Effect of thermal treatment on the mechanical and toughness properties of extruded SiCw/aluminium 6061 metal matrix composite

Mechanical, instrumented Charpy V-notch (CVN) energy and plane strain fracture toughness properties of SiC whisker reinforced-6061 aluminium metal matrix composite material from an extruded tube have been determined. The effect of thermal treatment and orientation have been studied. The mechanical strength properties are higher than wrought Al 6061 in the T6 condition. CVN energy values, however, were reduced by an order of magnitude.K _lc fracture toughness of the as-received, T6 and degassed + T6 thermal treatments were 50% of the wrought Al 6061 alloy. The effect of orientation showed that the orientation with the least amount of SiC whisker in the crack plane (i.e. greatest mean free path between reinforcements) yields the highest toughness value.     

Prototype evaluation of transformation toughened blast resistant naval hull steels: Part II

Application of a systems approach to computational materials design led to the theoretical design of a transformation toughened ultratough high-strength plate steel for blast-resistant naval hull applications. A first prototype alloy has achieved property goals motivated by projected naval hull applications requiring extreme fracture toughness (C _v > 85 ft-lbs or 115 J corresponding to K _Id≥ 200 ksi.in^1/2 or 220 MPa.m^1/2) at strength levels of 150–180 ksi (1,030–1,240 MPa) yield strength in weldable, formable plate steels. A continuous casting process was simulated by slab casting the prototype alloy as a 1.75′′ (4.45 cm) plate. Consistent with predictions, compositional banding in the plate was limited to an amplitude of 6–7.5 wt% Ni and 3.5–5 wt% Cu. Examination of the oxide scale showed no evidence of hot shortness in the alloy during hot working. Isothermal transformation kinetics measurements demonstrated achievement of 50% bainite in 4 min at 360 °C. Hardness and tensile tests confirmed predicted precipitation strengthening behavior in quench and tempered material. Multi-step tempering conditions were employed to achieve the optimal austenite stability resulting in significant increase of impact toughness to 130 ft-lb (176 J) at a strength level of 160 ksi (1,100 MPa). Comparison with the baseline toughness–strength combination determined by isochronal tempering studies indicates a transformation toughening increment of 65% in Charpy energy. Predicted Cu particle number densities and the heterogeneous nucleation of optimal stability high Ni 5 nm austenite on nanometer-scale copper precipitates in the multi-step tempered samples was confirmed using three-dimensional atom probe microanalysis. Charpy impact tests and fractography demonstrate ductile fracture with C _v > 80 ft-lbs (108 J) down to −40 °C, with a substantial toughness peak at 25 °C consistent with designed transformation toughening behavior. The properties demonstrated in this first prototype represent a substantial advance over existing naval hull steels. Achieving these improvements in a single design and prototyping iteration is a significant advance in computational materials design capability.     

An investigation of the microstructure and mechanical properties of the macro-interface in selectively reinforced aluminium castings

The ring groove areas of squeeze-cast Al-12% Si alloy pistons can be selectively reinforced with Saffil (Al₂O₃) fibres or SiC whiskers to provide local high temperature strength and wear resistance. Since the reinforced region and the unreinforced alloy typically have different coefficients of thermal expansion, cyclic residual stress may occur at the macro-interface between them when it experiences thermal cycling. This could conceivably result in fatigue induced damage at the macro-interface, making it susceptible to failure. To investigate this, the strength of the macro-interface has been measured before and after thermal cycling using bimaterial tensile samples. Prior to thermal exposure, samples typically failed at the macro-interface with an average strength less than that of the unreinforced alloy alone. The low initial strength has been attributed to several factors, including poor alloy-reinforcement bonding and an accumulation of brittle particles or other material at the macro-interface. After being thermally cycled 1000 times between 50 °C and 275 °C or given an equivalent isothermal exposure, samples typically failed in the unreinforced alloy or at the macro-interface with average strengths less than those measured prior to thermal exposure. However, there was no clear evidence that fatigue induced damage had occurred as a result of thermal cycling and the strength drop associated with thermal exposure has been attributed to alloy overageing.     

