Structure and elevated temperature properties of carbon-free ferritic alloys strengthened by a laves phase

A Laves phase, Fe₂Ta, was utilized to obtain good elevated temperature properties in a carbon-free iron alloy containing 1 at. pct Ta and 7 at. pct Cr. Room temperature embrittlement resulting from the precipitation of the Laves phase at grain boundaries was overcome by spheroidizing the precipitate. This was accomplished by thermally cycling the alloys through the α→γ transformation. The short-time yield strength of the alloys decreased very slowly with increase in test temperature up to 600°C, but above this temperature, the strength decreased rapidly. Results of constant load creep and stress rupture tests conducted at several temperatures and stresses indicated that the rupture and creep strengths of spheroidized 1 Ta-7 Cr alloy were higher than those of several commercial steels containing chromium and/or molybdenum carbides but lower than those of steels containing substantial amounts of tungsten and vanadium. When molybdenum was added to the base Fe-Ta-Cr alloy, the rupture and creep strengths were considerably increased. M. Dilip Bhandarkar, formerly with Lawrence Berkeley Laboratory.     

Creep and fracture of a laves phase strengthened ferritic alloy

The microstructural changes which occurred during creep were examined in a carbon-free ferritic alloy containing 1 at. pct Ta and 7 at. pct Cr. The alloy derived its creep resistance from a uniform dispersion of almost spherical particles of the Laves phase Fe₂Ta in a bee matrix. There was a lath-like substructure in this alloy, with dislocation tangles and subgrains within the laths. Partial recovery during creep led to the formation of a regular subgrain structure. The Laves phase particles prevented total recovery by pinning individual dislocations and subgrain boundaries. The apparent activation energy for creep, the estimated stress sensitivity of minimum creep rate, and the sub-structural features observed in crept specimens suggested that creep deformation of the alloy occurred by two or more independent processes. Fracture under creep conditions was initiated by void formation at precipitate-matrix interfaces. Intergranular void formation played an important role in the fracture of creep specimens tested at 1200°F (649°C), but not at lower temperatures. M. Dilip Bhandarkar was formerly associated.     

Creep and fracture of a Laves phase strengthened ferritic alloy

The microstructural changes which occurred during creep were examined in a carbon-free ferritic alloy containing 1 at. pct Ta and 7 at. pct Cr. The alloy derived its creep resistance from a uniform dispersion of almost spherical particles of the Laves phase Fe₂Ta in a bcc matrix. There was a lath-like substructure in this alloy, with dislocation tangles and subgrains within the laths. Partial recovery during creep led to the formation of a regular subgrain structure. The Laves phase particles prevented total recovery by pinning individual dislocations and subgrain boundaries. The apparent activation energy for creep, the estimated stress sensitivity of minimum creep rate, and the substructural features observed in crept specimens suggested that creep deformation of the alloy occurred by two or more independent processes. Fracture under creep conditions was initiated by void formation at precipitate-matrix interfaces. Intergranular void formation played an important role in the fracture of creep specimens tested at 1200°F (649°C), but not at lower temperatures.     

Texture and room temperature mechanical properties of dispersion strengthened Ni−Cr alloys

The effects of preferred crystallographic orientation on the elastic and plastic properties of dispersion-strengthened Ni-Cr alloys have been observed at room temperature. One alloy possessed a heavy cube-texture with a twin component while another alloy did not show evidence of a preferred crystallographic orientation. The elastic modulus was found to vary within the plane of the sheet of the textured alloy, depending on the direction of the axis of the tensile specimen, from 22.9×10⁶ psi (1.58×10¹² d per sq cm) to 33.1×10⁶ psi (2.28×10¹²) d per sq cm). The ultimate strength varied between 108×10³ psi (7.45×10⁹ d per sq cm) and 127×10³ psi (8.75×10⁹ d per sq cm) again depending upon the orientation of the tensile axis. These variations are related to theories of plastic and elastic behavior in materials. It is concluded that the presence of the preferred orientation acts to texture strengthen the alloy at room temperature. The tensile properties of the two alloys at 2000°F (1093°C) indicate that texture is possibly partially responsible for the excellent high temperature strengths and creep resistance of the dispersion strengthened alloys.     

