Fatigue deformation of TiAl base alloys

The fatigue behavior of Ti-36.3 wt pct Al and Ti-36.2 wt pct Al-4.65 wt pct Nb alloys was studied in the temperature range room temperature to 900°C. The microstructures of the alloys tested consisted predominantly of γ phase (TiAl) with a small volume fraction of γ phase (Ti₃Al) distributed in lamellar form. The alloys were tested to failure in alternate tension-compression fatigue at several constant load amplitudes with zero mean stress. Fracture modes and substructural changes resulting from fatigue deformation were studied by scanning electron microscopy and transmission electron miscroscopy respectively. The ratio of fatigue strength (at 10⁶ cycles) to ultimate tensile strength was found to be in the range 0.5 to 0.8 over the range of temperatures tested. The predominant mode of fracture changed from cleavage type at room temperature to intergranular type at temperatures above 600°C. The fatigue microstructure at low temperatures consisted of a high density of a/3 [111] faults and dislocation debris of predominantly a/2 [110] and a/2 [110] Burger's vectors with no preferential alignment of dislocations. At high temperatures, a dislocation braid structure consisting of all 〈110〉 slip vectors was observed. The changes in fracture behavior with temperature correlated well with changes in dislocation substructure developed during fatigue deformation. S. M. L. SASTRY was formerly NRC Research Associate in the Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH     

Microstructures of rapidly-solidified binary TiAl alloys

The microstructures of rapidly-solidified binary TiAl alloys containing 46–70 at.% Al have been studied using optical and analytical transmission electron microscopy (AEM). The phases present in the alloys and their distribution were found to be a sensitive function of composition. Essentially single-phase microstructures were seen for alloys with 46 at.% Al, 50–52 at.% Al and 60–65 at.% A. The primary solidification phases present in these alloys were α-Ti, ordered γ-TiAl and disordered cubic TiAl, respectively. The 60–65 at.% Al alloys showed indications of the solid-state formation of long-period superlattice structures based upon γ-TiAl, due to the excess Al. In other composition ranges, two-phase microstructures were seen. The 48 at.% Al alloy contained α₂-Ti₃Al + γ-TiAl, with α₂-Ti₃Al as the primary solidification phase. Alloys from 53 to 55 at.% Al were also α₂-Ti₃Al + γ-TiAl, but with γ-TiAl as the primary solidification phase. The 70 at.% Al alloy was two phase TiAl₂ + TiAl₃. A strong effect of interstitial oxygen content on the α₂-Ti₃Al + γ-TiAl phase relations was also seen. Comparison of these results with the equilibrium phase diagram and with ingot studies of the same alloys showed that most of the microstructures produced by rapid solidification were metastable. A possible metastable phase diagram for TiAl which is consistent with the results is proposed.     

Characterization of the effect of vanadium additions to TiAl base alloys

Ternary TiAl-base alloys containing V have been studied, using rapid-solidification processed materials. The mechanical behavior has been characterized and related to other results on microstructure, lattice parameters, site occupancy, phase stability, and deformation mode. It was found that V additions enhance the plasticity of duplex γ + α₂ alloys, altough not single-phase alloys. The suctibility increase which is attributable to the V addition is small (∼0.5%), compared with that attributable to the duplex structure effect (∼1%) as previously determined in a binary alloy study. The ductilization effect of V is partially due to its ability to occupy Al lattice sites and modify the TiAl bond. It is also partially due to its ability to promote twin formation, by modifying the Al partitioning and therefore the α₂/γ volume ratio in transformed regions.     

高铌TiAl合金的高温断裂韧性

利用紧凑拉伸试样通过预制疲劳裂纹研究近片层组织Ti-45Al-8Nb-0.2W-0.2B-0.1Y合金和全片层组织Ti-45Al-7Nb-0.2W-0.2Hf-0.3B-0.15C合金在750℃下的断裂韧性,并分析两种组织合金的断口形貌.结果表明,近片层组织和全片层组织高铌TiAl合金750℃时的断裂韧性分别为19.54和31.58 MPa·m^1/2,且近片层组织疲劳裂纹开始萌生时的最大疲劳载荷明显低于全片层组织.断口分析表明近片层组织中裂纹主要在等轴γ晶中萌生,裂纹扩展方式包括沿γ晶、穿γ晶及沿片层、穿片层;全片层组织中裂纹主要在垂直于加载方向的片层间萌生,裂纹以沿片层与穿片层的混合方式进行扩展且伴有二次裂纹的萌生.     