Effect of Re-Nb on the strength and toughness of 3Cr2MoNiWV cast hot working die steel

The effects of Re-Nb on the hardness, tensile strength, Charpy impact energy, and fracture toughness of 3Cr2MoNiWV cast hot working die steel have been investigated. It is shown that the tensile strengths at room temperature and at 600°C increased by 19% and 22% respectively while Charpy impact energy and fracture toughness increased to 109% and 70% respectively while the hardness remain unchanged by Re-Nb modification. The results showed that Re-Nb modification can refine the microstructure of the steel, increase the volume fraction of lath martensite and retained austenite in the tested steel and change the morphology of non-metallic inclusions from bar-like to a fine nodular type. The results are explained based on the modification of the microstructure due to Re–Nb modification.     

The influence of Al2O3 particulate reinforcement on cyclic stress response and fracture behavior of 6061 aluminum alloy

The cyclic stress response characteristics and cyclic fracture behavior of aluminum alloy 6061 discontinuously reinforced with particulates of Al₂O₃ are presented and discussed. The 6061/Al₂O₃ composite specimens and the unreinforced 6061 aluminum alloy were cyclically deformed using tension-compression loading under constant total strain amplitude control. Both the composite and the unreinforced alloy exhibited softening to failure from the onset of cyclic deformation. The degree of softening was observed to increase at the elevated test temperature for both the composite and the unreinforced counterpart. The intrisic micromechanisms controlling the stress response characteristics during fully-reversed cyclic straining are highlighted and rationale for the observed behavior is discussed. The cyclic fracture behavior of the composite is discussed in terms of the competing influences of intrinsic microstructural effects, deformation characteristics arising from a combination of mechanical and microstructural contributions, cyclic stress response, and test temperature.     

Microstructure and tensile properties of metal injection molding Co–29Cr–6Mo–0.23C alloy

The microstructure and tensile properties of a metal injection molding 0.23%C Co–Cr–Mo alloy (F75 alloy) were investigated. The as-sintered microstructure contains a significant amount of carbides, and is modified by solution annealing, the main effect being to reduce the amount of carbides. Ductility and ultimate tensile strength increase significantly, but yield strength decreases with solution annealing. Aging causes both intragranular and intergranular precipitation, which increases hardness and yield strength but decreases ductility excessively. In both as-sintered and solution-annealed conditions, the material displays noticeable work hardenability. By sintering at 1300 °C and solution annealing at 1220 °C, 440 MPa yield strength and 25% elongation at fracture are obtained.     

Evaluation of fracture toughness and impact toughness of laser rapid manufactured Inconel-625 structures and their co-relation

The present study involves evaluation of fracture toughness and Charpy impact toughness of Inconel 625 structures fabricated by laser based additive manufacturing. The results of crack tip opening displacement (CTOD) fracture toughness are close to those reported for the Inconel 625 weld metal. The nature of the load–time traces of instrumented Charpy impact tests revealed that the alloy Inconel 625 in laser fabricated condition was associated with fully ductile behavior with Charpy V-Notch impact energy in the range of 48–54 J. Stress relieving heat treatment at 950 °C for 1 h has resulted in marginal improvement in the impact toughness by about 10%, whereas no clear evidence of such improvement is seen in the CTOD fracture toughness. Fractographic examination of the Charpy specimens and the results of the instrumented impact tests imply that the mechanism of crack growth was propagation controlled under dynamic loading conditions. Dynamic fracture parameters were estimated from the instrumented impact test data and compared with the experimentally evaluated fracture toughness results.     

Effect of in-plane constraint on mechanical behaviour of a metallic composite in biaxial stress states

This investigation focuses on the effect of in-plane constraint, which is a measure of the hydrostatic tension at the plane stress condition, on the yielding, fracture and fatigue of thin-walled tubes of an alumina particle-reinforced 6061 aluminium alloy composite. The results are compared with that of the unreinforced alloy. The yield surface radius of the particulate reinforced metal matrix composite (PMMC) is larger than that of the alloy in all loading paths. The fracture strength of the PMMC in terms of equivalent stress value is less sensitive to the change of the in-plane constraint of biaxial loading than that of the unreinforced alloy. The fatigue lives of the composite under the same equivalent strain decrease with the increase of the in-plane constraint factor, but by a lesser amount in comparison to that of the alloy.     