Transient liquid phase bonding of ferritic oxide-dispersion-strengthened alloys

Joining of two ferritic oxide-dispersion-strengthened (ODS) alloys, MA956 and PM2000, using transient liquid phase (TLP) bonding is discussed. Thin-film boron coatings of different thicknesses were used as interlayers and different bond orientations with substrates cut along and normal to the direction of extrusion were studied, with postbond heat treatment and microscopic evaluation of the bonds. Microstructural continuity was achieved in bonds when joining fine grain substrates cut along the direction of extrusion.     

添加Y对富铜纳米相强化铁素体钢拉伸性能的改善

针对富Cu纳米相强化铁素体钢空气时效后拉伸脆断的问题,选取含与不含Y的2种对比合金钢进行相关研究。采用光镜(OM)、扫描电镜(SEM)观测了钢的基体组织、断口及氧化层;高分辨透射电镜(HRTEM)和X射线能谱(EDS)分析了纳米析出相的形貌、尺寸、数量密度、化学成分和晶体结构。结果表明:含Y钢中的含Y富Cu纳米析出粒子平均尺寸变大但数量密度降低,铁素体晶粒变细,钢的拉伸强度、塑韧性改善;Y的拖曳效应使氧化层变薄且钢基体-氧化层、氧化层之间结合牢固。富Cu纳米相强化铁素体钢空气时效后拉伸性能的改善,来源于Y的细化晶粒、抗氧化和净化效果。     

Structure and properties of rapidly solidified dispersion strengthened titanium alloys: Part II. tensile and creep properties

The effects of incoherent dispersoids on tensile and creep properties were determined in rapidly solidified Ti-Er and Ti-Nd alloys. Uniform distributions of. fine incoherent dispersoids in Ti matrix were produced by rapid solidification at cooling rates > 10³ °C per second and subsequent annealing at 700 to 800°C of Ti-1.0Er, Ti-2.0Er, Ti-1.5Nd, and Ti-3.0Nd alloys. The rapidly solidified particulates consolidated by vacuum hot pressing were isothermally forged, rolled, and annealed to produce fully recrystallized microstructures. The incoherent dispersoids in Ti-Er and Ti-Nd alloys increase by 40 to 110 pct the yield strength and ultimate tensile strength of Ti with no significant loss in ductility. The strength increments were analyzed in terms of the superposition of dispersion-, solid solution-, and fine grain-strengthening. Dispersion strengthening is offset to some extent by the reduction in interstitial oxygen solid solution strengthening caused by the scavenging of oxygen by Er and Nd. The dispersoids decrease the creep rates and increase the stress rupture lifetimes of Ti at 482 to 700 °C.     

Elevated temperature mechanical properties and residual tensile properties of two cast superalloys and several nickel-base oxide dispersion strengthened alloys

The elevated temperature tensile, stress-rupture and creep properties and residual tensile properties after creep straining have been determined for two cast superalloys and several wrought Ni-16Cr-4Al-yttria oxide dispersion strengthened (ODS) alloys. The creep behavior of the ODS alloys is similar to that of previously studied ODS nickel alloys. In general, the longitudinal direction is stronger than the long transverse direction, and creep is at least partially due to a diffusional creep mechanism as dispersoid-free zones were observed after creep-rupture testing. The tensile properties of the nickel-base superalloy B-1900 and cobalt-base superalloy MAR-M509 are not degraded by prior elevated temperature creep straining (at least up to 1 pct) between 1144 and 1366 K. On the other hand, the room temperature tensile properties of ODS nickel-base alloys can be reduced by prior creep strains of 0.5 pct or less between 1144 and 1477 K, with the long transverse direction being more susceptible to degradation than the longitudinal direction.     