热等静压保温保压时间对TiAl合金有关性能的影响

研究了热等静压时间对TiAl合金有关特性的影响.在其它条件不变的情况下,10 min保温保压后,TiAl合金的密度已经达到3.46g/cm~3.时间从10 min逐渐增加到70min,所得TiAl合金的密度有所增加,但增加不明显,所得TiAl合金是一种非稳定状态,时间的延长对物相的影响不大.同时随着时间的延长,TiAl合金内部产生了微裂纹.1380℃,保温1h热处理后,合金内部的微裂纹消失,物相组成也转变成稳定的TiAl相.     

Creep behavior of TiAl alloys with enhanced high-temperature capability

For high-temperature applications, creep strength is of major concern, in addition to oxidation and corrosion resistance, and determines the application range of titanium aluminide alloys in competition with other structural materials. Thus, this work was aimed at identifying mechanisms of creep deformation and microstructural degradation and at developing alloying concepts with respect to an enhanced high-temperature capability. The analysis shows that dislocation climb controls deformation in the range of the intended operation temperatures. Further, complex processes of phase transformations, recrystallization, and microstructural coarsening were observed, which contribute to microstructural degradation and limit component life in long-term service. By alloying with high contents of Nb, both room- and high-temperature strength properties can be improved as Nb increases the activation energy of diffusion and increases the propensity for twinning at ambient temperature. For alloys with enhanced high-temperature capability, microalloying with carbon is also of particular use, because carbide precipitates effectively hinder dislocation motion and are thought to increase microstructural stability. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Use of damping alloys to fight noise

Moscow Institute of Steel and Alloys. Translated from Metallurg, No. 2, p. 26, February, 1994.     

粉末冶金TiAl基合金高温变形行为

采用等温压缩试验,在变形温度为600～1050 ℃、应变速率为0.002～0.2 s^-1的条件下,研究了粉末冶金Ti-47.5Al-2.5V-1.0Cr合金的高温压缩性能与高温变形行为.结果表明:合金在高温压缩变形时,屈服强度随变形温度的升高、应变速率的降低而降低,塑性趋于升高.合金在高温塑性变形时,峰值流变应力、应变速率和变形温度之间较好地满足双曲正弦函数形式修正的Arrhenius关系,说明其变形受热激活控制.在800～1050℃/0.002～0.2 s^-1范围内,合金应变敏感系数m为0.152,高温变形激活能Q为376kJ·mol^-1.     

Use of a spectrophotometric hemoglobin assay to objectively quantify intracerebral hemorrhage in mice

BACKGROUND AND PURPOSE: There is great interest in developing novel anticoagulant or thrombolytic strategies to treat ischemic stroke. However, at present there are limited means to accurately assess the hemorrhagic potential of these agents. The present studies were designed to develop and validate a method to accurately quantify the degree of intracerebral hemorrhage (ICH) in murine models. METHODS: In a murine model, ICH was induced by stereotaxic intraparenchymal infusion of collagenase B alone (6 x 10(-6) U; n = 5) or collagenase B followed by intravenous recombinant tissue plasminogen activator (rt-PA) (0.1 mg/kg; n = 6). Controls consisted of either sham surgery with stereotaxic infusion of saline (n = 5) or untreated animals (n = 5). ICH was (1) graded by a scale based on maximal hemorrhage diameter on coronal sections and (2) quantified by a spectrophotometric assay measuring cyanomethemoglobin in chemically reduced extracts of homogenized murine brain. This spectrophotometric assay was validated with the use of known quantities of hemoglobin or autologous blood added to a separate cohort of homogenized brains. With this assay, the degree of hemorrhage after focal middle cerebral artery occlusion/reperfusion was quantified in mice treated with postocclusion high-dose intravenous rt-PA (10 mg/kg; n = 11) and control mice subjected to stroke but treated with physiological saline solution (n = 9). RESULTS: Known quantities of hemoglobin or autologous blood added to fresh whole brain tissue homogenates showed a linear relationship between the amount added and optical density (OD) at the absorbance peak of cyanomethemoglobin (r = 1.00 and .98, respectively). When in vivo studies were performed to quantify experimentally induced ICH, animals receiving intracerebral infusion of collagenase B had significantly higher ODs than saline-infused controls (2.1-fold, increase; P = .05). In a middle cerebral artery occlusion and reperfusion model of stroke, administration of rt-PA after reperfusion increased the OD by 1.8-fold compared with animals that received physiological saline solution (P < .001). When the two methods of measuring ICH (visual score and OD) were compared, there was a linear correlation (r = .88). Additional experiments demonstrated that triphenyltetrazolium staining, which is commonly used to stain viable brain tissue, does not interfere with the spectrophotometric quantification of ICH. CONCLUSIONS: These data demonstrate that the spectrophotometric assay accurately and reliably quantifies murine ICH. This new method should aid objective assessment of the hemorrhagic risks of novel anticoagulant or thrombolytic strategies to treat stroke and can facilitate quantification of other forms of ICH.     

