHA type. The results show that these cycles can be utilized to produce wrought products of high purity Al-Zn-Mg(-Cu) alloys characterized by equivalent toughness and ductility and much higher strength than conventionally processed commercial purity materials. Based on transmission electron microscopy studies, it was found that such improved behavior of FTMT material is attributable to the superposition of hardening effects, from aging precipitation and from dislocations. Preliminary stress-corrosion and fatigue tests indicate that these properties are not substantially influenced by T-AHA thermomechanical process. Further work is needed in this area, in order to better understand the directions to follow for developing better alloys.     

Residual grain-interaction stresses in zirconium alloys

Residual grain-interaction stresses develop during the thermomechanical treatment of Zr alloys due to the anisotropy of the mechanical and thermal properties of the hep lattice. The origin and characteristics of this type of residual stress are described in conjunction with underlying physical principles employed to measure grain-interaction strains by means of neutron diffraction. The effect of thermal treatments, deformation, and irradiation on the evolution of residual grain-interaction strains are reviewed, and the most up-to-date experimental results are presented. The effects of grain-interaction stresses on the plasticity and in-reactor deformation of ZIRCALOY-2 will be discussed. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25–29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Residual grain-interaction stresses in zirconium alloys

Residual grain-interaction stresses develop during the thermomechanical treatment of Zr alloys due to the anisotropy of the mechanical and thermal properties of the hep lattice. The origin and characteristics of this type of residual stress are described in conjunction with underlying physical principles employed to measure grain-interaction strains by means of neutron diffraction. The effect of thermal treatments, deformation, and irradiation on the evolution of residual grain-interaction strains are reviewed, and the most up-to-date experimental results are presented. The effects of grain-interaction stresses on the plasticity and in-reactor deformation of ZIRCALOY-2 will be discussed. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25–29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Regularities of sintering of zirconium diboride-molybenum alloys

Summary In the process of sintering of mixtures of zirconium diboride with 5, 10, and 15% Mo, specimen growth resulting from heterodiffusion is observed at the instant of formation of solid solution of Mo in ZrB₂ during slow heating to high temperatures or during the initial period of isothermal holding in the case of very rapid heating. At temperatures of up to 1700–1750°C, growth predominates over shrinkage, and specimen dimensions increase with increasing holding time; at temperatures above 1800°C, positive shrinkage takes place, but is very slight in the case of rapid heating to the isothermal holding temperature.During isothermal holding in the temperature range 1800–2200°C, very intensive shrinkage is observed during the initial period (20–30 min). Subsequently, this shrinkage slows down, and may be described as viscous flow caused by diffusional processes. The energy of activation of the densification process, calculated from the shear viscosity values obtained, was found to be 367±48, 352±28, and 379±46 kJ/mole for alloys of ZrB₂ with 5, 10, and 15% Mo, respectively, i.e., less than the energy of activation of densification of zirconium diboride (678±55 kJ/mole).Thus, the presence of molybdenum activates diffusion processes during sintering.     

Microstructure and properties of high strength aluminium alloys for structural applications

This article discusses the fundamental basis of high strength Al alloy design and describes the role of alloying elements, mechanical processing parameters and heat treatments toward the evolution of microstructure that controls the desired properties i.e. strength, fracture toughness, stress corrosion cracking (SCC) resistance, fatigue crack initiation and propagation resistance, and weldability in 7xxx series Al alloys. The beneficial effects of suitable micro/trace alloying elements, and deleterious effects of certain impurity elements on a variety of properties are further discussed within the present context.     