Mechanical properties and fracture behavior of high-strength steels

Typical thicknesses of high-strength steels (HSS) sheets used in the car industry are inapplicable for standardized testing procedures. The aim of this study is to propose an appropriate methodology for testing and comparing of thin HSS sheets. Microstructures were observed by means of light and scanning electron microscopy. The modified Charpy impact tests and fracture toughness tests were used in order to compare the fracture properties of three different HSS sheets (Docol 1200 M, Multiphase 1200 and BTR 165). Ductile-to-brittle transition curves and tearing resistance (J − Δa) curves were measured. From the fracture toughness linked to the specimen thicknesses the value of fracture toughness K_Ic was estimated. Fractographic analysis of broken specimens has revealed that due to the fine microstructure of mixed ferrite-martensite fracture mechanism remains ductile even at low temperatures (down to −100°C). __________ Translated from Problemy Prochnosti, No. 1, pp. 155–158, January–February, 2008.     

Influence of heat treatment and hot working on fracture toughness of cast aluminium base composites

Abstract

Evaluation of toughness in terms of the fracture energy E*, obtained using Charpy impact testing and the fracture toughness K_Ic obtained from bend tested specimens, has been carried out for various cast particle reinforced aluminium base composites, namely, A356–SiC, A357–SiC, 6061–Al₂O₃, and 2014–Al₂O₃. In practice, the first two are used in the as cast foundry condition and the last two in the cast and extruded condition. Hot extrusion or rolling to reduction ratios between 2 : 1 and 50: 1 was conducted on the 6061 and 2014 composites to characterise the influence of working processes. Heat treatment conditions considered included the as cast (or as worked), solid solution treated, and T6 temper. The results show that extrusion or rolling can markedly improve the toughness, but on thermal aging the toughness is reduced. The increase in total fracture energy by hot working is mainly caused by the increase of initiation energy, whereas the decrease of fracture energy by artificial aging is controlled by the propagation energy. The values of K_Ic obtained for these composites are from 15 to 25 MN m^?3/2. Comparisons and interpretations of the dynamic Charpy fracture energy, quasistatic fracture toughness, and fracture surface of the four composites are also presented.

MST/1806     

双屏蔽(B4C-W)/6061Al层状复合板设计与性能

基于B₄C和W良好的屏蔽中子和γ射线性能,采用6061铝合金作为基体,设计了一种新型双屏蔽(B₄C-W)/6061Al层状复合材料,通过放电等离子烧结后加热轧制成板材,对制备的复合材料微观组织和力学性能进行了研究。结果表明,屏蔽组元B₄C和W颗粒均匀地分布在6061Al基体中,层界面、B₄C/Al、W/Al异质界面之间结合良好,无空隙和裂纹。在颗粒与基体界面处形成扩散层,扩散层的厚度约为6 μm (W/Al)和4 μm (W/Al)。轧制态的(B₄C-W)/6061Al层状复合板的屈服强度(109 MPa)和极限抗拉强度(245 MPa)明显优于烧结态的复合材料,但断裂韧性降低。强度提高的原因主要是轧制后颗粒的二次分布、均匀性及界面结合强度提高,基体合金的晶粒尺寸减小,位错密度增加。层状复合板的断裂方式为基体合金的韧性断裂和颗粒的脆性断裂。      

Fatigue behavior of dissimilar friction stir welds between cast and wrought aluminum alloys

Cast aluminum alloy, AC4CH-T6, and wrought aluminum alloy, A6061-T6, were joined by means of friction stir welding (FSW) technique. The effect of microstructure and post heat treatment on fatigue behavior of the dissimilar joints was investigated. Near the weld centre, Vickers hardness was lower than in the parent metals and the hardness minima were observed along the trace route of FSW tool’s shoulder edge. Tensile fracture took place on A6061 side where the hardness was minimal, resulting in the lower static strength of the dissimilar joints than AC4CH or A6061. Fatigue fracture occurred on AC4CH side due to casting defects and the fatigue strength of the dissimilar joints was similar to that of AC4CH, but lower than that of A6061. Friction stir process (FSP) and post heat treatment successfully improved the fatigue strength of the dissimilar joints up to that of the parent metal, A6061. __________ Translated from Problemy Prochnosti, No. 1, pp. 150–154, January–February, 2008.     