Use of transverse microradiography to quantify mineral loss by erosion in bovine enamel

BACKGROUND: A double blind, placebo controlled study was undertaken to determine the effects of 104 weeks of intermittent cyclical etidronate therapy on bone mineral density (BMD) in patients undergoing long-term oral corticosteroid therapy. METHODS: Forty nine patients of mean age 59 years on long-term (> 6 months) corticosteroid treatment were randomised to receive either 400 mg/day etidronate or placebo for 14 days followed in both groups by calcium (equivalent to 97 mg elemental Ca/day) with vitamin D (400 IU) for 76 days. The cycle was repeated a total of eight times over a period of two years. Dual energy x ray absorptiometry (DEXA) measurements of the lumbar spine and hip BMD and biochemical bone marker analyses were performed at baseline and every six months. RESULTS: Twenty six patients (10 men) received cyclical etidronate and 23 (nine men) received placebo. The mean (SD) dose of corticosteroid (prednisone or equivalent) at baseline in the etidronate group was 8 (4) mg/day and in the placebo group was 7 (4) mg/day. Most of the patients (43%) suffered from asthma. Forty one patients completed the study (22 in the etidronate group and 19 in the placebo group). All had a low BMD at entry and with treatment a significant difference was observed between groups in the mean (SE) percentage change from baseline in lumbar spine BMD at week 104 of 4.5 (1.65)% (p = 0.007) with a 95% confidence interval (CI) of 1.12 to 7.87%. No clinically or statistically significant treatment differences were observed at the hip or with bone markers. The incidence of adverse events was similar in the two groups. CONCLUSIONS: The results show that intermittent cyclical etidronate therapy with calcium and vitamin D supplementation significantly increases lumbar spine BMD in patients with osteoporosis resulting from long-term treatment with corticosteroids.     

Study on notch fracture of TiAl alloys at room temperature

In-situ observations of fracture processes combined with one-to-one observations of fracture surfaces and finite-element method (FEM) calculations are carried out on notched tensile specimens of two-phase polycrystalline TiAl alloys. The results reveal that most cracks are initiated and propagated along the interfaces between lamellae before plastic deformation. The driving force for the fracture process is the tensile stress, which is consistent with a previous study.^[1] In specimens with a slit notch, most cracks are initiated directly from the notch root and extended along lamellar interfaces. The main crack can be stopped or deflected into a delamination mode by a barrier grain with a lamellar interface orientation deviated from the direction of crack propagation. In this case, new cracks are nucleated along lamellar interfaces of grains with favorable orientation ahead of the barrier grain. The main crack and a new crack are then linked by the translamellar cleavage fracture of the barrier grain with increasing applied load. In order to extend the main crack, further increases of the applied load are needed to move the high stress region into the ligament until catastrophic fracture. The FEM calculations reveal that the strength along lamellar interfaces (interlamellar fracture) is as low as 50 MPa and appreciably lower than the strength perpendicular to the lamellae (translamellar fracture), which shows a value higher than 120 MPa. This explains the reason why cracks nucleate and preferably extend along the lamellar interfaces.     