含Laves相TiCr2钛铬合金的高温氧化特性

研究了含铬18%～35%(质量分数)及Laves相TiCr2钛铬合金在650,700和780℃温度下的抗氧化性能.试验结果表明,含铬量对过共析钛铬合金的抗氧化性能有显著影响.在相同条件下,含铬量低于21%的合金的抗氧化性能不如纯钛,而含铬量达到26%以上时,抗氧化性能比纯钛高2～3倍.含TiCr2钛铬合金在高温下既发生了外氧化又发生了内氧化.外氧化层没有抗氧化保护作用.铬合金化提高含TiCr2钛铬合金的抗氧化性能主要是由于形成了连续、致密的含铬内氧化层.钛铬合金的高温抗氧化性能对氧化温度敏感,氧化温度提高,则抗氧化性能明显降低.     

Structure of alloys and equilibrium phase diagram of the Zr-Ru-Ir system. VI. Influence of iridium on the stability of the laves phase ZrRu2

Translated from Poroshkovaya Metallurgiya, No. 8(332), pp. 63–66, August, 1990.     

Exploratory study of elevated temperature tensile properties of alloys based on the intermetallic compound tini

The tensile properties and oxygen contamination behavior of TiNi and several of its alloys were examined in the temperature range between 800 and 1000 K (980 and 1340°F). Additions of aluminum, chromium, and silicon increased the elevated temperature tensile strength of TiNi up to threefold. TiNi and its alloys are less susceptible to elevated temperature oxygen contamination than is the Ti-6242 alloy. The elevated temperature tensile strength of TiNi is not affected by its 900 K (1160°F) allotropie transformation. Aluminum and possibly chromium promote solid-solution softening in TiNi at low solute concentrations. MARVIN GARFINKLE, formerly with NASA Lewis Research Center, Cleveland, Ohio 44135     

Structure and phase composition of Ti-Ni-Cu alloys

Conclusions Alloying with 4–10% Cu has an inoculating action on the structure of the eutectic titanium-nickel alloy, resulting in its refinement and a redistribution of its constituents, improves the wetting of diamond by the alloy, and lowers the latter's melting point. This opens up the possibility of using such materials in the formulation of diamond-containing composites. The addition of 20–30% Cu results in the formation of three-phase alloys of intermetallic character, which are characterized by a low melting point (930°C), a finegrained structure, and satisfactory adhesion to diamond, but also by low strength and ductility.Translated from Poroshkovaya Metallurgiya, No. 7 (259), pp. 52–57, July, 1984.     

Phase stability and mechanical properties of carbide and boride strengthened chromium-base alloys

Phase stability and mechanical properties of five carbide and two boride strengthened chromium-base alloys are presented. Compositions examined were Cr-0.5 TaC (mole pet), Cr-0.5 TiC, Cr-0.5 Cb(Nb)C, Cr-0.5 HfC, Cr-0.5 ZrC, Cr-0.5 CbB, and Cr-0.5 TaB. A transition in stability from the carbide of the principal alloying metal to Cr₂₃C₆, complete at approximately 2800°F, occurs in the Cr-0.5 TaC, Cr-0.5 TiC, and Cr-0.5 CbC alloys. Similarly, a change in phase stability from borides of columbium (niobium) and tantalum to Cr₄B occurs at ∼2800°F in the Cr-0.5 CbB and Cr-0.5 TaB compositions. The compounds HfC and ZrC, respectively, remained stable in the Cr-0.5 HfC and Cr-0.5 ZrC alloys at this temperature. Stress-rupture properties at 2100°F improved for several alloys when aged at this temperature to precipitate the carbide or boride of the principal alloying metal following higher temperature heat treatment to form the Cr₂₃C₆ or Cr₄B phases. Rupture life of the Cr-0.5 TaC alloy, for example, was increased at 15 ksi and 2100°F from 4 hr for as-fabricated material, to 186 hr after heat treatment. Improvement of rupture life for similar material and test conditions from 24 hr to 382 hr was observed in the Cr-0.5 TaB composition.     