热循环拉伸SiCw／6061Al复合材料变形与断裂行为研究

研究了热循环拉伸ＳｉＣｗ／６０６１Ａｌ复合材料变形与断裂过程，结果表明，热循环产生的界面应力有且于ＳｉＣｗ／６０６１Ａｌ复合材料的塑性变形；热循环低应力拉伸变形主要是位错的攀移；快速冷却时的过饱和空位既有助于位错的攀移，又能促进孔洞长大。     

The stamping behavior of an early-aged 6061 aluminum alloy

The stamping behavior of 6061 aluminum alloy with various conditions of early-aging is investigated in the present study. The relationship between the stamping performance, microstructure and mechanical property for this alloy is also discussed. Experimental results show that the 6061 aluminum alloy with a 10–30 min. early-aging at 160 °C will exhibit excellent stamping performance. The burnished surface of these treated alloys can reach a quite high value of 47%. Meanwhile, the mechanical strength and impact toughness have important effects on the stamping behavior of 6061 aluminum alloy. The moderate values of mechanical strength and toughness will exhibit an optimal stamping performance.     

Kinetics of precipitation hardening in SiC whisker-reinforced 6061 aluminium alloy

Ageing behaviour at 180 °C of 6061 aluminium alloy-SiC_w composites, drawn from bars obtained in various extruded ratios, and 6061 aluminium alloy used as matrix, have been compared. These materials were dissolved in a salt bath at 529 and 557 °C for 2 h, quenched in ice-water, and aged at 180 °C in an oil bath for increasing periods. Ageing kinetics were studied with Brinell hardness measurements and differential scanning calorimetry (DSC). Various samples of the composite, deriving from bars with Φ20, Φ35 and Φ50 mm in diameter, and 6061 aluminium alloy, show the same ageing mechanism; however, the ageing rates results increased for composites. While 6061 aluminium alloy shows its maximum hardness value after about 4–5 h at 180 °C, the 6061-SiC_w composites reach theirs in 2–3 h. Moreover, for composites hardness abruptly decreases after 3 h, while aluminium alloy keeps its maximum value for an ageing time as long as 6 h. Thermal analysis allows us to put together a definite DSC trace for every microstructural state. The highest hardness values are obtained as a result of the formation of a Guinier Preston (GP) needle-shaped zones, which progressively become more thermally stable with protracted isothermal treatment at 180 °C. The different ageing process rates observed for composites and for the 6061 alloy are correlated with the sizes of the reinforcements. Dimensional analysis of whiskers has been performed by light scattering and scanning electron microscopy. Ordinarily the longer the average length of the whiskers in the samples, the faster the ageing process. Higher temperatures are required for composite solutions than for 6061 alloy. On the other hand, 6061-SiC_w samples solutionized at higher temperature and then quenched sometimes show microcrack formation in the materials.     

Microstructure and toughness of coarse grain heat-affected zone of domestic X70 pipeline steel during in-service welding

The microstructures and mechanical properties of coarse grain heat-affected zone (CGHAZ) of domestic X70 pipeline were investigated. The weld CGHAZ thermal cycles having different cooling time Δt _8/5 were simulated with the Gleeble-1500 thermal/mechanical simulator. The Charpy impact absorbed energy for toughness was measured, and the corresponding fractographs, optical micrographs, and electron micrographs were systematically investigated to study the effect of cooling time on microstructure, impact toughness, and fracture morphology in the CGHAZ of domestic X70 pipeline steel during in-service welding. The results of simulated experiment show that the microstructure of CGHAZ of domestic X70 pipeline steel during in-service welding mainly consists of granular bainite and lath bainite. Martensite–austenite (M–A) constituents are observed at the lath boundaries. With increase in cooling time, the M–A constituents change from elongated shape to massive shape. The reduction of toughness may be affected by not only the M–A constituents but also the coarse bainite sheaves. Accelerating cooling with cooling time Δt _8/5 of 8 s can be chosen in the field in-service welding X70 pipeline to control microstructures and improve toughness.     