Use of flow cytometric CD34 analysis to quantify hematopoietic progenitor cells

This review summarizes our experiments on flow cytometric analysis of CD34 positive mononuclear cells (MNC) and on colony formation of myeloid hematopoietic progenitor cells in the clonogenic assay. We examined MNC isolated by density centrifugation of bone marrow, cord blood and peripheral blood. The latter samples originated either from patients recovering from myelosuppressive treatment who received no growth factors or from patients treated with G-CSF or GM-CSF. We attempted to correlate the results obtained by CD34 analysis with the cloning efficiency determined after a 14 day culture period in the methylcellulose-based clonogenic assay. The highest cloning efficacy (60%-100%) was observed in cord blood, however, a good correlation was found in both untreated and GM-CSF treated peripheral blood samples in which a mean of 50% and 20% of the number of CD34 positive MNC gave rise to myeloid colonies. In bone marrow, the cloning efficacy was generally lower and ranged between 5% and 15%. The lowest values were observed in G-CSF treated peripheral blood in which colonies were grown from only 1%-9% of the CD34+ MNC. Due to the variable numbers of CD34+ lymphoid and/or more committed myeloid precursors which form either no colonies or only clusters, there was a greater variation and a lower cloning efficiency in the latter two cell sources. In conclusion, one colour CD34 analysis of cord blood MNC and untreated or GM-CSF treated peripheral blood MNC provides reliable results with respect to the content of myeloid progenitors. Analysis of bone marrow MNC and G-CSF treated peripheral blood MNC requires two colour staining using CD34 and CD45RA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Use of the Psychiatric Evaluation Form to quantify children's interview data.

Two interview reports for each of 207 children (aged 4–17 yrs) were available as a result of litigation requesting damages for "psychic impairment" of the survivors of a flood. Evidence was present that certain of the symptom scales on the Psychiatric Evaluation Form (PEF) could be reliably rated from the diagnostic reports by nonclinicians. Ratings from interviews conducted 6 mo apart by interviewers with different styles and purposes were significantly correlated. Information was also given about relationships among the scales, and a suggestion was made for clustering them. Clusters from each interview were significantly correlated with a global clinical impairment rating made at the time of the 2nd interview. Clusters from the 2 reports were also differentially predictive of 2 physical symptoms, enuresis and obesity. Evidence thus suggests that the PEF scales are a useful and valid means of quantifying children's interview data and can be used to examine research questions in clinical settings. (14 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

On the grain boundary statistics in metals and alloys susceptible to annealing twinning

Grain boundary distribution which includes grain boundary character distribution (i.e. distribution of boundaries by the reciprocal density of coincidence sites Σ) as well as distributions of boundaries by misorientation angles and axes is an important parameter describing polycrystalline microstructure. Numerous experimental data on grain boundary distributions in low stacking fault energy f.c.c. materials that are susceptible to annealing twinning have been analyzed and it has been established that there is a certain stable grain boundary distribution characterized by the dominance of Σ3ⁿ boundaries in all statically recrystallized materials of this class. computer modelling based on the assumption that multiple twinning is the main process controlling structure formation has confirmed this conclusion. It has been also found that distribution of lengths of different types of grain boundaries is more sensitive to the stacking fault energy and treatment of the material. Relation between grain boundary distributions and grain orientation distribution has been studied both experimentally and by computer modelling. It has been established that the grain boundary distribution is not completely determined by texture but is only influenced by it, because the grain boundary spectrum is primarily dependent on the orientation correlations which may exist between various crystallites of a polycrystal. Control of grain boundary distributions by means of various treatments has been demonstrated and possibilities of grain boundary design for improving the bulk properties of polycrystalline materials are discussed.     

Ni-Fe-X基合金抗氧化性能的研究

用热压法制备了3种成分的Ni-Fe-X基合金,并在1 000℃和1 200℃进行了大气氧化试验。XRD,SEM,EDX和金相分析表明,Ni-11Fe-10Cu-6Al-3Sn(其中数据为质量分数)的氧化层最致密,主要由深色、浅色2种不同形貌和成分的混合氧化物组成,合金中的Cu具有较强的抗氧化性能,氧化铝的存在有效改善了氧化物结构。Fe-Ni合金氧化膜疏松,抗热震性弱,氧化膜与基体间有明显的裂纹,氧化膜主要是铁氧化物。