高强度管线钢板X70的开发

对高强度管线钢的市场需求和国内外发展进行了分析,通过系列技术研究,韶钢开发的高强度管线钢X70实现批量供货．X70强度韧性指标达到西气东输二线要求．     

Heat treating characteristics of high strength Al-Zn-Mg-Cu alloys with and without silver additions

Effects of the addition of 0.35 pct Ag on tensile properties of commercially-fabricated 2 in. thick plate were investigated with two 7075-type alloys and similar chromium-free compositions containing 0.35 pct Mn. Both rates of cooling during the quench from the solution heat treatment and rates of heating to precipitation heat-treating temperatures strongly affected relative strengths of the alloys. Alloys containing silver developed substantially higher strengths than the control alloys without silver when 0.125 in. (3.2 mm) thick specimens from the plate were rapidly quenched and rapidly heated to precipitation temperatures above 200∮F (92°) and isothermally precipitated. When the rapid quench was followed by slower heating to the precipitation temperature, however, both silver-free and silver-containing alloys developed comparable high strengths. When the full thickness plate was heat treated employing cooling and heating rates established by section size and standard commercial processing, silver had,relatively little effect on the mechanical properties. With lower quenching rates, alloys without silver developed higher strengths than their counterparts with silver. Formerly Assistant Director, Alcoa Research Laboratories, is now retired     

Sintering of alloys of zirconium diboride with molybdenum disilicide

Summary The sintering of zirconium diboride with molybdenum disilicide is accompanied by the formation of a solid solution based on zirconium diboride, formation of a liquid phase at temperatures above 1800°C, and partial vaporization of silicon in the ZrB₂+15% MoSi₂ alloy. At temperatures up to 1800°C, solidphase sintering takes place; at low temperatures, this is accompanied by specimen growth due to heterodiffusion processes resulting from the difference in the partial diffusion coefficients of the components and to the vaporization of excess silicon in the case of the ZrB₂+15% MoSi₂ alloy.At temperatures above 1800°C, shrinkage is caused by the formation of a liquid phase, which disappears during sintering. Under these conditions, grain recrystallization and growth in the solid solution of Mo and Si in zirconium diboride in the case of 15% MoSi₂ alloys are not completed even after 4-h holding at temperatures of 1800, 1900, and 2000°C.Translated from Poroshkovaya Metallurgiya, No. 9(45), pp. 11–16, September, 1966.     

Creep strength of magnesium-based alloys

The high-temperature creep resistance of magnesium alloys was discussed, with special reference to Mg-Al and Mg-Y alloys. Mg-Al solid-solution alloys are superior to Al-Mg solid-solution alloys in terms of creep resistance. This is attributed to the high internal stress typical of an hcp structure having only two independent basal slip systems. Although magnesium has a smaller shear modulus than aluminum, the inherent creep resistance of Mg alloys is better than that of Al alloys. The creep resistance of Mg alloys is improved substantially by the addition of Y. Solid-solution hardening is the principal mechanism of the strengthening, but the details of the mechanism have not been elucidated yet. Forest dislocation hardening in concentrated alloys and dynamic precipitation in a Mg-2.4 pct Y alloy also contribute to the strengthening. An addition of a very small amount of Zn raises the dislocation density and significantly improves the creep resistance of Mg-Y alloys. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Creep strength of magnesium-based alloys

The high-temperature creep resistance of magnesium alloys was discussed, with special reference to Mg-Al and Mg-Y alloys. Mg-Al solid-solution alloys are superior to Al-Mg solid-solution alloys in terms of creep resistance. This is attributed to the high internal stress typical of an hcp structure having only two independent basal slip systems. Although magnesium has a smaller shear modulus than aluminum, the inherent creep resistance of Mg alloys is better than that of Al alloys. The creep resistance of Mg alloys is improved substantially by the addition of Y. Solid-solution hardening is the principal mechanism of the strengthening, but the details of the mechanism have not been elucidated yet. Forest dislocation hardening in concentrated alloys and dynamic precipitation in a Mg-2.4 pct Y alloy also contribute to the strengthening. An addition of a very small amount of Zn raises the dislocation density and significantly improves the creep resistance of Mg-Y alloys. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

屈服强度500MPa级高强度高韧性耐候钢的研制

采用低碳+复合微合金化的方法,通过Cu-Ni-Cr-Si的成分体系增强耐候性能,结合新一代控制轧制与控制冷却(TMCP)工艺,成功研制出厚度达32 mm、屈服强度500 MPa级的高强度高韧性耐候钢板.试验钢板具备高强度、高的低温韧性、优异的焊接和良好耐腐蚀性能,其组织为细小的针状铁素体.     