Improvement of the fatigue characteristics of VT1-0 titanium alloy by the surface mechanical and rapid thermal treatment

We study the influence of different modes of thermomechanical treatment of the surface of commercially pure titanium followed by its rapid thermal treatment on the characteristics of strength of the surface layers. It is shown that the highest strength of the surface layers is observed after treatment performed according to the following scheme: stabilizing annealing in a vacuum of about 10⁻⁵ Pa at 800°C for 1 h → deformation of the surface by ultrasonic impact treatment → rapid induction annealing of the surface at 600°C. The structural changes induced in the surface layers of VT1-0 alloy by this treatment are analyzed. It is discovered that the fatigue limit of the VT1-0 alloy increases by 50% as compared with the annealed state. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 3, pp. 85–91, May–June, 2006.     

The effect of microstructure on the fracture toughness of a metal-matrix composite

The fracture behaviour and plane strain fracture toughness, K_IC, of four 8090-based metal-matrix composites containing 20 weight % SiC particles, 3, 6 and 23 μm in diameter, has been evaluated as a function of matrix ageing condition. Toughness values are found to be almost independent of reinforcement size. Ageing at 170°C results in a monotonic decrease in toughness with increasing strength up to the peak condition, with no subsequent recovery in toughness on overageing. However, unlike reinforced 8090, the composites are not found to be susceptible to intergranular embrittlement on overageing. The observed trends are found to be independent of reinforcement size. These findings are explained in terms of the strength, work hardening behaviour and nature and distribution of void-nucleating particles in the matrix.     

Deformation behaviour and microstructure of a 20% Al2O3 reinforced 6061 Al composite

Deformation and microstructural behaviours of a 20% (volume percent) particle reinforced 6061 Al matrix composite have been studied by torsion from 25 to 540°C with strain rates of 0.1, 1 and 5 s⁻¹. The logarithmic stress versus reciprocal temperature relationship exhibits two slopes indicating different deformation mechanisms. The 20% Al₂O₃/6061 Al composite shows a greater hardening behaviour than those of the 10% Al₂O₃/6061 Al composite and of the monolithic alloy. Above 250°C, TEM investigations reveal much smaller subgrain size and higher volume of non-cellular substructures, as well as dynamic recrystallization nuclei in the 20% Al₂O₃/6061 Al composite in comparison to those of the 10% Al₂O₃/6061 Al composite and matrix alloy the same test condition. The torsion fracture surface was studied and compared to the three point bending failure specimens.     

Effect of the La<Subscript>2</Subscript>O<Subscript>3</Subscript> sol–gel coating on the alumina scale adherence on a model Fe–20Cr–5Al alloy at 1100 °C

The effect of lanthanum sol–gel coatings was studied in order to improve the alumina scale adherence during the model Fe–20Cr–5Al alloy oxidation, at 1100 °C, in air. Various sol–gel coating procedures were applied. Argon annealing of the lanthanum sol–gel coating was tested at temperatures ranging between 600 and 1000 °C. The coating crystallographic nature was characterized by X-ray diffraction (XRD) depending on the annealing temperature. The oxidation process has been examined at 1100 °C by in situ XRD on blank Fe–20Cr–5Al, sol–gel coated and argon-annealed specimens. This study shows that the coating argon annealing at 1000 °C leads to the preferential formation of LaAlO₃ instead of La₂O₃. This coating procedure leads to an alumina scale formation showing the best adherence under thermal cycling conditions at 1100 °C.