Creep and tensile properties of several oxide dispersion strengthened nickel base alloys

The room temperature and 1365 K tensile properties and 1365 K tensile creep properties at low strain rates were measured for several oxide dispersion strengthened (ODS) alloys. The alloys examined included ODS Ni, ODS Ni-20Cr and ODS Ni-16Cr-4J5Al. Metallography of creep tested, large grain size ODS alloys indicated that creep of these alloys is an inhomogeneous process. All alloys appear to possess a threshold stress for creep. It is believed that the threshold stress is associated with diffusional creep in the large grain size ODS alloys and normal dislocation motion in perfect single crystalline ODS alloys. Threshold stresses for large grain size ODS Ni-20Cr and Ni-16Cr-4J5A1 type alloys are dependent on the grain aspect ratio. Because of the deleterious effect of prior creep on room temperature mechanical properties of large grain size ODS alloys, it is speculated that the threshold stress may be the design-limiting creep strength property.     

Structure and properties of Nb-Al alloys prepared by powder metallurgy

The structure and short-time strength of Nb-Al alloys of two compositions prepared by powder metallurgy are studied. The mechanical alloying of niobium with aluminum in a planetary ball mill in air is shown to result in simultaneous alloying of niobium with oxygen. During subsequent vacuum high-temperature sintering, disperse particles of a complex oxide, whose tentative composition is (AlNb)₂O₃, form in the alloy structure. The short-time strength at 1250°C of the prepared alloys exceeds that of nickel-aluminum superalloys.     

Effect of prior creep at 1365 K on the room temperature tensile properties of several oxide dispersion strengthened alloys

A study was undertaken to determine if oxide dispersion strengthened (ODS) Ni-base alloys in wrought bar form are subject to a loss of room temperature tensile properties after elevated temperature creep similar to that found in a thin gage ODS alloy sheet. The bar products evaluated included ODS-Ni, ODS-NiCr, and advanced ODS-NiCrAl types. Tensile type test specimens were creep exposed in air at various stress levels at 1365 K and then tensile tested at room temperature. Low residual tensile properties, change in fracture mode, the appearance of dispersoid free bands, grain boundary cavitation, and/or internal oxidation in the microstructure were interpreted as creep degradation effects. This work has shown that many ODS alloys are subject to creep damage. Degradation of tensile properties occurs after very small amounts (≲0.2 pct) of creep strain; ductility being the most sensitive property. The amount of degradation is dependent on the creep strain and is essentially independent of the alloy system. All the ODS alloys which were creep damaged possessed a large grain size (>100 μm). Creep damage appears to be due to diffusional creep which produces dispersoid free bands around boundaries acting as vacancy sources. Low angle and, possibly, twin boundaries were found to act as vacancy sources. The residual tensile properties of two alloys were not affected by prior creep parallel to the extrusion axis. One of these alloys, DS-NiCr(S), was single crystalline. The other alloy, TD-Ni, possessed a small, elongated grain structure which minimized the thickness of the dispersoid free bands produced by diffusional creep.     

Effect of yttrium oxide volume fraction and particle size on elevated temperature strength of a dispersic strengthened superalloy

The effect of yttrium oxide dispersoid volume fraction and particle size on the 1400†F (1033 K) and 1900†F (1311 K) rupture strength of a dispersion strengthened nickel-base superalloy, made by mechanical alloying, was investigated. Yttrium oxide Contents ranged from 0 pct to 4.5 pct by volume, and average oxide particle sizes varied from about 150Å (15 nm) to 580Å (15 nm) to 580Å (58 nm). High volume fractions and small particle sizes gave low grain aspect ratios and poor 1900†F (1311 K) and 1400†F (1033 K) stress rupture properties following heat treatment. Rupture strengths at 1400†F (1033 K) were otherwise relatively unaffected by dispersoid parameters. At grain aspect ratios less than 6.0, 1900†F (1311 K) rupture strength was controlled by grain geometry while at higher values rupture strength was directly influenced by dispersoid parameters.