Influence of high pressure during solidification on the microstructure and strength of Mg-Zn-Y alloys

The effect of high pressure during solidification on the microstructure and mechanical property of Mg-6Zn-1Y and Mg-6Zn-3Y was investigated using optical microscopy, scanning electronic microscopy, X-ray diffraction(XRD) and Vickers-hardness testing. Under atmospheric-pressure solidification, Mg-6Zn-1Y consisted of α-Mg, Mg7Zn3 and Mg_3YZn_6; whilst Mg-6Zn-3Y consisted of α-Mg, Mg_3Y_2Zn_3 and Mg_3YZn_6. Under 6 GPa high-pressure solidification, both alloy consisted of α-Mg, MgZ n and Mg12 YZn. The shape of the main second phase changed from a lamellar structure formed for atmospheric-pressure solidification to small particles formed for solidification at 6 GPa pressure. The dendrite microstructure was refined and was more regular, and the length of the primary dendrite arm increased under 6 GPa high-pressure solidification, which was attributed to increasing thermal undercooling, compositional undercooling and kinetics undercooling. After solidification at 6 GPa pressure, the solid solubility of Y in the second phase and the Vickers-hardness increased from 15 wt.% and 69 MPa for Mg-6Zn-1Y to 49 wt.% and 97 MPa; and from 19 wt.% and 71 MPa for Mg-6Zn-3Y alloy to 20 wt.% and 92 MPa, respectively.     

高强韧铝合金非等温时效工艺的研究进展

非等温时效工艺作为一种新兴的时效处理方法,能够有效地提高高强韧铝合金的综合性能。通过简要归纳近些年来应用于高强韧铝合金的非等温时效工艺,总结出经不同非等温时效处理后高强韧铝合金析出相的特征、合金力学性能和腐蚀性能的变化情况。非等温时效工艺的效率相较于传统时效工艺有很大提高,并且能够同时调控高强韧铝合金内基体析出相和晶界析出相的种类、尺寸和分布情况,使高强韧铝合金兼具与T6峰值时效态相差不多的力学性能和近T7x过时效态的腐蚀性能。最后,对未来高强韧铝合金非等温时效工艺的研究和应用进行了展望。      

Powder metallurgy approach for control of microstructure and properties in high strength aluminum alloys

High-strength products made from atomized Al-Zn-Mg-Cu-Co alloy powders have good combinations of strength, ductility, resistance to stress-corrosion cracking and fracture toughness. Powder Metallurgy (PJM) methods produce fine metallurgical structures and compositions which cannot be produced by Ingot Metallurgy (IJM) methods. Fine structures result from very rapid solidification and from the effect of fine dispersoids in restricting grain growth. Stress-corrosion cracking (SCC) performance is favored by grain morphology of PJM products. Co₂Al₉ particles in PJM products are 0.02 to 2.0 μm spheroids occurring frequently on grain boundaries where they may serve several functions in slowing SCC attack. Oxide particles are irregular shapes, 0.01 to 0.04 μm in size, occurring in clusters at grain boundaries and in grain bodies. Some of the oxide particles are magnesium oxide and alter the environment in a SCC crack to arrest attack. Porosity is not a significant factor in the structure of PJM products made by a vacuum compacting process. P/M wrought products have superior combinations of high strength and stress-corrosion cracking resistance compared to IJM 7075 and 7050 alloys. While equaling the fracture toughness of 7075 alloy, the PJM products at present have somewhat lower fracture toughness than 7050 alloy, due in part to a larger amount of second-phase particles in the form of Co₂Al₉ and oxide. This paper is based on an invited presentation made at a symposium on “Advances in the Physical Metallurgy of Aluminum Alloys” held at the Spring Meeting of TMS-IMD in Philadelphia, Pennsylvania, on May 29 to June 1, 1973. The symposium was co-sponsored by the Physical Metallurgy Committee and the Non-Ferrous Metals Committee of TMS-IMD.